765 lines
24 KiB
C
765 lines
24 KiB
C
/* Target-dependent code for GDB, the GNU debugger.
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Copyright 1986, 1987, 1989, 1991, 1992, 1993, 1994, 1995, 1996, 1997,
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2000, 2001 Free Software Foundation, Inc.
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This file is part of GDB.
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This program is free software; you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 2 of the License, or
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(at your option) any later version.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program; if not, write to the Free Software
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Foundation, Inc., 59 Temple Place - Suite 330,
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Boston, MA 02111-1307, USA. */
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#include "defs.h"
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#include "frame.h"
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#include "inferior.h"
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#include "symtab.h"
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#include "target.h"
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#include "gdbcore.h"
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#include "gdbcmd.h"
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#include "symfile.h"
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#include "objfiles.h"
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#include "regcache.h"
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#include "ppc-tdep.h"
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/* The following two instructions are used in the signal trampoline
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code on linux/ppc */
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#define INSTR_LI_R0_0x7777 0x38007777
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#define INSTR_SC 0x44000002
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/* Since the *-tdep.c files are platform independent (i.e, they may be
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used to build cross platform debuggers), we can't include system
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headers. Therefore, details concerning the sigcontext structure
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must be painstakingly rerecorded. What's worse, if these details
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ever change in the header files, they'll have to be changed here
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as well. */
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/* __SIGNAL_FRAMESIZE from <asm/ptrace.h> */
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#define PPC_LINUX_SIGNAL_FRAMESIZE 64
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/* From <asm/sigcontext.h>, offsetof(struct sigcontext_struct, regs) == 0x1c */
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#define PPC_LINUX_REGS_PTR_OFFSET (PPC_LINUX_SIGNAL_FRAMESIZE + 0x1c)
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/* From <asm/sigcontext.h>,
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offsetof(struct sigcontext_struct, handler) == 0x14 */
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#define PPC_LINUX_HANDLER_PTR_OFFSET (PPC_LINUX_SIGNAL_FRAMESIZE + 0x14)
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/* From <asm/ptrace.h>, values for PT_NIP, PT_R1, and PT_LNK */
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#define PPC_LINUX_PT_R0 0
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#define PPC_LINUX_PT_R1 1
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#define PPC_LINUX_PT_R2 2
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#define PPC_LINUX_PT_R3 3
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#define PPC_LINUX_PT_R4 4
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#define PPC_LINUX_PT_R5 5
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#define PPC_LINUX_PT_R6 6
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#define PPC_LINUX_PT_R7 7
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#define PPC_LINUX_PT_R8 8
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#define PPC_LINUX_PT_R9 9
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#define PPC_LINUX_PT_R10 10
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#define PPC_LINUX_PT_R11 11
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#define PPC_LINUX_PT_R12 12
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#define PPC_LINUX_PT_R13 13
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#define PPC_LINUX_PT_R14 14
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#define PPC_LINUX_PT_R15 15
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#define PPC_LINUX_PT_R16 16
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#define PPC_LINUX_PT_R17 17
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#define PPC_LINUX_PT_R18 18
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#define PPC_LINUX_PT_R19 19
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#define PPC_LINUX_PT_R20 20
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#define PPC_LINUX_PT_R21 21
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#define PPC_LINUX_PT_R22 22
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#define PPC_LINUX_PT_R23 23
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#define PPC_LINUX_PT_R24 24
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#define PPC_LINUX_PT_R25 25
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#define PPC_LINUX_PT_R26 26
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#define PPC_LINUX_PT_R27 27
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#define PPC_LINUX_PT_R28 28
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#define PPC_LINUX_PT_R29 29
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#define PPC_LINUX_PT_R30 30
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#define PPC_LINUX_PT_R31 31
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#define PPC_LINUX_PT_NIP 32
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#define PPC_LINUX_PT_MSR 33
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#define PPC_LINUX_PT_CTR 35
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#define PPC_LINUX_PT_LNK 36
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#define PPC_LINUX_PT_XER 37
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#define PPC_LINUX_PT_CCR 38
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#define PPC_LINUX_PT_MQ 39
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#define PPC_LINUX_PT_FPR0 48 /* each FP reg occupies 2 slots in this space */
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#define PPC_LINUX_PT_FPR31 (PPC_LINUX_PT_FPR0 + 2*31)
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#define PPC_LINUX_PT_FPSCR (PPC_LINUX_PT_FPR0 + 2*32 + 1)
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static int ppc_linux_at_sigtramp_return_path (CORE_ADDR pc);
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/* Determine if pc is in a signal trampoline...
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Ha! That's not what this does at all. wait_for_inferior in infrun.c
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calls IN_SIGTRAMP in order to detect entry into a signal trampoline
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just after delivery of a signal. But on linux, signal trampolines
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are used for the return path only. The kernel sets things up so that
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the signal handler is called directly.
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If we use in_sigtramp2() in place of in_sigtramp() (see below)
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we'll (often) end up with stop_pc in the trampoline and prev_pc in
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the (now exited) handler. The code there will cause a temporary
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breakpoint to be set on prev_pc which is not very likely to get hit
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again.
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If this is confusing, think of it this way... the code in
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wait_for_inferior() needs to be able to detect entry into a signal
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trampoline just after a signal is delivered, not after the handler
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has been run.
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So, we define in_sigtramp() below to return 1 if the following is
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true:
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1) The previous frame is a real signal trampoline.
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- and -
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2) pc is at the first or second instruction of the corresponding
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handler.
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Why the second instruction? It seems that wait_for_inferior()
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never sees the first instruction when single stepping. When a
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signal is delivered while stepping, the next instruction that
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would've been stepped over isn't, instead a signal is delivered and
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the first instruction of the handler is stepped over instead. That
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puts us on the second instruction. (I added the test for the
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first instruction long after the fact, just in case the observed
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behavior is ever fixed.)
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IN_SIGTRAMP is called from blockframe.c as well in order to set
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the signal_handler_caller flag. Because of our strange definition
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of in_sigtramp below, we can't rely on signal_handler_caller getting
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set correctly from within blockframe.c. This is why we take pains
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to set it in init_extra_frame_info(). */
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int
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ppc_linux_in_sigtramp (CORE_ADDR pc, char *func_name)
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{
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CORE_ADDR lr;
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CORE_ADDR sp;
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CORE_ADDR tramp_sp;
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char buf[4];
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CORE_ADDR handler;
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lr = read_register (PPC_LR_REGNUM);
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if (!ppc_linux_at_sigtramp_return_path (lr))
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return 0;
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sp = read_register (SP_REGNUM);
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if (target_read_memory (sp, buf, sizeof (buf)) != 0)
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return 0;
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tramp_sp = extract_unsigned_integer (buf, 4);
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if (target_read_memory (tramp_sp + PPC_LINUX_HANDLER_PTR_OFFSET, buf,
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sizeof (buf)) != 0)
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return 0;
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handler = extract_unsigned_integer (buf, 4);
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return (pc == handler || pc == handler + 4);
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}
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/*
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* The signal handler trampoline is on the stack and consists of exactly
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* two instructions. The easiest and most accurate way of determining
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* whether the pc is in one of these trampolines is by inspecting the
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* instructions. It'd be faster though if we could find a way to do this
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* via some simple address comparisons.
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*/
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static int
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ppc_linux_at_sigtramp_return_path (CORE_ADDR pc)
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{
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char buf[12];
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unsigned long pcinsn;
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if (target_read_memory (pc - 4, buf, sizeof (buf)) != 0)
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return 0;
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/* extract the instruction at the pc */
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pcinsn = extract_unsigned_integer (buf + 4, 4);
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return (
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(pcinsn == INSTR_LI_R0_0x7777
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&& extract_unsigned_integer (buf + 8, 4) == INSTR_SC)
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||
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(pcinsn == INSTR_SC
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&& extract_unsigned_integer (buf, 4) == INSTR_LI_R0_0x7777));
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}
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CORE_ADDR
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ppc_linux_skip_trampoline_code (CORE_ADDR pc)
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{
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char buf[4];
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struct obj_section *sect;
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struct objfile *objfile;
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unsigned long insn;
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CORE_ADDR plt_start = 0;
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CORE_ADDR symtab = 0;
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CORE_ADDR strtab = 0;
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int num_slots = -1;
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int reloc_index = -1;
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CORE_ADDR plt_table;
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CORE_ADDR reloc;
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CORE_ADDR sym;
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long symidx;
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char symname[1024];
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struct minimal_symbol *msymbol;
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/* Find the section pc is in; return if not in .plt */
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sect = find_pc_section (pc);
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if (!sect || strcmp (sect->the_bfd_section->name, ".plt") != 0)
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return 0;
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objfile = sect->objfile;
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/* Pick up the instruction at pc. It had better be of the
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form
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li r11, IDX
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where IDX is an index into the plt_table. */
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if (target_read_memory (pc, buf, 4) != 0)
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return 0;
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insn = extract_unsigned_integer (buf, 4);
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if ((insn & 0xffff0000) != 0x39600000 /* li r11, VAL */ )
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return 0;
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reloc_index = (insn << 16) >> 16;
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/* Find the objfile that pc is in and obtain the information
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necessary for finding the symbol name. */
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for (sect = objfile->sections; sect < objfile->sections_end; ++sect)
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{
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const char *secname = sect->the_bfd_section->name;
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if (strcmp (secname, ".plt") == 0)
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plt_start = sect->addr;
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else if (strcmp (secname, ".rela.plt") == 0)
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num_slots = ((int) sect->endaddr - (int) sect->addr) / 12;
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else if (strcmp (secname, ".dynsym") == 0)
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symtab = sect->addr;
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else if (strcmp (secname, ".dynstr") == 0)
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strtab = sect->addr;
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}
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/* Make sure we have all the information we need. */
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if (plt_start == 0 || num_slots == -1 || symtab == 0 || strtab == 0)
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return 0;
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/* Compute the value of the plt table */
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plt_table = plt_start + 72 + 8 * num_slots;
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/* Get address of the relocation entry (Elf32_Rela) */
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if (target_read_memory (plt_table + reloc_index, buf, 4) != 0)
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return 0;
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reloc = extract_address (buf, 4);
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sect = find_pc_section (reloc);
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if (!sect)
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return 0;
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if (strcmp (sect->the_bfd_section->name, ".text") == 0)
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return reloc;
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/* Now get the r_info field which is the relocation type and symbol
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index. */
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if (target_read_memory (reloc + 4, buf, 4) != 0)
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return 0;
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symidx = extract_unsigned_integer (buf, 4);
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/* Shift out the relocation type leaving just the symbol index */
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/* symidx = ELF32_R_SYM(symidx); */
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symidx = symidx >> 8;
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/* compute the address of the symbol */
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sym = symtab + symidx * 4;
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/* Fetch the string table index */
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if (target_read_memory (sym, buf, 4) != 0)
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return 0;
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symidx = extract_unsigned_integer (buf, 4);
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/* Fetch the string; we don't know how long it is. Is it possible
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that the following will fail because we're trying to fetch too
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much? */
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if (target_read_memory (strtab + symidx, symname, sizeof (symname)) != 0)
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return 0;
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/* This might not work right if we have multiple symbols with the
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same name; the only way to really get it right is to perform
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the same sort of lookup as the dynamic linker. */
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msymbol = lookup_minimal_symbol_text (symname, NULL, NULL);
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if (!msymbol)
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return 0;
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return SYMBOL_VALUE_ADDRESS (msymbol);
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}
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/* The rs6000 version of FRAME_SAVED_PC will almost work for us. The
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signal handler details are different, so we'll handle those here
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and call the rs6000 version to do the rest. */
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CORE_ADDR
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ppc_linux_frame_saved_pc (struct frame_info *fi)
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{
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if (fi->signal_handler_caller)
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{
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CORE_ADDR regs_addr =
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read_memory_integer (fi->frame + PPC_LINUX_REGS_PTR_OFFSET, 4);
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/* return the NIP in the regs array */
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return read_memory_integer (regs_addr + 4 * PPC_LINUX_PT_NIP, 4);
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}
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else if (fi->next && fi->next->signal_handler_caller)
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{
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CORE_ADDR regs_addr =
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read_memory_integer (fi->next->frame + PPC_LINUX_REGS_PTR_OFFSET, 4);
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/* return LNK in the regs array */
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return read_memory_integer (regs_addr + 4 * PPC_LINUX_PT_LNK, 4);
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}
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else
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return rs6000_frame_saved_pc (fi);
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}
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void
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ppc_linux_init_extra_frame_info (int fromleaf, struct frame_info *fi)
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{
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rs6000_init_extra_frame_info (fromleaf, fi);
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if (fi->next != 0)
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{
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/* We're called from get_prev_frame_info; check to see if
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this is a signal frame by looking to see if the pc points
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at trampoline code */
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if (ppc_linux_at_sigtramp_return_path (fi->pc))
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fi->signal_handler_caller = 1;
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else
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fi->signal_handler_caller = 0;
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}
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}
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int
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ppc_linux_frameless_function_invocation (struct frame_info *fi)
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{
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/* We'll find the wrong thing if we let
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rs6000_frameless_function_invocation () search for a signal trampoline */
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if (ppc_linux_at_sigtramp_return_path (fi->pc))
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return 0;
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else
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return rs6000_frameless_function_invocation (fi);
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}
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void
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ppc_linux_frame_init_saved_regs (struct frame_info *fi)
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{
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if (fi->signal_handler_caller)
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{
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CORE_ADDR regs_addr;
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int i;
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if (fi->saved_regs)
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return;
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frame_saved_regs_zalloc (fi);
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regs_addr =
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read_memory_integer (fi->frame + PPC_LINUX_REGS_PTR_OFFSET, 4);
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fi->saved_regs[PC_REGNUM] = regs_addr + 4 * PPC_LINUX_PT_NIP;
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fi->saved_regs[PPC_PS_REGNUM] = regs_addr + 4 * PPC_LINUX_PT_MSR;
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fi->saved_regs[PPC_CR_REGNUM] = regs_addr + 4 * PPC_LINUX_PT_CCR;
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fi->saved_regs[PPC_LR_REGNUM] = regs_addr + 4 * PPC_LINUX_PT_LNK;
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fi->saved_regs[PPC_CTR_REGNUM] = regs_addr + 4 * PPC_LINUX_PT_CTR;
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fi->saved_regs[PPC_XER_REGNUM] = regs_addr + 4 * PPC_LINUX_PT_XER;
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fi->saved_regs[PPC_MQ_REGNUM] = regs_addr + 4 * PPC_LINUX_PT_MQ;
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for (i = 0; i < 32; i++)
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fi->saved_regs[PPC_GP0_REGNUM + i] = regs_addr + 4 * PPC_LINUX_PT_R0 + 4 * i;
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for (i = 0; i < 32; i++)
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fi->saved_regs[FP0_REGNUM + i] = regs_addr + 4 * PPC_LINUX_PT_FPR0 + 8 * i;
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}
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else
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rs6000_frame_init_saved_regs (fi);
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}
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CORE_ADDR
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ppc_linux_frame_chain (struct frame_info *thisframe)
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{
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/* Kernel properly constructs the frame chain for the handler */
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if (thisframe->signal_handler_caller)
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return read_memory_integer ((thisframe)->frame, 4);
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else
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return rs6000_frame_chain (thisframe);
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}
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/* FIXME: Move the following to rs6000-tdep.c (or some other file where
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it may be used generically by ports which use either the SysV ABI or
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the EABI */
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/* round2 rounds x up to the nearest multiple of s assuming that s is a
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power of 2 */
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#undef round2
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#define round2(x,s) ((((long) (x) - 1) & ~(long)((s)-1)) + (s))
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/* Pass the arguments in either registers, or in the stack. Using the
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ppc sysv ABI, the first eight words of the argument list (that might
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be less than eight parameters if some parameters occupy more than one
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word) are passed in r3..r10 registers. float and double parameters are
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passed in fpr's, in addition to that. Rest of the parameters if any
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are passed in user stack.
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If the function is returning a structure, then the return address is passed
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in r3, then the first 7 words of the parametes can be passed in registers,
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starting from r4. */
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CORE_ADDR
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ppc_sysv_abi_push_arguments (int nargs, value_ptr *args, CORE_ADDR sp,
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int struct_return, CORE_ADDR struct_addr)
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{
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int argno;
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int greg, freg;
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int argstkspace;
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int structstkspace;
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int argoffset;
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int structoffset;
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value_ptr arg;
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struct type *type;
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int len;
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char old_sp_buf[4];
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CORE_ADDR saved_sp;
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greg = struct_return ? 4 : 3;
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freg = 1;
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argstkspace = 0;
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structstkspace = 0;
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/* Figure out how much new stack space is required for arguments
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which don't fit in registers. Unlike the PowerOpen ABI, the
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SysV ABI doesn't reserve any extra space for parameters which
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are put in registers. */
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for (argno = 0; argno < nargs; argno++)
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{
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arg = args[argno];
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type = check_typedef (VALUE_TYPE (arg));
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len = TYPE_LENGTH (type);
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if (TYPE_CODE (type) == TYPE_CODE_FLT)
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{
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if (freg <= 8)
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freg++;
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else
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{
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/* SysV ABI converts floats to doubles when placed in
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memory and requires 8 byte alignment */
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if (argstkspace & 0x4)
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argstkspace += 4;
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argstkspace += 8;
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}
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}
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else if (TYPE_CODE (type) == TYPE_CODE_INT && len == 8) /* long long */
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{
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if (greg > 9)
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{
|
|
greg = 11;
|
|
if (argstkspace & 0x4)
|
|
argstkspace += 4;
|
|
argstkspace += 8;
|
|
}
|
|
else
|
|
{
|
|
if ((greg & 1) == 0)
|
|
greg++;
|
|
greg += 2;
|
|
}
|
|
}
|
|
else
|
|
{
|
|
if (len > 4
|
|
|| TYPE_CODE (type) == TYPE_CODE_STRUCT
|
|
|| TYPE_CODE (type) == TYPE_CODE_UNION)
|
|
{
|
|
/* Rounding to the nearest multiple of 8 may not be necessary,
|
|
but it is safe. Particularly since we don't know the
|
|
field types of the structure */
|
|
structstkspace += round2 (len, 8);
|
|
}
|
|
if (greg <= 10)
|
|
greg++;
|
|
else
|
|
argstkspace += 4;
|
|
}
|
|
}
|
|
|
|
/* Get current SP location */
|
|
saved_sp = read_sp ();
|
|
|
|
sp -= argstkspace + structstkspace;
|
|
|
|
/* Allocate space for backchain and callee's saved lr */
|
|
sp -= 8;
|
|
|
|
/* Make sure that we maintain 16 byte alignment */
|
|
sp &= ~0x0f;
|
|
|
|
/* Update %sp before proceeding any further */
|
|
write_register (SP_REGNUM, sp);
|
|
|
|
/* write the backchain */
|
|
store_address (old_sp_buf, 4, saved_sp);
|
|
write_memory (sp, old_sp_buf, 4);
|
|
|
|
argoffset = 8;
|
|
structoffset = argoffset + argstkspace;
|
|
freg = 1;
|
|
greg = 3;
|
|
/* Fill in r3 with the return structure, if any */
|
|
if (struct_return)
|
|
{
|
|
char val_buf[4];
|
|
store_address (val_buf, 4, struct_addr);
|
|
memcpy (®isters[REGISTER_BYTE (greg)], val_buf, 4);
|
|
greg++;
|
|
}
|
|
/* Now fill in the registers and stack... */
|
|
for (argno = 0; argno < nargs; argno++)
|
|
{
|
|
arg = args[argno];
|
|
type = check_typedef (VALUE_TYPE (arg));
|
|
len = TYPE_LENGTH (type);
|
|
|
|
if (TYPE_CODE (type) == TYPE_CODE_FLT)
|
|
{
|
|
if (freg <= 8)
|
|
{
|
|
if (len > 8)
|
|
printf_unfiltered (
|
|
"Fatal Error: a floating point parameter #%d with a size > 8 is found!\n", argno);
|
|
memcpy (®isters[REGISTER_BYTE (FP0_REGNUM + freg)],
|
|
VALUE_CONTENTS (arg), len);
|
|
freg++;
|
|
}
|
|
else
|
|
{
|
|
/* SysV ABI converts floats to doubles when placed in
|
|
memory and requires 8 byte alignment */
|
|
/* FIXME: Convert floats to doubles */
|
|
if (argoffset & 0x4)
|
|
argoffset += 4;
|
|
write_memory (sp + argoffset, (char *) VALUE_CONTENTS (arg), len);
|
|
argoffset += 8;
|
|
}
|
|
}
|
|
else if (TYPE_CODE (type) == TYPE_CODE_INT && len == 8) /* long long */
|
|
{
|
|
if (greg > 9)
|
|
{
|
|
greg = 11;
|
|
if (argoffset & 0x4)
|
|
argoffset += 4;
|
|
write_memory (sp + argoffset, (char *) VALUE_CONTENTS (arg), len);
|
|
argoffset += 8;
|
|
}
|
|
else
|
|
{
|
|
if ((greg & 1) == 0)
|
|
greg++;
|
|
|
|
memcpy (®isters[REGISTER_BYTE (greg)],
|
|
VALUE_CONTENTS (arg), 4);
|
|
memcpy (®isters[REGISTER_BYTE (greg + 1)],
|
|
VALUE_CONTENTS (arg) + 4, 4);
|
|
greg += 2;
|
|
}
|
|
}
|
|
else
|
|
{
|
|
char val_buf[4];
|
|
if (len > 4
|
|
|| TYPE_CODE (type) == TYPE_CODE_STRUCT
|
|
|| TYPE_CODE (type) == TYPE_CODE_UNION)
|
|
{
|
|
write_memory (sp + structoffset, VALUE_CONTENTS (arg), len);
|
|
store_address (val_buf, 4, sp + structoffset);
|
|
structoffset += round2 (len, 8);
|
|
}
|
|
else
|
|
{
|
|
memset (val_buf, 0, 4);
|
|
memcpy (val_buf, VALUE_CONTENTS (arg), len);
|
|
}
|
|
if (greg <= 10)
|
|
{
|
|
*(int *) ®isters[REGISTER_BYTE (greg)] = 0;
|
|
memcpy (®isters[REGISTER_BYTE (greg)], val_buf, 4);
|
|
greg++;
|
|
}
|
|
else
|
|
{
|
|
write_memory (sp + argoffset, val_buf, 4);
|
|
argoffset += 4;
|
|
}
|
|
}
|
|
}
|
|
|
|
target_store_registers (-1);
|
|
return sp;
|
|
}
|
|
|
|
/* ppc_linux_memory_remove_breakpoints attempts to remove a breakpoint
|
|
in much the same fashion as memory_remove_breakpoint in mem-break.c,
|
|
but is careful not to write back the previous contents if the code
|
|
in question has changed in between inserting the breakpoint and
|
|
removing it.
|
|
|
|
Here is the problem that we're trying to solve...
|
|
|
|
Once upon a time, before introducing this function to remove
|
|
breakpoints from the inferior, setting a breakpoint on a shared
|
|
library function prior to running the program would not work
|
|
properly. In order to understand the problem, it is first
|
|
necessary to understand a little bit about dynamic linking on
|
|
this platform.
|
|
|
|
A call to a shared library function is accomplished via a bl
|
|
(branch-and-link) instruction whose branch target is an entry
|
|
in the procedure linkage table (PLT). The PLT in the object
|
|
file is uninitialized. To gdb, prior to running the program, the
|
|
entries in the PLT are all zeros.
|
|
|
|
Once the program starts running, the shared libraries are loaded
|
|
and the procedure linkage table is initialized, but the entries in
|
|
the table are not (necessarily) resolved. Once a function is
|
|
actually called, the code in the PLT is hit and the function is
|
|
resolved. In order to better illustrate this, an example is in
|
|
order; the following example is from the gdb testsuite.
|
|
|
|
We start the program shmain.
|
|
|
|
[kev@arroyo testsuite]$ ../gdb gdb.base/shmain
|
|
[...]
|
|
|
|
We place two breakpoints, one on shr1 and the other on main.
|
|
|
|
(gdb) b shr1
|
|
Breakpoint 1 at 0x100409d4
|
|
(gdb) b main
|
|
Breakpoint 2 at 0x100006a0: file gdb.base/shmain.c, line 44.
|
|
|
|
Examine the instruction (and the immediatly following instruction)
|
|
upon which the breakpoint was placed. Note that the PLT entry
|
|
for shr1 contains zeros.
|
|
|
|
(gdb) x/2i 0x100409d4
|
|
0x100409d4 <shr1>: .long 0x0
|
|
0x100409d8 <shr1+4>: .long 0x0
|
|
|
|
Now run 'til main.
|
|
|
|
(gdb) r
|
|
Starting program: gdb.base/shmain
|
|
Breakpoint 1 at 0xffaf790: file gdb.base/shr1.c, line 19.
|
|
|
|
Breakpoint 2, main ()
|
|
at gdb.base/shmain.c:44
|
|
44 g = 1;
|
|
|
|
Examine the PLT again. Note that the loading of the shared
|
|
library has initialized the PLT to code which loads a constant
|
|
(which I think is an index into the GOT) into r11 and then
|
|
branchs a short distance to the code which actually does the
|
|
resolving.
|
|
|
|
(gdb) x/2i 0x100409d4
|
|
0x100409d4 <shr1>: li r11,4
|
|
0x100409d8 <shr1+4>: b 0x10040984 <sg+4>
|
|
(gdb) c
|
|
Continuing.
|
|
|
|
Breakpoint 1, shr1 (x=1)
|
|
at gdb.base/shr1.c:19
|
|
19 l = 1;
|
|
|
|
Now we've hit the breakpoint at shr1. (The breakpoint was
|
|
reset from the PLT entry to the actual shr1 function after the
|
|
shared library was loaded.) Note that the PLT entry has been
|
|
resolved to contain a branch that takes us directly to shr1.
|
|
(The real one, not the PLT entry.)
|
|
|
|
(gdb) x/2i 0x100409d4
|
|
0x100409d4 <shr1>: b 0xffaf76c <shr1>
|
|
0x100409d8 <shr1+4>: b 0x10040984 <sg+4>
|
|
|
|
The thing to note here is that the PLT entry for shr1 has been
|
|
changed twice.
|
|
|
|
Now the problem should be obvious. GDB places a breakpoint (a
|
|
trap instruction) on the zero value of the PLT entry for shr1.
|
|
Later on, after the shared library had been loaded and the PLT
|
|
initialized, GDB gets a signal indicating this fact and attempts
|
|
(as it always does when it stops) to remove all the breakpoints.
|
|
|
|
The breakpoint removal was causing the former contents (a zero
|
|
word) to be written back to the now initialized PLT entry thus
|
|
destroying a portion of the initialization that had occurred only a
|
|
short time ago. When execution continued, the zero word would be
|
|
executed as an instruction an an illegal instruction trap was
|
|
generated instead. (0 is not a legal instruction.)
|
|
|
|
The fix for this problem was fairly straightforward. The function
|
|
memory_remove_breakpoint from mem-break.c was copied to this file,
|
|
modified slightly, and renamed to ppc_linux_memory_remove_breakpoint.
|
|
In tm-linux.h, MEMORY_REMOVE_BREAKPOINT is defined to call this new
|
|
function.
|
|
|
|
The differences between ppc_linux_memory_remove_breakpoint () and
|
|
memory_remove_breakpoint () are minor. All that the former does
|
|
that the latter does not is check to make sure that the breakpoint
|
|
location actually contains a breakpoint (trap instruction) prior
|
|
to attempting to write back the old contents. If it does contain
|
|
a trap instruction, we allow the old contents to be written back.
|
|
Otherwise, we silently do nothing.
|
|
|
|
The big question is whether memory_remove_breakpoint () should be
|
|
changed to have the same functionality. The downside is that more
|
|
traffic is generated for remote targets since we'll have an extra
|
|
fetch of a memory word each time a breakpoint is removed.
|
|
|
|
For the time being, we'll leave this self-modifying-code-friendly
|
|
version in ppc-linux-tdep.c, but it ought to be migrated somewhere
|
|
else in the event that some other platform has similar needs with
|
|
regard to removing breakpoints in some potentially self modifying
|
|
code. */
|
|
int
|
|
ppc_linux_memory_remove_breakpoint (CORE_ADDR addr, char *contents_cache)
|
|
{
|
|
unsigned char *bp;
|
|
int val;
|
|
int bplen;
|
|
char old_contents[BREAKPOINT_MAX];
|
|
|
|
/* Determine appropriate breakpoint contents and size for this address. */
|
|
bp = BREAKPOINT_FROM_PC (&addr, &bplen);
|
|
if (bp == NULL)
|
|
error ("Software breakpoints not implemented for this target.");
|
|
|
|
val = target_read_memory (addr, old_contents, bplen);
|
|
|
|
/* If our breakpoint is no longer at the address, this means that the
|
|
program modified the code on us, so it is wrong to put back the
|
|
old value */
|
|
if (val == 0 && memcmp (bp, old_contents, bplen) == 0)
|
|
val = target_write_memory (addr, contents_cache, bplen);
|
|
|
|
return val;
|
|
}
|