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binutils-gdb/gdb/ppc-linux-tdep.c

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/* Target-dependent code for GDB, the GNU debugger.
Copyright 1986, 1987, 1989, 1991, 1992, 1993, 1994, 1995, 1996,
1997, 2000, 2001, 2002 Free Software Foundation, Inc.
This file is part of GDB.
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 59 Temple Place - Suite 330,
Boston, MA 02111-1307, USA. */
#include "defs.h"
#include "frame.h"
#include "inferior.h"
#include "symtab.h"
#include "target.h"
#include "gdbcore.h"
#include "gdbcmd.h"
#include "symfile.h"
#include "objfiles.h"
#include "regcache.h"
#include "value.h"
#include "solib-svr4.h"
#include "ppc-tdep.h"
/* The following two instructions are used in the signal trampoline
code on GNU/Linux PPC. */
#define INSTR_LI_R0_0x7777 0x38007777
#define INSTR_SC 0x44000002
/* Since the *-tdep.c files are platform independent (i.e, they may be
used to build cross platform debuggers), we can't include system
headers. Therefore, details concerning the sigcontext structure
must be painstakingly rerecorded. What's worse, if these details
ever change in the header files, they'll have to be changed here
as well. */
/* __SIGNAL_FRAMESIZE from <asm/ptrace.h> */
#define PPC_LINUX_SIGNAL_FRAMESIZE 64
/* From <asm/sigcontext.h>, offsetof(struct sigcontext_struct, regs) == 0x1c */
#define PPC_LINUX_REGS_PTR_OFFSET (PPC_LINUX_SIGNAL_FRAMESIZE + 0x1c)
/* From <asm/sigcontext.h>,
offsetof(struct sigcontext_struct, handler) == 0x14 */
#define PPC_LINUX_HANDLER_PTR_OFFSET (PPC_LINUX_SIGNAL_FRAMESIZE + 0x14)
/* From <asm/ptrace.h>, values for PT_NIP, PT_R1, and PT_LNK */
#define PPC_LINUX_PT_R0 0
#define PPC_LINUX_PT_R1 1
#define PPC_LINUX_PT_R2 2
#define PPC_LINUX_PT_R3 3
#define PPC_LINUX_PT_R4 4
#define PPC_LINUX_PT_R5 5
#define PPC_LINUX_PT_R6 6
#define PPC_LINUX_PT_R7 7
#define PPC_LINUX_PT_R8 8
#define PPC_LINUX_PT_R9 9
#define PPC_LINUX_PT_R10 10
#define PPC_LINUX_PT_R11 11
#define PPC_LINUX_PT_R12 12
#define PPC_LINUX_PT_R13 13
#define PPC_LINUX_PT_R14 14
#define PPC_LINUX_PT_R15 15
#define PPC_LINUX_PT_R16 16
#define PPC_LINUX_PT_R17 17
#define PPC_LINUX_PT_R18 18
#define PPC_LINUX_PT_R19 19
#define PPC_LINUX_PT_R20 20
#define PPC_LINUX_PT_R21 21
#define PPC_LINUX_PT_R22 22
#define PPC_LINUX_PT_R23 23
#define PPC_LINUX_PT_R24 24
#define PPC_LINUX_PT_R25 25
#define PPC_LINUX_PT_R26 26
#define PPC_LINUX_PT_R27 27
#define PPC_LINUX_PT_R28 28
#define PPC_LINUX_PT_R29 29
#define PPC_LINUX_PT_R30 30
#define PPC_LINUX_PT_R31 31
#define PPC_LINUX_PT_NIP 32
#define PPC_LINUX_PT_MSR 33
#define PPC_LINUX_PT_CTR 35
#define PPC_LINUX_PT_LNK 36
#define PPC_LINUX_PT_XER 37
#define PPC_LINUX_PT_CCR 38
#define PPC_LINUX_PT_MQ 39
#define PPC_LINUX_PT_FPR0 48 /* each FP reg occupies 2 slots in this space */
#define PPC_LINUX_PT_FPR31 (PPC_LINUX_PT_FPR0 + 2*31)
#define PPC_LINUX_PT_FPSCR (PPC_LINUX_PT_FPR0 + 2*32 + 1)
static int ppc_linux_at_sigtramp_return_path (CORE_ADDR pc);
/* Determine if pc is in a signal trampoline...
Ha! That's not what this does at all. wait_for_inferior in
infrun.c calls PC_IN_SIGTRAMP in order to detect entry into a
signal trampoline just after delivery of a signal. But on
GNU/Linux, signal trampolines are used for the return path only.
The kernel sets things up so that the signal handler is called
directly.
If we use in_sigtramp2() in place of in_sigtramp() (see below)
we'll (often) end up with stop_pc in the trampoline and prev_pc in
the (now exited) handler. The code there will cause a temporary
breakpoint to be set on prev_pc which is not very likely to get hit
again.
If this is confusing, think of it this way... the code in
wait_for_inferior() needs to be able to detect entry into a signal
trampoline just after a signal is delivered, not after the handler
has been run.
So, we define in_sigtramp() below to return 1 if the following is
true:
1) The previous frame is a real signal trampoline.
- and -
2) pc is at the first or second instruction of the corresponding
handler.
Why the second instruction? It seems that wait_for_inferior()
never sees the first instruction when single stepping. When a
signal is delivered while stepping, the next instruction that
would've been stepped over isn't, instead a signal is delivered and
the first instruction of the handler is stepped over instead. That
puts us on the second instruction. (I added the test for the
first instruction long after the fact, just in case the observed
behavior is ever fixed.)
PC_IN_SIGTRAMP is called from blockframe.c as well in order to set
the signal_handler_caller flag. Because of our strange definition
of in_sigtramp below, we can't rely on signal_handler_caller
getting set correctly from within blockframe.c. This is why we
take pains to set it in init_extra_frame_info(). */
int
ppc_linux_in_sigtramp (CORE_ADDR pc, char *func_name)
{
CORE_ADDR lr;
CORE_ADDR sp;
CORE_ADDR tramp_sp;
char buf[4];
CORE_ADDR handler;
lr = read_register (gdbarch_tdep (current_gdbarch)->ppc_lr_regnum);
if (!ppc_linux_at_sigtramp_return_path (lr))
return 0;
sp = read_register (SP_REGNUM);
if (target_read_memory (sp, buf, sizeof (buf)) != 0)
return 0;
tramp_sp = extract_unsigned_integer (buf, 4);
if (target_read_memory (tramp_sp + PPC_LINUX_HANDLER_PTR_OFFSET, buf,
sizeof (buf)) != 0)
return 0;
handler = extract_unsigned_integer (buf, 4);
return (pc == handler || pc == handler + 4);
}
/*
* The signal handler trampoline is on the stack and consists of exactly
* two instructions. The easiest and most accurate way of determining
* whether the pc is in one of these trampolines is by inspecting the
* instructions. It'd be faster though if we could find a way to do this
* via some simple address comparisons.
*/
static int
ppc_linux_at_sigtramp_return_path (CORE_ADDR pc)
{
char buf[12];
unsigned long pcinsn;
if (target_read_memory (pc - 4, buf, sizeof (buf)) != 0)
return 0;
/* extract the instruction at the pc */
pcinsn = extract_unsigned_integer (buf + 4, 4);
return (
(pcinsn == INSTR_LI_R0_0x7777
&& extract_unsigned_integer (buf + 8, 4) == INSTR_SC)
||
(pcinsn == INSTR_SC
&& extract_unsigned_integer (buf, 4) == INSTR_LI_R0_0x7777));
}
CORE_ADDR
ppc_linux_skip_trampoline_code (CORE_ADDR pc)
{
char buf[4];
struct obj_section *sect;
struct objfile *objfile;
unsigned long insn;
CORE_ADDR plt_start = 0;
CORE_ADDR symtab = 0;
CORE_ADDR strtab = 0;
int num_slots = -1;
int reloc_index = -1;
CORE_ADDR plt_table;
CORE_ADDR reloc;
CORE_ADDR sym;
long symidx;
char symname[1024];
struct minimal_symbol *msymbol;
/* Find the section pc is in; return if not in .plt */
sect = find_pc_section (pc);
if (!sect || strcmp (sect->the_bfd_section->name, ".plt") != 0)
return 0;
objfile = sect->objfile;
/* Pick up the instruction at pc. It had better be of the
form
li r11, IDX
where IDX is an index into the plt_table. */
if (target_read_memory (pc, buf, 4) != 0)
return 0;
insn = extract_unsigned_integer (buf, 4);
if ((insn & 0xffff0000) != 0x39600000 /* li r11, VAL */ )
return 0;
reloc_index = (insn << 16) >> 16;
/* Find the objfile that pc is in and obtain the information
necessary for finding the symbol name. */
for (sect = objfile->sections; sect < objfile->sections_end; ++sect)
{
const char *secname = sect->the_bfd_section->name;
if (strcmp (secname, ".plt") == 0)
plt_start = sect->addr;
else if (strcmp (secname, ".rela.plt") == 0)
num_slots = ((int) sect->endaddr - (int) sect->addr) / 12;
else if (strcmp (secname, ".dynsym") == 0)
symtab = sect->addr;
else if (strcmp (secname, ".dynstr") == 0)
strtab = sect->addr;
}
/* Make sure we have all the information we need. */
if (plt_start == 0 || num_slots == -1 || symtab == 0 || strtab == 0)
return 0;
/* Compute the value of the plt table */
plt_table = plt_start + 72 + 8 * num_slots;
/* Get address of the relocation entry (Elf32_Rela) */
if (target_read_memory (plt_table + reloc_index, buf, 4) != 0)
return 0;
reloc = extract_address (buf, 4);
sect = find_pc_section (reloc);
if (!sect)
return 0;
if (strcmp (sect->the_bfd_section->name, ".text") == 0)
return reloc;
/* Now get the r_info field which is the relocation type and symbol
index. */
if (target_read_memory (reloc + 4, buf, 4) != 0)
return 0;
symidx = extract_unsigned_integer (buf, 4);
/* Shift out the relocation type leaving just the symbol index */
/* symidx = ELF32_R_SYM(symidx); */
symidx = symidx >> 8;
/* compute the address of the symbol */
sym = symtab + symidx * 4;
/* Fetch the string table index */
if (target_read_memory (sym, buf, 4) != 0)
return 0;
symidx = extract_unsigned_integer (buf, 4);
/* Fetch the string; we don't know how long it is. Is it possible
that the following will fail because we're trying to fetch too
much? */
if (target_read_memory (strtab + symidx, symname, sizeof (symname)) != 0)
return 0;
/* This might not work right if we have multiple symbols with the
same name; the only way to really get it right is to perform
the same sort of lookup as the dynamic linker. */
msymbol = lookup_minimal_symbol_text (symname, NULL, NULL);
if (!msymbol)
return 0;
return SYMBOL_VALUE_ADDRESS (msymbol);
}
/* The rs6000 version of FRAME_SAVED_PC will almost work for us. The
signal handler details are different, so we'll handle those here
and call the rs6000 version to do the rest. */
CORE_ADDR
ppc_linux_frame_saved_pc (struct frame_info *fi)
{
if (fi->signal_handler_caller)
{
CORE_ADDR regs_addr =
read_memory_integer (fi->frame + PPC_LINUX_REGS_PTR_OFFSET, 4);
/* return the NIP in the regs array */
return read_memory_integer (regs_addr + 4 * PPC_LINUX_PT_NIP, 4);
}
else if (fi->next && fi->next->signal_handler_caller)
{
CORE_ADDR regs_addr =
read_memory_integer (fi->next->frame + PPC_LINUX_REGS_PTR_OFFSET, 4);
/* return LNK in the regs array */
return read_memory_integer (regs_addr + 4 * PPC_LINUX_PT_LNK, 4);
}
else
return rs6000_frame_saved_pc (fi);
}
void
ppc_linux_init_extra_frame_info (int fromleaf, struct frame_info *fi)
{
rs6000_init_extra_frame_info (fromleaf, fi);
if (fi->next != 0)
{
/* We're called from get_prev_frame_info; check to see if
this is a signal frame by looking to see if the pc points
at trampoline code */
if (ppc_linux_at_sigtramp_return_path (fi->pc))
fi->signal_handler_caller = 1;
else
fi->signal_handler_caller = 0;
}
}
int
ppc_linux_frameless_function_invocation (struct frame_info *fi)
{
/* We'll find the wrong thing if we let
rs6000_frameless_function_invocation () search for a signal trampoline */
if (ppc_linux_at_sigtramp_return_path (fi->pc))
return 0;
else
return rs6000_frameless_function_invocation (fi);
}
void
ppc_linux_frame_init_saved_regs (struct frame_info *fi)
{
if (fi->signal_handler_caller)
{
CORE_ADDR regs_addr;
int i;
if (fi->saved_regs)
return;
frame_saved_regs_zalloc (fi);
regs_addr =
read_memory_integer (fi->frame + PPC_LINUX_REGS_PTR_OFFSET, 4);
fi->saved_regs[PC_REGNUM] = regs_addr + 4 * PPC_LINUX_PT_NIP;
fi->saved_regs[gdbarch_tdep (current_gdbarch)->ppc_ps_regnum] =
regs_addr + 4 * PPC_LINUX_PT_MSR;
fi->saved_regs[gdbarch_tdep (current_gdbarch)->ppc_cr_regnum] =
regs_addr + 4 * PPC_LINUX_PT_CCR;
fi->saved_regs[gdbarch_tdep (current_gdbarch)->ppc_lr_regnum] =
regs_addr + 4 * PPC_LINUX_PT_LNK;
fi->saved_regs[gdbarch_tdep (current_gdbarch)->ppc_ctr_regnum] =
regs_addr + 4 * PPC_LINUX_PT_CTR;
fi->saved_regs[gdbarch_tdep (current_gdbarch)->ppc_xer_regnum] =
regs_addr + 4 * PPC_LINUX_PT_XER;
fi->saved_regs[gdbarch_tdep (current_gdbarch)->ppc_mq_regnum] =
regs_addr + 4 * PPC_LINUX_PT_MQ;
for (i = 0; i < 32; i++)
fi->saved_regs[gdbarch_tdep (current_gdbarch)->ppc_gp0_regnum + i] =
regs_addr + 4 * PPC_LINUX_PT_R0 + 4 * i;
for (i = 0; i < 32; i++)
fi->saved_regs[FP0_REGNUM + i] = regs_addr + 4 * PPC_LINUX_PT_FPR0 + 8 * i;
}
else
rs6000_frame_init_saved_regs (fi);
}
CORE_ADDR
ppc_linux_frame_chain (struct frame_info *thisframe)
{
/* Kernel properly constructs the frame chain for the handler */
if (thisframe->signal_handler_caller)
return read_memory_integer ((thisframe)->frame, 4);
else
return rs6000_frame_chain (thisframe);
}
/* FIXME: Move the following to rs6000-tdep.c (or some other file where
it may be used generically by ports which use either the SysV ABI or
the EABI */
/* Until November 2001, gcc was not complying to the SYSV ABI for
returning structures less than or equal to 8 bytes in size. It was
returning everything in memory. When this was corrected, it wasn't
fixed for native platforms. */
int
ppc_sysv_abi_broken_use_struct_convention (int gcc_p, struct type *value_type)
{
if (TYPE_LENGTH (value_type) == 16
&& TYPE_VECTOR (value_type))
return 0;
return generic_use_struct_convention (gcc_p, value_type);
}
/* Structures 8 bytes or less long are returned in the r3 & r4
registers, according to the SYSV ABI. */
int
ppc_sysv_abi_use_struct_convention (int gcc_p, struct type *value_type)
{
if (TYPE_LENGTH (value_type) == 16
&& TYPE_VECTOR (value_type))
return 0;
return (TYPE_LENGTH (value_type) > 8);
}
/* round2 rounds x up to the nearest multiple of s assuming that s is a
power of 2 */
#undef round2
#define round2(x,s) ((((long) (x) - 1) & ~(long)((s)-1)) + (s))
/* Pass the arguments in either registers, or in the stack. Using the
ppc sysv ABI, the first eight words of the argument list (that might
be less than eight parameters if some parameters occupy more than one
word) are passed in r3..r10 registers. float and double parameters are
passed in fpr's, in addition to that. Rest of the parameters if any
are passed in user stack.
If the function is returning a structure, then the return address is passed
in r3, then the first 7 words of the parametes can be passed in registers,
starting from r4. */
CORE_ADDR
ppc_sysv_abi_push_arguments (int nargs, struct value **args, CORE_ADDR sp,
int struct_return, CORE_ADDR struct_addr)
{
int argno;
/* Next available general register for non-float, non-vector arguments. */
int greg;
/* Next available floating point register for float arguments. */
int freg;
/* Next available vector register for vector arguments. */
int vreg;
int argstkspace;
int structstkspace;
int argoffset;
int structoffset;
struct value *arg;
struct type *type;
int len;
char old_sp_buf[4];
CORE_ADDR saved_sp;
greg = struct_return ? 4 : 3;
freg = 1;
vreg = 2;
argstkspace = 0;
structstkspace = 0;
/* Figure out how much new stack space is required for arguments
which don't fit in registers. Unlike the PowerOpen ABI, the
SysV ABI doesn't reserve any extra space for parameters which
are put in registers. */
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)
freg++;
else
{
/* SysV ABI converts floats to doubles when placed in
memory and requires 8 byte alignment */
if (argstkspace & 0x4)
argstkspace += 4;
argstkspace += 8;
}
}
else if (TYPE_CODE (type) == TYPE_CODE_INT && len == 8) /* long long */
{
if (greg > 9)
{
greg = 11;
if (argstkspace & 0x4)
argstkspace += 4;
argstkspace += 8;
}
else
{
if ((greg & 1) == 0)
greg++;
greg += 2;
}
}
else if (!TYPE_VECTOR (type))
{
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;
}
else
{
if (len == 16
&& TYPE_CODE (type) == TYPE_CODE_ARRAY
&& TYPE_VECTOR (type))
{
if (vreg <= 13)
vreg++;
else
{
/* Vector arguments must be aligned to 16 bytes on
the stack. */
argstkspace += round2 (argstkspace, 16);
argstkspace += 16;
}
}
}
}
/* 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;
vreg = 2;
/* Fill in r3 with the return structure, if any */
if (struct_return)
{
char val_buf[4];
store_address (val_buf, 4, struct_addr);
memcpy (&registers[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 (&registers[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 (&registers[REGISTER_BYTE (greg)],
VALUE_CONTENTS (arg), 4);
memcpy (&registers[REGISTER_BYTE (greg + 1)],
VALUE_CONTENTS (arg) + 4, 4);
greg += 2;
}
}
else if (!TYPE_VECTOR (type))
{
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)
{
memcpy (&registers[REGISTER_BYTE (greg)], val_buf, 4);
greg++;
}
else
{
write_memory (sp + argoffset, val_buf, 4);
argoffset += 4;
}
}
else
{
if (len == 16
&& TYPE_CODE (type) == TYPE_CODE_ARRAY
&& TYPE_VECTOR (type))
{
struct gdbarch_tdep *tdep = gdbarch_tdep (current_gdbarch);
char *v_val_buf = alloca (16);
memset (v_val_buf, 0, 16);
memcpy (v_val_buf, VALUE_CONTENTS (arg), len);
if (vreg <= 13)
{
memcpy (&registers[REGISTER_BYTE (tdep->ppc_vr0_regnum
+ vreg)],
v_val_buf, 16);
vreg++;
}
else
{
write_memory (sp + argoffset, v_val_buf, 16);
argoffset += 16;
}
}
}
}
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)
{
const 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;
}
/* Fetch (and possibly build) an appropriate link_map_offsets
structure for GNU/Linux PPC targets using the struct offsets
defined in link.h (but without actual reference to that file).
This makes it possible to access GNU/Linux PPC shared libraries
from a GDB that was not built on an GNU/Linux PPC host (for cross
debugging). */
struct link_map_offsets *
ppc_linux_svr4_fetch_link_map_offsets (void)
{
static struct link_map_offsets lmo;
static struct link_map_offsets *lmp = NULL;
if (lmp == NULL)
{
lmp = &lmo;
lmo.r_debug_size = 8; /* The actual size is 20 bytes, but
this is all we need. */
lmo.r_map_offset = 4;
lmo.r_map_size = 4;
lmo.link_map_size = 20; /* The actual size is 560 bytes, but
this is all we need. */
lmo.l_addr_offset = 0;
lmo.l_addr_size = 4;
lmo.l_name_offset = 4;
lmo.l_name_size = 4;
lmo.l_next_offset = 12;
lmo.l_next_size = 4;
lmo.l_prev_offset = 16;
lmo.l_prev_size = 4;
}
return lmp;
}
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