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MIPS Kprobes: Support branch instructions probing 

This patch provides support for kprobes on branch instructions. The branch
instruction at the probed address is actually emulated and not executed
out-of-line like other normal instructions. Instead the delay-slot instruction
is copied and single stepped out of line.

At the time of probe hit, the original branch instruction is evaluated
and the target cp0_epc is computed similar to compute_retrun_epc(). It
is also checked if the delay slot instruction can be skipped, which is
true if there is a NOP in delay slot or branch is taken in case of
branch likely instructions. Once the delay slot instruction is single
stepped the normal execution resume with the cp0_epc updated the earlier
computed cp0_epc as per the branch instructions.

Signed-off-by: Maneesh Soni <manesoni@cisco.com>
Signed-off-by: Victor Kamensky <kamensky@cisco.com>
Cc: David Daney <david.daney@cavium.com>
Cc: ananth@in.ibm.com
Cc: linux-kernel@vger.kernel.org
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/2914/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
This commit is contained in:
Maneesh Soni 2011-11-08 17:08:26 +05:30 committed by Ralf Baechle
parent d8d4e3ae0b
commit 6457a396bb
2 changed files with 117 additions and 33 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

5
arch/mips/include/asm/kprobes.h
View File

@ -74,6 +74,8 @@ struct prev_kprobe {
: MAX_JPROBES_STACK_SIZE)
#define SKIP_DELAYSLOT 0x0001
/* per-cpu kprobe control block */
struct kprobe_ctlblk {
unsigned long kprobe_status;
@ -82,6 +84,9 @@ struct kprobe_ctlblk {
unsigned long kprobe_saved_epc;
unsigned long jprobe_saved_sp;
struct pt_regs jprobe_saved_regs;
/* Per-thread fields, used while emulating branches */
unsigned long flags;
unsigned long target_epc;
u8 jprobes_stack[MAX_JPROBES_STACK_SIZE];
struct prev_kprobe prev_kprobe;
};

145
arch/mips/kernel/kprobes.c
View File

@ -30,6 +30,7 @@
#include <linux/slab.h>
#include <asm/ptrace.h>
#include <asm/branch.h>
#include <asm/break.h>
#include <asm/inst.h>
@ -152,13 +153,6 @@ int __kprobes arch_prepare_kprobe(struct kprobe *p)
goto out;
}
if (insn_has_delayslot(insn)) {
pr_notice("Kprobes for branch and jump instructions are not"
"supported\n");
ret = -EINVAL;
goto out;
}
if ((probe_kernel_read(&prev_insn, p->addr - 1,
sizeof(mips_instruction)) == 0) &&
insn_has_delayslot(prev_insn)) {
@ -178,9 +172,20 @@ int __kprobes arch_prepare_kprobe(struct kprobe *p)
* In the kprobe->ainsn.insn[] array we store the original
* instruction at index zero and a break trap instruction at
* index one.
*
* On MIPS arch if the instruction at probed address is a
* branch instruction, we need to execute the instruction at
* Branch Delayslot (BD) at the time of probe hit. As MIPS also
* doesn't have single stepping support, the BD instruction can
* not be executed in-line and it would be executed on SSOL slot
* using a normal breakpoint instruction in the next slot.
* So, read the instruction and save it for later execution.
*/
if (insn_has_delayslot(insn))
memcpy(&p->ainsn.insn[0], p->addr + 1, sizeof(kprobe_opcode_t));
else
memcpy(&p->ainsn.insn[0], p->addr, sizeof(kprobe_opcode_t));
memcpy(&p->ainsn.insn[0], p->addr, sizeof(kprobe_opcode_t));
p->ainsn.insn[1] = breakpoint2_insn;
p->opcode = *p->addr;
@ -231,16 +236,96 @@ static void set_current_kprobe(struct kprobe *p, struct pt_regs *regs,
kcb->kprobe_saved_epc = regs->cp0_epc;
}
static void prepare_singlestep(struct kprobe *p, struct pt_regs *regs)
/**
* evaluate_branch_instrucion -
*
* Evaluate the branch instruction at probed address during probe hit. The
* result of evaluation would be the updated epc. The insturction in delayslot
* would actually be single stepped using a normal breakpoint) on SSOL slot.
*
* The result is also saved in the kprobe control block for later use,
* in case we need to execute the delayslot instruction. The latter will be
* false for NOP instruction in dealyslot and the branch-likely instructions
* when the branch is taken. And for those cases we set a flag as
* SKIP_DELAYSLOT in the kprobe control block
*/
static int evaluate_branch_instruction(struct kprobe *p, struct pt_regs *regs,
struct kprobe_ctlblk *kcb)
{
union mips_instruction insn = p->opcode;
long epc;
int ret = 0;
epc = regs->cp0_epc;
if (epc & 3)
goto unaligned;
if (p->ainsn.insn->word == 0)
kcb->flags |= SKIP_DELAYSLOT;
else
kcb->flags &= ~SKIP_DELAYSLOT;
ret = __compute_return_epc_for_insn(regs, insn);
if (ret < 0)
return ret;
if (ret == BRANCH_LIKELY_TAKEN)
kcb->flags |= SKIP_DELAYSLOT;
kcb->target_epc = regs->cp0_epc;
return 0;
unaligned:
pr_notice("%s: unaligned epc - sending SIGBUS.\n", current->comm);
force_sig(SIGBUS, current);
return -EFAULT;
}
static void prepare_singlestep(struct kprobe *p, struct pt_regs *regs,
struct kprobe_ctlblk *kcb)
{
int ret = 0;
regs->cp0_status &= ~ST0_IE;
/* single step inline if the instruction is a break */
if (p->opcode.word == breakpoint_insn.word ||
p->opcode.word == breakpoint2_insn.word)
regs->cp0_epc = (unsigned long)p->addr;
else
regs->cp0_epc = (unsigned long)&p->ainsn.insn[0];
else if (insn_has_delayslot(p->opcode)) {
ret = evaluate_branch_instruction(p, regs, kcb);
if (ret < 0) {
pr_notice("Kprobes: Error in evaluating branch\n");
return;
}
}
regs->cp0_epc = (unsigned long)&p->ainsn.insn[0];
}
/*
* Called after single-stepping. p->addr is the address of the
* instruction whose first byte has been replaced by the "break 0"
* instruction. To avoid the SMP problems that can occur when we
* temporarily put back the original opcode to single-step, we
* single-stepped a copy of the instruction. The address of this
* copy is p->ainsn.insn.
*
* This function prepares to return from the post-single-step
* breakpoint trap. In case of branch instructions, the target
* epc to be restored.
*/
static void __kprobes resume_execution(struct kprobe *p,
struct pt_regs *regs,
struct kprobe_ctlblk *kcb)
{
if (insn_has_delayslot(p->opcode))
regs->cp0_epc = kcb->target_epc;
else {
unsigned long orig_epc = kcb->kprobe_saved_epc;
regs->cp0_epc = orig_epc + 4;
}
}
static int __kprobes kprobe_handler(struct pt_regs *regs)
@ -279,8 +364,13 @@ static int __kprobes kprobe_handler(struct pt_regs *regs)
save_previous_kprobe(kcb);
set_current_kprobe(p, regs, kcb);
kprobes_inc_nmissed_count(p);
prepare_singlestep(p, regs);
prepare_singlestep(p, regs, kcb);
kcb->kprobe_status = KPROBE_REENTER;
if (kcb->flags & SKIP_DELAYSLOT) {
resume_execution(p, regs, kcb);
restore_previous_kprobe(kcb);
preempt_enable_no_resched();
}
return 1;
} else {
if (addr->word != breakpoint_insn.word) {
@ -324,8 +414,16 @@ static int __kprobes kprobe_handler(struct pt_regs *regs)
}
ss_probe:
prepare_singlestep(p, regs);
kcb->kprobe_status = KPROBE_HIT_SS;
prepare_singlestep(p, regs, kcb);
if (kcb->flags & SKIP_DELAYSLOT) {
kcb->kprobe_status = KPROBE_HIT_SSDONE;
if (p->post_handler)
p->post_handler(p, regs, 0);
resume_execution(p, regs, kcb);
preempt_enable_no_resched();
} else
kcb->kprobe_status = KPROBE_HIT_SS;
return 1;
no_kprobe:
@ -334,25 +432,6 @@ no_kprobe:
}
/*
* Called after single-stepping. p->addr is the address of the
* instruction whose first byte has been replaced by the "break 0"
* instruction. To avoid the SMP problems that can occur when we
* temporarily put back the original opcode to single-step, we
* single-stepped a copy of the instruction. The address of this
* copy is p->ainsn.insn.
*
* This function prepares to return from the post-single-step
* breakpoint trap.
*/
static void __kprobes resume_execution(struct kprobe *p,
struct pt_regs *regs,
struct kprobe_ctlblk *kcb)
{
unsigned long orig_epc = kcb->kprobe_saved_epc;
regs->cp0_epc = orig_epc + 4;
}
static inline int post_kprobe_handler(struct pt_regs *regs)
{
struct kprobe *cur = kprobe_running();