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x86/kprobe: Add comments to arch_{,un}optimize_kprobes()

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Add a few words describing how it is safe to overwrite the 4 bytes
after a kprobe. In specific it is possible the JMP.d32 required for
the optimized kprobe overwrites multiple instructions.

Tested-by: Alexei Starovoitov <ast@kernel.org>
Tested-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20191111132458.401696663@infradead.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Peter Zijlstra 2019-11-11 14:02:10 +01:00 committed by Ingo Molnar
parent 04ae87a520
commit f2cb4f95b7
1 changed files with 12 additions and 2 deletions
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14
arch/x86/kernel/kprobes/opt.c
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@ -414,8 +414,12 @@ err:
}
/*
* Replace breakpoints (int3) with relative jumps.
* Replace breakpoints (INT3) with relative jumps (JMP.d32).
* Caller must call with locking kprobe_mutex and text_mutex.
*
* The caller will have installed a regular kprobe and after that issued
* syncrhonize_rcu_tasks(), this ensures that the instruction(s) that live in
* the 4 bytes after the INT3 are unused and can now be overwritten.
*/
void arch_optimize_kprobes(struct list_head *oplist)
{
@ -441,7 +445,13 @@ void arch_optimize_kprobes(struct list_head *oplist)
}
}
/* Replace a relative jump with a breakpoint (int3). */
/*
* Replace a relative jump (JMP.d32) with a breakpoint (INT3).
*
* After that, we can restore the 4 bytes after the INT3 to undo what
* arch_optimize_kprobes() scribbled. This is safe since those bytes will be
* unused once the INT3 lands.
*/
void arch_unoptimize_kprobe(struct optimized_kprobe *op)
{
arch_arm_kprobe(&op->kp);