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    Subject[tip: core/kprobes] x86/kprobe: Add comments to arch_{,un}optimize_kprobes()
    The following commit has been merged into the core/kprobes branch of tip:

    Commit-ID: 654920255149b034ad5a8aee6b02cfa16e65e9c0
    Gitweb: https://git.kernel.org/tip/654920255149b034ad5a8aee6b02cfa16e65e9c0
    Author: Peter Zijlstra <peterz@infradead.org>
    AuthorDate: Mon, 11 Nov 2019 14:02:10 +01:00
    Committer: Ingo Molnar <mingo@kernel.org>
    CommitterDate: Tue, 19 Nov 2019 13:13:08 +01:00

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

    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>
    ---
    arch/x86/kernel/kprobes/opt.c | 14 ++++++++++++--
    1 file changed, 12 insertions(+), 2 deletions(-)

    diff --git a/arch/x86/kernel/kprobes/opt.c b/arch/x86/kernel/kprobes/opt.c
    index 26e0d6c..3f45b5c 100644
    --- a/arch/x86/kernel/kprobes/opt.c
    +++ b/arch/x86/kernel/kprobes/opt.c
    @@ -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);
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