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    Subject[PATCH -tip v8 10/13] x86/kprobes: Push a fake return address at kretprobe_trampoline
    Date
    This changes x86/kretprobe stack frame on kretprobe_trampoline
    a bit, which now push the kretprobe_trampoline as a fake return
    address at the bottom of the stack frame. With this fix, the ORC
    unwinder will see the kretprobe_trampoline as a return address.

    Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
    Suggested-by: Josh Poimboeuf <jpoimboe@redhat.com>
    Tested-by: Andrii Nakryik <andrii@kernel.org>
    Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
    ---
    arch/x86/kernel/kprobes/core.c | 31 ++++++++++++++++++++++---------
    1 file changed, 22 insertions(+), 9 deletions(-)

    diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
    index 74f049b6e77f..4d040aaf969b 100644
    --- a/arch/x86/kernel/kprobes/core.c
    +++ b/arch/x86/kernel/kprobes/core.c
    @@ -1041,28 +1041,31 @@ asm(
    ".global kretprobe_trampoline\n"
    ".type kretprobe_trampoline, @function\n"
    "kretprobe_trampoline:\n"
    - /* We don't bother saving the ss register */
    #ifdef CONFIG_X86_64
    - " pushq %rsp\n"
    + /* Push fake return address to tell the unwinder it's a kretprobe */
    + " pushq $kretprobe_trampoline\n"
    UNWIND_HINT_FUNC
    + /* Save the sp-8, this will be fixed later */
    + " pushq %rsp\n"
    " pushfq\n"
    SAVE_REGS_STRING
    " movq %rsp, %rdi\n"
    " call trampoline_handler\n"
    - /* Replace saved sp with true return address. */
    - " movq %rax, 19*8(%rsp)\n"
    RESTORE_REGS_STRING
    + " addq $8, %rsp\n"
    " popfq\n"
    #else
    - " pushl %esp\n"
    + /* Push fake return address to tell the unwinder it's a kretprobe */
    + " pushl $kretprobe_trampoline\n"
    UNWIND_HINT_FUNC
    + /* Save the sp-4, this will be fixed later */
    + " pushl %esp\n"
    " pushfl\n"
    SAVE_REGS_STRING
    " movl %esp, %eax\n"
    " call trampoline_handler\n"
    - /* Replace saved sp with true return address. */
    - " movl %eax, 15*4(%esp)\n"
    RESTORE_REGS_STRING
    + " addl $4, %esp\n"
    " popfl\n"
    #endif
    " ret\n"
    @@ -1073,8 +1076,10 @@ NOKPROBE_SYMBOL(kretprobe_trampoline);
    /*
    * Called from kretprobe_trampoline
    */
    -__used __visible void *trampoline_handler(struct pt_regs *regs)
    +__used __visible void trampoline_handler(struct pt_regs *regs)
    {
    + unsigned long *frame_pointer;
    +
    /* fixup registers */
    regs->cs = __KERNEL_CS;
    #ifdef CONFIG_X86_32
    @@ -1082,8 +1087,16 @@ __used __visible void *trampoline_handler(struct pt_regs *regs)
    #endif
    regs->ip = (unsigned long)&kretprobe_trampoline;
    regs->orig_ax = ~0UL;
    + regs->sp += sizeof(long);
    + frame_pointer = ((unsigned long *)&regs->sp) + 1;

    - return (void *)kretprobe_trampoline_handler(regs, &regs->sp);
    + /* Replace fake return address with real one. */
    + *frame_pointer = kretprobe_trampoline_handler(regs, frame_pointer);
    + /*
    + * Move flags to sp so that kretprobe_trapmoline can return
    + * right after popf.
    + */
    + regs->sp = regs->flags;
    }
    NOKPROBE_SYMBOL(trampoline_handler);

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