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    SubjectRe: [PATCH v3 3/7] riscv: Fixup kprobes handler couldn't change pc
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    On Mon, 13 Jul 2020 16:39:18 PDT (-0700), guoren@kernel.org wrote:
    > From: Guo Ren <guoren@linux.alibaba.com>
    >
    > The "Changing Execution Path" section in the Documentation/kprobes.txt
    > said:
    >
    > Since kprobes can probe into a running kernel code, it can change the
    > register set, including instruction pointer.
    >
    > Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
    > Cc: Masami Hiramatsu <mhiramat@kernel.org>
    > Cc: Palmer Dabbelt <palmerdabbelt@google.com>
    > ---
    > arch/riscv/kernel/mcount-dyn.S | 3 ++-
    > 1 file changed, 2 insertions(+), 1 deletion(-)
    >
    > diff --git a/arch/riscv/kernel/mcount-dyn.S b/arch/riscv/kernel/mcount-dyn.S
    > index 35a6ed7..4b58b54 100644
    > --- a/arch/riscv/kernel/mcount-dyn.S
    > +++ b/arch/riscv/kernel/mcount-dyn.S
    > @@ -123,6 +123,7 @@ ENDPROC(ftrace_caller)
    > sd ra, (PT_SIZE_ON_STACK+8)(sp)
    > addi s0, sp, (PT_SIZE_ON_STACK+16)
    >
    > + sd ra, PT_EPC(sp)
    > sd x1, PT_RA(sp)
    > sd x2, PT_SP(sp)
    > sd x3, PT_GP(sp)

    So that's definately not going to be EPC any more. I'm not sure that field is
    sanely named, though, as it's really just the PC when it comes to other ptrace
    stuff.

    > @@ -157,6 +158,7 @@ ENDPROC(ftrace_caller)
    > .endm
    >
    > .macro RESTORE_ALL
    > + ld ra, PT_EPC(sp)
    > ld x1, PT_RA(sp)

    x1 is ra, so loading it twice doesn't seem reasonable.

    > ld x2, PT_SP(sp)
    > ld x3, PT_GP(sp)
    > @@ -190,7 +192,6 @@ ENDPROC(ftrace_caller)
    > ld x31, PT_T6(sp)
    >
    > ld s0, (PT_SIZE_ON_STACK)(sp)
    > - ld ra, (PT_SIZE_ON_STACK+8)(sp)
    > addi sp, sp, (PT_SIZE_ON_STACK+16)
    > .endm

    If you're dropping the load you should drop the store above as well. In
    general this seems kind of mixed up, both before and after this patch.

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