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Date:   Fri, 14 Aug 2020 15:36:06 -0700 (PDT)
Subject:     Re: [PATCH v3 3/7] riscv: Fixup kprobes handler couldn't change pc
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CC:     Paul Walmsley <paul.walmsley@sifive.com>, mhiramat@kernel.org,
        oleg@redhat.com, linux-riscv@lists.infradead.org,
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        zong.li@sifive.com, guoren@kernel.org, me@packi.ch,
        Bjorn Topel <bjorn.topel@gmail.com>, guoren@linux.alibaba.com
From:   Palmer Dabbelt <palmerdabbelt@google.com>
To:     guoren@kernel.org
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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.