Re: [PATCH 03/15] x86/kprobes: Fix frame pointer annotations

[Masami Hiramatsu]

On Wed, 05 Jun 2019 15:07:56 +0200
Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:

> The kprobe trampolines have a FRAME_POINTER annotation that makes no
> sense. It marks the frame in the middle of pt_regs, at the place of
> saving BP.

commit ee213fc72fd67 introduced this code, and this is for unwinder which
uses frame pointer. I think current code stores the address of previous
(original context's) frame pointer into %rbp. So with that, if unwinder
tries to decode frame pointer, it can get the original %rbp value,
instead of &pt_regs from current %rbp.

> 
> Change it to mark the pt_regs frame as per the ENCODE_FRAME_POINTER
> from the respective entry_*.S.
> 

With this change, I think stack unwinder can not get the original %rbp
value. Peter, could you check the above commit?

Thank you,

> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> ---
>  arch/x86/kernel/kprobes/common.h |   24 ++++++++++++------------
>  1 file changed, 12 insertions(+), 12 deletions(-)
> 
> --- a/arch/x86/kernel/kprobes/common.h
> +++ b/arch/x86/kernel/kprobes/common.h
> @@ -5,15 +5,10 @@
>  /* Kprobes and Optprobes common header */
>  
>  #include <asm/asm.h>
> -
> -#ifdef CONFIG_FRAME_POINTER
> -# define SAVE_RBP_STRING "	push %" _ASM_BP "\n" \
> -			 "	mov  %" _ASM_SP ", %" _ASM_BP "\n"
> -#else
> -# define SAVE_RBP_STRING "	push %" _ASM_BP "\n"
> -#endif
> +#include <asm/frame.h>
>  
>  #ifdef CONFIG_X86_64
> +
>  #define SAVE_REGS_STRING			\
>  	/* Skip cs, ip, orig_ax. */		\
>  	"	subq $24, %rsp\n"		\
> @@ -27,11 +22,13 @@
>  	"	pushq %r10\n"			\
>  	"	pushq %r11\n"			\
>  	"	pushq %rbx\n"			\
> -	SAVE_RBP_STRING				\
> +	"	pushq %rbp\n"			\
>  	"	pushq %r12\n"			\
>  	"	pushq %r13\n"			\
>  	"	pushq %r14\n"			\
> -	"	pushq %r15\n"
> +	"	pushq %r15\n"			\
> +	ENCODE_FRAME_POINTER
> +
>  #define RESTORE_REGS_STRING			\
>  	"	popq %r15\n"			\
>  	"	popq %r14\n"			\
> @@ -51,19 +48,22 @@
>  	/* Skip orig_ax, ip, cs */		\
>  	"	addq $24, %rsp\n"
>  #else
> +
>  #define SAVE_REGS_STRING			\
>  	/* Skip cs, ip, orig_ax and gs. */	\
> -	"	subl $16, %esp\n"		\
> +	"	subl $4*4, %esp\n"		\
>  	"	pushl %fs\n"			\
>  	"	pushl %es\n"			\
>  	"	pushl %ds\n"			\
>  	"	pushl %eax\n"			\
> -	SAVE_RBP_STRING				\
> +	"	pushl %ebp\n"			\
>  	"	pushl %edi\n"			\
>  	"	pushl %esi\n"			\
>  	"	pushl %edx\n"			\
>  	"	pushl %ecx\n"			\
> -	"	pushl %ebx\n"
> +	"	pushl %ebx\n"			\
> +	ENCODE_FRAME_POINTER
> +
>  #define RESTORE_REGS_STRING			\
>  	"	popl %ebx\n"			\
>  	"	popl %ecx\n"			\
> 
> 


-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>