Re: [PATCH] Fix for "dis" command to correctly display the offset of disassembly code

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On 2023/02/21 12:03, Lianbo Jiang wrote:
> For gdb-10.2, the disassembly code may start with "=>", which needs to
> be stripped when calculating the address. Otherwise, parsing the address
> will fail because the current code always assumes that it starts with the
> "0x". For example:
> 
>    crash> gdb disassemble 0xffffffffa2317add
>    Dump of assembler code for function native_queued_spin_lock_slowpath:
>       0xffffffffa2317ab0 <+0>:     nopl   0x0(%rax,%rax,1)
>       0xffffffffa2317ab5 <+5>:     push   %rbp
>       0xffffffffa2317ab6 <+6>:     mov    %rsp,%rbp
>       ...
>       0xffffffffa2317ad3 <+35>:    mov    %edx,%eax
>       0xffffffffa2317ad5 <+37>:    lock cmpxchg %ecx,(%rdi)
>    => 0xffffffffa2317ad9 <+41>:    cmp    %eax,%edx
>       0xffffffffa2317adb <+43>:    jne    0xffffffffa2317ac0 <native_queued_spin_lock_slowpath+16>
>       0xffffffffa2317add <+45>:    pop    %rbp
>       0xffffffffa2317ade <+46>:    xchg   %ax,%ax
>       ...
Probably the following prints the "=> ".
Out of curiosity, what did you do before the gdb disas?

/* Return a prefix for instruction address:
    "=> " for current instruction, else "   ".  */

const char *
pc_prefix (CORE_ADDR addr)
{
   if (has_stack_frames ())
     {
       struct frame_info *frame;
       CORE_ADDR pc;

       frame = get_selected_frame (NULL);
       if (get_frame_pc_if_available (frame, &pc) && pc == addr)
         return "=> ";
     }
   return "   ";
}

Anyway, the patch looks good to me.

Acked-by: Kazuhito Hagio <k-hagio-ab@xxxxxxx>

Thanks,
Kazu

> 
> Without the patch:
>    crash> dis 0xffffffffa2317add -r | tail -5
>    0xffffffffa2317ad3 <native_queued_spin_lock_slowpath+35>:	mov    %edx,%eax
>    0xffffffffa2317ad5 <native_queued_spin_lock_slowpath+37>:	lock cmpxchg %ecx,(%rdi)
>    0xffffffffa2317ad5 <native_queued_spin_lock_slowpath+37>:	cmp    %eax,%edx
>                                                         ^^^
>    0xffffffffa2317adb <native_queued_spin_lock_slowpath+43>:	jne    0xffffffffa2317ac0 <native_queued_spin_lock_slowpath+16>
>    0xffffffffa2317add <native_queued_spin_lock_slowpath+45>:	pop    %rbp
> 
> With the patch:
>    crash> dis 0xffffffffa2317add -r | tail -5
>    0xffffffffa2317ad3 <native_queued_spin_lock_slowpath+35>:	mov    %edx,%eax
>    0xffffffffa2317ad5 <native_queued_spin_lock_slowpath+37>:	lock cmpxchg %ecx,(%rdi)
>    0xffffffffa2317ad9 <native_queued_spin_lock_slowpath+41>:	cmp    %eax,%edx
>    0xffffffffa2317adb <native_queued_spin_lock_slowpath+43>:	jne    0xffffffffa2317ac0 <native_queued_spin_lock_slowpath+16>
>    0xffffffffa2317add <native_queued_spin_lock_slowpath+45>:	pop    %rbp
> 
> Reported-by: Vernon Lovejoy <vlovejoy@xxxxxxxxxx>
> Signed-off-by: Lianbo Jiang <lijiang@xxxxxxxxxx>
> ---
>   kernel.c | 4 ++++
>   1 file changed, 4 insertions(+)
> 
> diff --git a/kernel.c b/kernel.c
> index a42e6ad7d78c..6e98f5f6f6b1 100644
> --- a/kernel.c
> +++ b/kernel.c
> @@ -2112,6 +2112,10 @@ cmd_dis(void)
>   			rewind(pc->tmpfile);
>   
>   		while (fgets(buf2, BUFSIZE, pc->tmpfile)) {
> +
> +			if (STRNEQ(buf2, "=>"))
> +				shift_string_left(buf2, 2);
> +
>   			strip_beginning_whitespace(buf2);
>   
>   			if (do_load_module_filter)
--
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