[Crash-utility] command dis fails to display name of function

[Dave Anderson]

----- Original Message -----
> Hello Dave,
> 
> When using dis command on RHEL5.11, I found a weird thing. Please check the
> following execution.
> 
> <cut>
> crash> dis do_wp_page | grep 591
> 0xffffffff8001115c <do_wp_page+591>:	callq  0xffffffff800623d0
> <__sched_text_start>
> crash> dis schedule
> 0xffffffff800623d0 <__sched_text_start>:        push   %rbp
> 0xffffffff800623d1 <schedule+1>:        mov    %rsp,%rbp
> ...
> crash> dis do_wp_page | grep 591
> 0xffffffff8001115c <do_wp_page+591>:	callq  0xffffffff800623d0 <schedule>
> crash> dis do_wp_page | grep 591
> 0xffffffff8001115c <do_wp_page+591>:	callq  0xffffffff800623d0
> <__sched_text_start>
> <cut>
> 
> schedule and __sched_text_start are always shifting.
> 
> After some investigation, I found the following sentences in
> arch/x86_64/kernel/vmlinux.lds.S
> 
> <cut>
> SECTIONS
> {
> ...
> SCHED_TEXT
> LOCK_TEXT
> KPROBES_TEXT
> ...
> <cut>
> 
> the SCHED_TEXT is defined like below
> <cut>
> #define SCHED_TEXT                                                      \
>                  ALIGN_FUNCTION();                                       \
>                  VMLINUX_SYMBOL(__sched_text_start) = .;                 \
>                  *(.sched.text)                                          \
>                  VMLINUX_SYMBOL(__sched_text_end) = .;
> <cut>
> 
> So symbol __sched_text_start may have the same address as the first function
> of *(.sched.text). And LOCK_TEXT/KPROBES_TEXT/IRQENTRY_TEXT has the same
> problems. The attached patch is used to fix this.
 

> diff --git a/symbols.c b/symbols.c
> index cebff52..461348c 100755
> --- a/symbols.c
> +++ b/symbols.c
> @@ -4527,6 +4527,15 @@ value_search(ulong value, ulong *offset)
>                           (spnext->value == value))
>                               sp = spnext;
>
> +#define STRNVEQ(A, B)        (string_exists((char *)A) && string_exists((char *)B) && \
> +                      strstr(A, B) && (strstr(A, B) - A) == \
> +                      (strlen((char *)(A)) - strlen((char *)(B))) )
> +
> +                     if (STRNVEQ(sp->name, "_text_start") &&
> +                         ((spnext = sp+1) < st->symend) &&
> +                         (spnext->value == value))
> +                             sp = spnext;
> +
>                          return((struct syment *)sp);
>                  }
>                  if (sp->value > value) {

Good catch -- thanks!  

The "shifting" is due to the caching in symval_hash_search().  But since sp->name
is guaranteed to exist, I think the pre-existing strstr_rightmost() function makes
it a little easier to understand:

--- crash-7.0.9/symbols.c.orig
+++ crash-7.0.9/symbols.c
@@ -4527,6 +4527,11 @@ value_search(ulong value, ulong *offset)
 			    (spnext->value == value))
 				sp = spnext;
 
+			if (strstr_rightmost(sp->name, "_text_start") &&
+			    ((spnext = sp+1) < st->symend) &&
+			    (spnext->value == value))
+				sp = spnext;
+
                         return((struct syment *)sp);
                 }
                 if (sp->value > value) {

and I'll add a comment above it.

Thanks,
  Dave