[PATCH v4 07/16] Print backtrace in a signal-safe manner

[Peter Hutterer]

On Mon, May 14, 2012 at 02:14:28PM -0700, Chase Douglas wrote:
> Backtraces are often printed in signal context, such as when a segfault
> occurs.
> 
> Signed-off-by: Chase Douglas <chase.douglas at canonical.com>
> ---
>  os/backtrace.c |   49 +++++++++++++++++++++++++++++--------------------
>  1 file changed, 29 insertions(+), 20 deletions(-)
> 
> diff --git a/os/backtrace.c b/os/backtrace.c
> index 81348f4..d41c3f3 100644
> --- a/os/backtrace.c
> +++ b/os/backtrace.c
> @@ -45,29 +45,37 @@ xorg_backtrace(void)
>      int size, i;
>      Dl_info info;
>  
> -    ErrorF("\n");
> -    ErrorF("Backtrace:\n");
> +    ErrorSigSafe("\n");
> +    ErrorSigSafe("Backtrace:\n");
>      size = backtrace(array, 64);
>      for (i = 0; i < size; i++) {
>          int rc = dladdr(array[i], &info);
>  
>          if (rc == 0) {
> -            ErrorF("%d: ?? [%p]\n", i, array[i]);
> +            ErrorSigSafe("%u: ?? [%p]\n", i, array[i]);
>              continue;
>          }
>          mod = (info.dli_fname && *info.dli_fname) ? info.dli_fname : "(vdso)";
>          if (info.dli_saddr)
> -            ErrorF("%d: %s (%s+0x%lx) [%p]\n", i, mod,
> -                   info.dli_sname,
> -                   (long unsigned int) ((char *) array[i] -
> -                                        (char *) info.dli_saddr), array[i]);
> +            ErrorSigSafe(
> +                "%u: %s (%s+0x%x) [%p]\n",
> +                i,
> +                mod,
> +                info.dli_sname,
> +                (long unsigned int)((char *) array[i] -
> +                                    (char *) info.dli_saddr),
> +                array[i]);
>          else
> -            ErrorF("%d: %s (%p+0x%lx) [%p]\n", i, mod,
> -                   info.dli_fbase,
> -                   (long unsigned int) ((char *) array[i] -
> -                                        (char *) info.dli_fbase), array[i]);
> +            ErrorSigSafe(
> +                "%u: %s (%s+0x%x) [%p]\n",

typo: this should be "%u: %s (%p+0x%lx) [%p]\n" (note the %p, not %s). The
backtrace looks like garbage otherwise.

Reviewed-by: Peter Hutterer <peter.hutterer at who-t.net> otherwise

Cheers,
  Peter

> +                i,
> +                mod,
> +                info.dli_fbase,
> +                (long unsigned int)((char *) array[i] -
> +                                    (char *) info.dli_fbase),
> +                array[i]);
>      }
> -    ErrorF("\n");
> +    ErrorSigSafe("\n");
>  }
>  
>  #else                           /* not glibc or glibc < 2.1 */
> @@ -105,7 +113,7 @@ xorg_backtrace_frame(uintptr_t pc, int signo, void *arg)
>              strcpy(signame, "unknown");
>          }
>  
> -        ErrorF("** Signal %d (%s)\n", signo, signame);
> +        ErrorSigSafe("** Signal %u (%s)\n", signo, signame);
>      }
>  
>      snprintf(header, sizeof(header), "%d: 0x%lx", depth, pc);
> @@ -123,7 +131,8 @@ xorg_backtrace_frame(uintptr_t pc, int signo, void *arg)
>              symname = "<section start>";
>              offset = pc - (uintptr_t) dlinfo.dli_fbase;
>          }
> -        ErrorF("%s: %s:%s+0x%lx\n", header, dlinfo.dli_fname, symname, offset);
> +        ErrorSigSafe("%s: %s:%s+0x%x\n", header, dlinfo.dli_fname, symname,
> +                     offset);
>  
>      }
>      else {
> @@ -131,7 +140,7 @@ xorg_backtrace_frame(uintptr_t pc, int signo, void *arg)
>           * probably poke elfloader here, but haven't written that code yet,
>           * so we just print the pc.
>           */
> -        ErrorF("%s\n", header);
> +        ErrorSigSafe("%s\n", header);
>      }
>  
>      return 0;
> @@ -183,7 +192,7 @@ xorg_backtrace_pstack(void)
>  
>              if (bytesread > 0) {
>                  btline[bytesread] = 0;
> -                ErrorF("%s", btline);
> +                ErrorSigSafe("%s", btline);
>              }
>              else if ((bytesread < 0) || ((errno != EINTR) && (errno != EAGAIN)))
>                  done = 1;
> @@ -203,8 +212,8 @@ void
>  xorg_backtrace(void)
>  {
>  
> -    ErrorF("\n");
> -    ErrorF("Backtrace:\n");
> +    ErrorSigSafe("\n");
> +    ErrorSigSafe("Backtrace:\n");
>  
>  #ifdef HAVE_PSTACK
>  /* First try fork/exec of pstack - otherwise fall back to walkcontext
> @@ -221,9 +230,9 @@ xorg_backtrace(void)
>              walkcontext(&u, xorg_backtrace_frame, &depth);
>          else
>  #endif
> -            ErrorF("Failed to get backtrace info: %s\n", strerror(errno));
> +            ErrorSigSafe("Failed to get backtrace info: %s\n", strerror(errno));
>      }
> -    ErrorF("\n");
> +    ErrorSigSafe("\n");
>  }
>  
>  #else
> -- 
> 1.7.9.5
>