Re: [Libunwind-devel] [PATCH] elf: Follow .gnu_debuglink when resolving function names

[Saleem Abdulrasool]

I agree the existing hard-coded "/usr/lib/debug" should be removed.

Yeah, it's certainly more complicated to properly process the split debug information.  I think that a proper hook for accessing debug information might be worth it.  It may be possible that the debug information is in dbx (or stabs?).  

On Sun, Feb 5, 2017 at 12:02 PM, Bert Wesarg <address@hidden> wrote:

Dave,

On Sat, Feb 4, 2017 at 9:15 PM, Dave Watson <address@hidden> wrote:
> Currently we only look at the current image to resolve function names,
> which requires debug info.  Often the debug info is split to a different
> file.  os-linux.c does look in some distro's path in /usr/lib/debug%s,
> this diff will also follow the commonly used .gnu_debuglink section.
> ---
>  src/elfxx.c | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
>
> diff --git a/src/elfxx.c b/src/elfxx.c
> index 25bd5a2..7b05f3f 100644
> --- a/src/elfxx.c
> +++ b/src/elfxx.c
> @@ -312,11 +312,27 @@ elf_w (get_proc_name) (unw_addr_space_t as, pid_t pid, unw_word_t ip,
>    unsigned long segbase, mapoff;
>    struct elf_image ei;
>    int ret;
> +  Elf_W (Shdr) *shdr;
>
>    ret = tdep_get_elf_image (&ei, pid, ip, &segbase, &mapoff, NULL, 0);
>    if (ret < 0)
>      return ret;
>
> +  shdr = elf_w (find_section) (&ei, ".gnu_debuglink");
> +  if (shdr) {
> +    char debug_path[PATH_MAX];
> +    char *link = ((uint8_t *) ei.image) + shdr->sh_offset;
> +    size_t link_len = shdr->sh_size;
> +
> +    strncpy(debug_path, link, link_len);

Unfurtnatly, interpreting .gnu_debuglink is not that simple. I refer here to

https://sourceware.org/gdb/onlinedocs/gdb/Separate-Debug-Files.html

At least it is important, the the .gnu_debuglink should not contain
any path components at all. Thus using .gnu_debuglink verbatime wont
result in any sane path.

Also I think the code in os-linux.c should loose its current
/usr/lib/debug code.

Bert

> +    Debug(1, "Found debuglink section, following %s len %i\n", debug_path, link_len);
> +    munmap (ei.image, ei.size);
> +
> +    ret = elf_map_image(&ei, debug_path);
> +    if (ret < 0)
> +      return ret;
> +  }
> +
>    ret = elf_w (get_proc_name_in_image) (as, &ei, segbase, mapoff, ip, buf, buf_len, offp);
>
>    munmap (ei.image, ei.size);
> --
> 2.8.0-rc2
>

--

Saleem Abdulrasool
compnerd (at) compnerd (dot) org