bugs in external debug info support in libbacktrace

[Milian Wolff]

Hey Ian, others,

I was made aware that libbacktrace got support for external debug info with 
[1], great work! I have just synced the latest libbacktrace into heaptrack [2] 
in a local branch and played around with it and noticed two limitations:

[1]: https://github.com/gcc-mirror/gcc/commit/
b919941efc58035debbcf69b645c072b7dd6ba4e
[2]: https://github.com/KDE/heaptrack

a) elf_open_debugfile_by_debuglink checks the crc, even if it is not provided 
by the debug file. I.e. I have a file where `debuglink_crc == 0`, but the 
got_crc calculated from elf_crc32_file is non-zero. I have patched this 
locally with the following to make it work for me:

diff --git a/libbacktrace/elf.c b/libbacktrace/elf.c
index 06823fcf59b..24bf58728fd 100644
--- a/libbacktrace/elf.c
+++ b/libbacktrace/elf.c
@@ -1005,7 +1005,7 @@ elf_open_debugfile_by_debuglink (struct backtrace_state 
*state,
   if (ddescriptor < 0)
     return -1;
 
-  got_crc = elf_crc32_file (state, ddescriptor, error_callback, data);
+  got_crc = debuglink_crc ? elf_crc32_file (state, ddescriptor, 
error_callback, data) : 0;
   if (got_crc != debuglink_crc)
     {
       backtrace_close (ddescriptor, error_callback, data);

b) elf_add guards the code to inspect the symtab-shndx with a `&& !debuginfo` 
check in loc 2797. This results in all files with separate debug info yielding 
`found_sym = 0` when calling elf_add, and symbol resolution is broken. 
Personally I have patched this check out to make symbol resolution work for 
me:

diff --git a/libbacktrace/elf.c b/libbacktrace/elf.c
index 06823fcf59b..6876bd3ed8e 100644
--- a/libbacktrace/elf.c
+++ b/libbacktrace/elf.c
@@ -2794,7 +2794,7 @@ elf_add (struct backtrace_state *state, const char 
*filename, int descriptor,
 
   if (symtab_shndx == 0)
     symtab_shndx = dynsym_shndx;
-  if (symtab_shndx != 0 && !debuginfo)
+  if (symtab_shndx != 0)
     {
       const b_elf_shdr *symtab_shdr;
       unsigned int strtab_shndx;

Could you please check whether the two patches above could be upstreamed?

Thanks a lot for your work
-- 
Milian Wolff | milian.wolff@kdab.com | Senior Software Engineer
KDAB (Deutschland) GmbH&Co KG, a KDAB Group company
Tel: +49-30-521325470
KDAB - The Qt Experts
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