[Bug binutils/29075] objdump -S does not support debuginfod

[amerey at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=29075

--- Comment #30 from Aaron Merey <amerey at redhat dot com> ---
(In reply to Martin Liska from comment #29)
> Thanks for the implementation. However, I think one piece is missing and it's
> find_separate_debug and find_separate_debug rely on fact the debugginfo is
> present for a abfd, but can't load it from debuginfod:
> 
> $ DEBUGINFOD_VERBOSE=1
> /home/marxin/Programming/binutils/objdir/binutils/objdump -S `which true`
> (does not get any source file)
> 
> while installing coreutils-debuginfo (and *not* coreutils-debugsource) I get:
> 
> $ DEBUGINFOD_VERBOSE=1
> /home/marxin/Programming/binutils/objdir/binutils/objdump -S `which true`
[...]
>   char const *node = program;
>   struct infomap const *map_prog = infomap;
>     249c:     48 89 e3                mov    %rsp,%rbx

It looks like this happens because of some peculiarities regarding how objdump
and libbfd handle the default settings for following debug links. 

For the case where coreutils-debuginfo is not installed: if you explicitly tell
objdump to process links with the '-L' option then debuginfod is used to
download the missing debuginfo as expected. If '-L' is not given then objdump
is configured to not attempt to look for any separate debuginfo unless
'--dwarf=follow-links' is given without being enabled by default or
'--dwarf=follow-links' is enabled by default AND objdump is given a command
line option that requires the use of debuginfo like '--debugging'. Currently
'-S' is not taken to be such an option. So 'objdump -S' does not bother to
check for separate debuginfo unless one of these other conditions met.
Consequently objdump will not have the information needed to download any
source files either.

So why does 'objdump -S' download source files when coreutils-debuginfo is
installed? Objdump itself does not look for the debuginfo in this case, as it
follows the same logic I described above. Instead the search for the debuginfo
occurs within libbfd. It does this on its own terms, even if you run 'objdump
-S --dwarf=no-follow-links' libbfd still searches for the separate debuginfo
file (maybe this is a bug?). If libbfd can find the debuginfo, it passes line
information back to objdump which it uses to download any missing source files.

The simplest way to address this is probably to modify the behavior of '-S' so
that it enables the processing of debuglinks when '--dwarf=follow-links' is
enabled, without requiring any additional conditions to be met. However this
will not stop libbfd from finding locally-installed debuginfo even when you
tell objdump to not follow links.

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