Re: i386-linux signal backtraces broken

[Mark Kettenis <kettenis@chello.nl>]

Daniel Jacobowitz <drow@mvista.com> writes:

> There's only one problem here.  On my desktop (Debian GNU/Linux, glibc
> 2.2.5), there are two copies of sigaction in a dynamically linked
> executable.  One of them's in libc.so.6 and the other is in ld-linux.so.2.
> The only __restore symbol we find is in ld-linux.so.2; this seems to be
> because we leave a symbol table in ld-linux.so.2 (probably for the
> debugger's benefit, so that it can find _dl_debug_state) - but we strip
> libc.so.6.

How unfortunate.  I'd recommend using an unstripped libc.so.6 when
doing any serious debugging, but I guess that won't trick the Debian
folks into distributing an unstripped libc.

> Unfortunately, the application gets the copy of __restore that is in
> libc.so.6.  Which is right after a function whose name appears in the
> dynamic symbol table (sigaction).  So it's considered to be part of
> sigaction, and NAME is "sigaction".
> 
> We have two choices, that I see:
>   - Call the code inspection functions always
>   - Call the code inspection functions if the name is sigaction, taking
>     advantage of the glibc implementation detail that sigaction is the
>     only exported name for this function that I can see, and they are
>     implemented right after it in the same file.

We could also modify glibc such that __restore and __restore_rt get
included in libc.so's dynamic symbol table.  Or perhaps we could
modify GDB such that it scans libc.so.6 for signal trampolines when it
is loaded.

> Option (A) is a performance hit.  Option (B) is, well, a little fragile.

I don't think implementing (B) makes the code more fragile than it
already is.

Mark