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Re: [RFA/symtab] Handle weak functions as global
- From: Elena Zannoni <ezannoni at redhat dot com>
- To: Daniel Jacobowitz <drow at false dot org>
- Cc: gdb-patches at sources dot redhat dot com, Elena Zannoni <ezannoni at redhat dot com>
- Date: Sun, 28 Nov 2004 21:08:32 -0500
- Subject: Re: [RFA/symtab] Handle weak functions as global
- References: <20041101173014.GA20449@nevyn.them.org>
Daniel Jacobowitz writes:
> This patch is a one liner, but requires a bit of explanation. If you have a
> version of glibc available with debugging symbols (in both ld.so and
> libc.so), you can see the problem like this:
>
> - Compile a program which calls open ().
> - Run to main.
> - Set a breakpoint on open.
> - Continue - the breakpoint will not be hit.
>
> [Separate debug files will show the bug if you point GDB at the right
> directory; on Debian this is /usr/lib/debug, not sure where it is on RH
> systems.]
>
> What happens, in my setup at least, is that gdb searches first the
> executable, then libc.so.6, then ld.so for "open". It either finds a
> trampoline in the executable or nothing at all (depending on your version of
> binutils). Then, it finds open in libc.so.6, which has type mst_file_text.
> Then it finds open in ld.so, which also has a symbol named open, which also
> has type mst_file_text. We did not find any mst_text symbol, so we settle
> for returning the most recent mst_file_text symbol - which is the wrong one.
>
> There's two strange things here. The first is that we return the second
> file symbol instead of the first; it doesn't much matter, since we'll be
> wrong half the time anyway. The second is that the symbol in libc.so is
> marked as mst_file_text. This turned out to be because BSF_WEAK handling
> was added to elfread for non-code symbols, but not for code symbols; and
> open is a weak entry point to libc.so.6 for historical reasons. If we treat
> it as global, paralleling the non-code symbol handling, then GDB will
> resolve breakpoints on "open" reliably to libc.so.6 (assuming no other
> shared library defines it). This gives the debugger much better chances of
> finding the right symbol - any situation more complicated would require
> setting breakpoints at all functions named "open", which is a To-Do-Later
> issue.
>
Interesting. This is just gdb not following in the symbol searches any
of the ELF scoping rules. Can you add a small testcase?
How about:
file1.c
---
void
bar (void)
{
puts ("bar in u1");
}
file2.c
---
void bar (void);
void
foo (void)
{
bar ();
}
void
bar (void)
{
puts ("bar in u2");
}
foo.c
---
int
main()
{
foo ();
return 0;
}
If you compile the files with bar() as shared libraries, what version
of bar does gdb pick from main?
> Is this patch OK?
>
Does it effect the testcase above?
> --
> Daniel Jacobowitz
>
> 2004-11-01 Daniel Jacobowitz <dan@debian.org>
>
> * elfread.c (elf_symtab_read): Treat weak functions as global.
>
> Index: elfread.c
> ===================================================================
> RCS file: /big/fsf/rsync/src-cvs/src/gdb/elfread.c,v
> retrieving revision 1.47
> diff -u -p -r1.47 elfread.c
> --- elfread.c 23 Oct 2004 16:18:08 -0000 1.47
> +++ elfread.c 1 Nov 2004 17:00:39 -0000
> @@ -305,7 +305,7 @@ elf_symtab_read (struct objfile *objfile
> }
> else if (sym->section->flags & SEC_CODE)
> {
> - if (sym->flags & BSF_GLOBAL)
> + if (sym->flags & (BSF_GLOBAL | BSF_WEAK))
> {
> ms_type = mst_text;
> }