Does the LD --wrap feature work for library internal references?

[Sebastian Huber]

On 18/01/2019 14:41, Alan Modra wrote:
> On Fri, Jan 18, 2019 at 10:04:36AM +0100, Sebastian Huber wrote:
>> On 18/01/2019 01:24, Alan Modra wrote:
>>> No, -ffunction-sections will make no difference.  Really, --wrap was
>>> intended for wrapping system functions, which I guess is why the
>>> feature was implemented only for undefined symbols.  I don't see a
>>> fundamental reason why --wrap couldn't be made to work with defined
>>> symbols, provided the compiler and assembler don't optimise references
>>> to local functions.
>> In case it is acceptable to extend --wrap to work also for defined symbol
>> references, then I would have a look at this and try to implement it. If you
>> already have an idea which functions needs to be touched for this new
>> feature, then this would be helpful for me.
> It might just be a matter of calling bfd_wrapped_link_hash_lookup in
> more places where we currently call bfd_link_hash_lookup and its
> derivatives like elf_link_hash_lookup.  Knowing which places to change
> is the difficult part.  Anything involved with relocation processing
> for a start.
>

I think to wrap defined references you have to add things to other 
places than spots which call bfd_link_hash_lookup() or 
bfd_wrapped_link_hash_lookup(). I built ld with -O0 -g and used the 
following test program:

void f(void)
{
}

void g(void);

void _start(void)
{
     f();
     g();
}

I use this GDB script:

b bfd_wrapped_link_hash_lookup  if string[0] != '.' && (string[0] == 'f' 
|| string[7] == 'f' || string[0] == 'g' || string[7] == 'g')
commands
c
end
b bfd_link_hash_lookup if string[0] != '.' && (string[0] == 'f' || 
string[7] == 'f' || string[0] == 'g' || string[7] == 'g')
commands
c
end
r

This yields:

gdb --command=main.gdb --args ld-new -wrap=f -wrap=g main.o -o main.exe
Breakpoint 2, bfd_link_hash_lookup (table=0x8df4f0, string=0x8f2bd0 "f", 
create=1, copy=0, follow=0) at ../../bfd/linker.c:511
511       if (table == NULL || string == NULL)

Breakpoint 2, bfd_link_hash_lookup (table=0x8df4f0, string=0x8f2bd2 
"_start", create=1, copy=0, follow=0) at ../../bfd/linker.c:511
511       if (table == NULL || string == NULL)

Breakpoint 1, bfd_wrapped_link_hash_lookup (abfd=0x8eec40, info=0x8c2de0 
<link_info>, string=0x8f2bd9 "g", create=1, copy=0, follow=0) at 
../../bfd/linker.c:541
541       if (info->wrap_hash != NULL)

Breakpoint 2, bfd_link_hash_lookup (table=0x8df4f0, string=0x8f35c0 
"__wrap_g", create=1, copy=1, follow=0) at ../../bfd/linker.c:511
511       if (table == NULL || string == NULL)
/home/EB/sebastian_h/archive/binutils-git/build/ld/ld-new: main.o: in 
function `_start':
main.c:(.text+0x11): undefined reference to `__wrap_g'

The lookup is performed only once for "f". I think this is when ld finds 
the definition of f(). For the call in _start() we end up in other areas 
of ld.

-- 
Sebastian Huber, embedded brains GmbH

Address : Dornierstr. 4, D-82178 Puchheim, Germany
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