[LLVMdev] [lld] ELF weak aliases

[Michael Spencer]

On Tue, Jan 8, 2013 at 8:56 PM,  <shankare at codeaurora.org> wrote:
> Hi Michael,
>
> Does ELF support aliasing ?
>
> How is the relationship captured in ELF symbol table, that one symbol is a
> alias of another symbol ?

It is not explicitly captured. It's an implicit relationship due to
the symbols having the same address.

>
>> Note that __stdout_used is the last symbol in the .rodata section.
>> This means that the reader assigns the data (16 bytes of 0) to
>> __stdout_used. Because dummy_file and the other __stdx_used symbols
>> come before it, they end up in the right place in the final file.
>
> Did you change the Reader too ?

No. I just made another symbol to steal the actual content.

>
> The Reader doesnot allocate any space for __stdout_used. The size of the
> current symbol = (value of next symbol - current symbol). In this case its
> zero.

__stdout_used is the last symbol at that address, so it gets the data.
The hack was to make __stdout_used not get the data.

>
>>
>> This works great until another object file provides a definition of
>> __stdout_used. The weak definition of it gets totally removed, meaning
>> so does the content for the other __stdx_used symbols.
>
> When the other object provides a definition for __stdout_used, the atom
> gets the property of the other object which defines the atom isnt it, and
> so as the ordinal too riht ?
>
> Couldnt follow how did the others move ?

I'm not quite sure what you mean here.

>
> This is what I see with binutils/ld :-
>
> $cat 1.c
> #include "stdio_impl.h"
>
> static FILE *const dummy_file = 0;
> weak_alias(dummy_file, __stdin_used);
> weak_alias(dummy_file, __stdout_used);
> weak_alias(dummy_file, __stderr_used);
>
> $cat 2.c
> int __stdout_used = 10;
> $readelf -s 1.o | grep -E 'used|dummy_file'
>      6: 0000000000000000     8 OBJECT  LOCAL  DEFAULT    4 dummy_file
>      9: 0000000000000000     8 OBJECT  WEAK   DEFAULT    4 __stdin_used
>     10: 0000000000000000     8 OBJECT  WEAK   DEFAULT    4 __stdout_used
>     11: 0000000000000000     8 OBJECT  WEAK   DEFAULT    4 __stderr_used
> $readelf -s 2.o | grep -E 'used|dummy_file'
>      7: 0000000000000000     4 OBJECT  GLOBAL DEFAULT    2 __stdout_used
> $ld 1.o 2.o
> ld: warning: cannot find entry symbol _start; defaulting to 00000000004000e8
> $readelf -s a.out | grep -E 'used|dummy_file'
>      5: 00000000004000e8     8 OBJECT  LOCAL  DEFAULT    1 dummy_file
>      7: 00000000006000f0     4 OBJECT  GLOBAL DEFAULT    2 __stdout_used
>      8: 00000000004000e8     8 OBJECT  WEAK   DEFAULT    1 __stdin_used
>     13: 00000000004000e8     8 OBJECT  WEAK   DEFAULT    1 __stderr_used
>
> Thanks
>
> Shankar Easwaran
>

Yes, which is what we want. Currently we get a dummy_file,
__stdin_used, __stderr_used all as 0 size.

- Michael Spencer