[LLVMdev] [lld] ELF weak aliases

[Michael Spencer]

So I just got lua to link and run and work on x86-64 Linux with musl
and lld. It did require one change to hack around incorrect handling
of ELF weak aliases.

In musl __stdio_exit.c
<http://git.musl-libc.org/cgit/musl/tree/src/stdio/__stdio_exit.c> we
have:

static FILE *const dummy_file = 0;
weak_alias(dummy_file, __stdin_used);
weak_alias(dummy_file, __stdout_used);
weak_alias(dummy_file, __stderr_used);

weak_alias(old, new) is defined as: extern __typeof(old) new
__attribute__((weak, alias(#old)))

This generates the following object file:
mspencer at mspencer-vm:~/Projects/test$ objdump -st
../musl/src/stdio/__stdio_exit.o

../musl/src/stdio/__stdio_exit.o:     file format elf64-x86-64

SYMBOL TABLE:
0000000000000000 l    df *ABS*  0000000000000000 src/stdio/__stdio_exit.c
0000000000000044 l     F .text  0000000000000049 close_file
0000000000000000 l     O .rodata        0000000000000008 dummy_file
0000000000000000 l    d  .text  0000000000000000 .text
0000000000000000 l    d  .data  0000000000000000 .data
0000000000000000 l    d  .bss   0000000000000000 .bss
0000000000000000 l    d  .rodata        0000000000000000 .rodata
0000000000000000 l    d  .note.GNU-stack        0000000000000000 .note.GNU-stack
0000000000000000  w    O .rodata        0000000000000008 __stderr_used
0000000000000000  w    O .rodata        0000000000000008 __stdin_used
0000000000000000 g     F .text  0000000000000044 __stdio_exit
0000000000000000  w    O .rodata        0000000000000008 __stdout_used
0000000000000000         *UND*  0000000000000000 __libc
0000000000000000         *UND*  0000000000000000 __lock
0000000000000000         *UND*  0000000000000000 __lockfile

Contents of section .text:
 0000 53833d00 00000000 740abf00 000000e8  S.=.....t.......
 0010 00000000 488b1d00 000000eb 0c4889df  ....H........H..
 0020 e81f0000 00488b5b 704885db 75ef488b  .....H.[pH..u.H.
 0030 3d000000 00e80a00 0000488b 3d000000  =.........H.=...
 0040 005beb00 534889fb 4885db74 3e83bb8c  .[..SH..H..t>...
 0050 00000000 78084889 dfe80000 0000488b  ....x.H.......H.
 0060 4328483b 4338760a 4889df31 f631d2ff  C(H;C8v.H..1.1..
 0070 5348488b 7308482b 7310730f 488b4350  SHH.s.H+s.s.H.CP
 0080 4889dfba 01000000 5bffe05b c3        H.......[..[.
Contents of section .rodata:
 0000 00000000 00000000

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.

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.

I fixed this by adding weak_alias(dummy_file,
__zinurfilestealinurdata); to __stdio_exit.c which allocated the 16
bytes to __zinurfilestealinurdata.

Another way to fix this it to, in the reader, assign all the data to
the non-weak symbol (dummy_file in this case) when multiple symbols
share the same location. However, this fails to work if you have a
weak symbol pointing in to the middle of a non weak symbol's data. In
this case we actually need to move the data over to the non-weak
symbol (or create an anonymous local symbol to hold the data).
However, this only needs to happen in specific cases.

- Michael Spencer