[llvm-dev] question about --emit-relocs with lld

[Tom Stellard via llvm-dev]

On 04/04/2019 01:21 PM, Kees Cook via llvm-dev wrote:
> Hi,
> 
> While doing Linux kernel builds linked with lld, I've tracked down a
> difference that breaks relocation of the kernel image (e.g. under
> KASLR[1]). Some relocations are changed to ABS (weirdly, all are in
> .rodata section). Note the difference below in the resulting linked
> output.

Can you file a bug for this.

-Tom

> .L__const._start.instance becomes *ABS* only under lld:
> 
> $ cat minimal.c
> struct minimal {
>         void *pointer;
>         int value;
> };
> 
> void _start(void)
> {
>         struct minimal instance = {
>                 .value = 1,
>         };
> }
> $ llvm-build/x86/bin/clang -c minimal.c
> $ /usr/bin/ld.bfd --emit-relocs minimal.o -o minimal.bfd
> $ llvm-build/x86/bin/ld.lld --emit-relocs minimal.o -o minimal.lld
> $ objdump -Sdr minimal.bfd
> ...
> 00000000004000b0 <_start>:
>   4000b0:       55                      push   %rbp
>   4000b1:       48 89 e5                mov    %rsp,%rbp
>   4000b4:       48 8b 04 25 d0 00 40    mov    0x4000d0,%rax
>   4000bb:       00
>                         4000b8: R_X86_64_32S    .rodata
>   4000bc:       48 89 45 f0             mov    %rax,-0x10(%rbp)
>   4000c0:       48 8b 04 25 d8 00 40    mov    0x4000d8,%rax
>   4000c7:       00
>                         4000c4: R_X86_64_32S    .L__const._start.instance+0x8
>   4000c8:       48 89 45 f8             mov    %rax,-0x8(%rbp)
>   4000cc:       5d                      pop    %rbp
>   4000cd:       c3                      retq
> 
> $ objdump -Sdr minimal.lld
> ...
> 0000000000201000 <_start>:
>   201000:       55                      push   %rbp
>   201001:       48 89 e5                mov    %rsp,%rbp
>   201004:       48 8b 04 25 20 01 20    mov    0x200120,%rax
>   20100b:       00
>                         201008: R_X86_64_32S    .rodata
>   20100c:       48 89 45 f0             mov    %rax,-0x10(%rbp)
>   201010:       48 8b 04 25 28 01 20    mov    0x200128,%rax
>   201017:       00
>                         201014: R_X86_64_32S    *ABS*+0x8
>   201018:       48 89 45 f8             mov    %rax,-0x8(%rbp)
>   20101c:       5d                      pop    %rbp
>   20101d:       c3                      retq
> 
> I'm not sure where to start looking for solving this...
> 
> Thanks!
> 
> -Kees
> 
> [1] https://github.com/ClangBuiltLinux/linux/issues/404
>