[lld/ELF] eh_frame_hdr generation only supports 32-bit values

[aeubanks]

trying the large code model on some binaries, we're running into lld complaining about eh_frame_hdr values not fitting in 32 bits:

$ cat /tmp/a.c
int main() {}
$ cat /tmp/bigtext.lds 
SECTIONS
{
    .ltext :
    {
        *(.ltext)
        *(.ltext.*)

        . = ALIGN(4);

        . = MAX(., 0x100000000);

    }
}
$ clang -fuse-ld=lld -mcmodel=large -T /tmp/bigtext.lds /tmp/a.c -o /tmp/a
ld.lld: error: /tmp/a-bdc8cb.o:(.eh_frame): PC offset is too large: 0xffffffff000002ac

according to https://refspecs.linuxfoundation.org/LSB_1.3.0/gLSB/gLSB/ehframehdr.html, 64 bit values are allowed by specifying DW_EH_PE_sdata8

lld code that assumes 32 bit eh_frame_hdr values:

llvm-project/lld/ELF/SyntheticSections.cpp

Line 616 in 796fafe

if (!isInt<32>(pc - va)) {

llvm-project/lld/ELF/SyntheticSections.cpp

Line 638 in 796fafe

hdrBuf[1] = DW_EH_PE_pcrel | DW_EH_PE_sdata4; // eh_frame_ptr_enc

would lld be ok with going to 64 bit values in eh_frame_hdr when any value doesn't fit in 32 bits?

[llvmbot]

@llvm/issue-subscribers-lld-elf

Author: Arthur Eubanks (aeubanks)

Details

trying the large code model on some binaries, we're running into lld complaining about eh_frame_hdr values not fitting in 32 bits:

$ cat /tmp/a.c
int main() {}
$ cat /tmp/bigtext.lds 
SECTIONS
{
    .ltext :
    {
        *(.ltext)
        *(.ltext.*)

        . = ALIGN(4);

        . = MAX(., 0x100000000);

    }
}
$ clang -fuse-ld=lld -mcmodel=large -T /tmp/bigtext.lds /tmp/a.c -o /tmp/a
ld.lld: error: /tmp/a-bdc8cb.o:(.eh_frame): PC offset is too large: 0xffffffff000002ac

according to https://refspecs.linuxfoundation.org/LSB_1.3.0/gLSB/gLSB/ehframehdr.html, 64 bit values are allowed by specifying DW_EH_PE_sdata8

lld code that assumes 32 bit eh_frame_hdr values:

llvm-project/lld/ELF/SyntheticSections.cpp

Line 616 in 796fafe

if (!isInt<32>(pc - va)) {

llvm-project/lld/ELF/SyntheticSections.cpp

Line 638 in 796fafe

hdrBuf[1] = DW_EH_PE_pcrel | DW_EH_PE_sdata4; // eh_frame_ptr_enc

would lld be ok with going to 64 bit values in eh_frame_hdr when any value doesn't fit in 32 bits?

[aeubanks]

Either autodetect when any value doesn't fit in 32 bits and then switch from sdata4 to sdata8, or a separate flag that forces sdata8 instead of sdata4