[lld][ELF] lld doesn't check RISC-V e_flags for ET_DYN inputs

[mysterymath]

RISC-V linkers check e_flags for ABI compatibility between input files. LLD seems to do this only for ET_REL (.o) files. GNU (BFD) ld also checks ET_DYN (.so) files.

This led to an issue in Fuchsia WRT llvm-ifs. (See the corresponding Fuchsia issue for details.)

This reproducer succeeds on LLD, but it produces the following on GNU LD:

% ./prebuilt/third_party/gcc/linux-x64/bin/riscv64-elf-ld.bfd -o foo foo.o libbar.so
./prebuilt/third_party/gcc/linux-x64/bin/riscv64-elf-ld.bfd: libbar.so: can't link soft-float modules with double-float modules
./prebuilt/third_party/gcc/linux-x64/bin/riscv64-elf-ld.bfd: failed to merge target specific data of file libbar.so

Originally filed by @frobtech downstream; adding for context/discussion.

[llvmbot]

@llvm/issue-subscribers-lld-elf

Author: Daniel Thornburgh (mysterymath)

RISC-V linkers check `e_flags` for ABI compatibility between input files. LLD seems to do this only for ET_REL (.o) files. GNU (BFD) ld also checks ET_DYN (.so) files.

This led to an issue in Fuchsia WRT llvm-ifs. (See the corresponding Fuchsia issue for details.)

This reproducer succeeds on LLD, but it produces the following on GNU LD:

% ./prebuilt/third_party/gcc/linux-x64/bin/riscv64-elf-ld.bfd -o foo foo.o libbar.so
./prebuilt/third_party/gcc/linux-x64/bin/riscv64-elf-ld.bfd: libbar.so: can't link soft-float modules with double-float modules
./prebuilt/third_party/gcc/linux-x64/bin/riscv64-elf-ld.bfd: failed to merge target specific data of file libbar.so

Originally filed by @frobtech downstream; adding for context/discussion.

[lenary]

I do think this is an LLD bug.

Right now, the ABI does have a section on "Policy for Merge Objects With Different File Headers", which refers to "inputs [sic] files", and "object files". When I read "object files", I did first think "relocatable object files", but the document is not specific, and shared libraries are also (shared) object files.

The issue looks to be with the code here:

llvm-project/lld/ELF/Arch/RISCV.cpp

Line 156 in 631769f

for (InputFile *f : ctx.objectFiles) {

We should probably also be iterating over ctx.sharedFiles as well here.

Given compatibility is also based on the gnu properties, are these also being preserved by llvm-ifs?