Fedora Rawhide kernel builds fails with `unkown symbol errors`.

[tstellar]

After upgrading from clang-17 to clang-18, the Fedora Rawhide kernel builds started to fail with many errors like:

nfsv4.ko needs unknown symbol __pcpu_unique_nfs4_callback_count1

Full build log is here

[tstellar]

@nathanchance Here is the config file we were using for the last successful build with clang-17: config.txt

[nikic]

I suspect that the key part of the config is CONFIG_DEBUG_FORCE_WEAK_PER_CPU=y.

[nikic]

test.c:

__attribute__((__section__(".discard"))) __attribute__((unused)) char __pcpu_unique_foo;

module.lds:

SECTIONS {
 /DISCARD/ : {
  *(.discard)
  *(.discard.*)
 }
}

Then:

build/bin/clang test.c -c
build/bin/ld.lld -r -T module.lds test.o -o test
objdump -t test

Produces:

0000000000000000       O *UND*	0000000000000000 __pcpu_unique_foo

With old ld.lld and ld.bfd, the undefined symbol was not present.

[nikic]

This regressed in llvm/llvm-project@1981b1b, cc @MaskRay

[nikic]

Filed an upstream issue: llvm/llvm-project#85048

[nikic]

Issue has been fixed and backported to the 18.x branch, so this can be closed.