[llvm-bugs] [Bug 35929] New: lld unable to link compiler-rt on aarch64 (hwasan)

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=35929

            Bug ID: 35929
           Summary: lld unable to link compiler-rt on aarch64 (hwasan)
           Product: lld
           Version: unspecified
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: ELF
          Assignee: unassignedbugs at nondot.org
          Reporter: peter.smith at linaro.org
                CC: llvm-bugs at lists.llvm.org

It is currently not possible to link the sanitizers on AArch64 using lld due to
relocations in the text section that can't be resolved with -znotext. These
relocations come from an AArch64 only sanitizer hwasan
(https://clang.llvm.org/docs/HardwareAssistedAddressSanitizerDesign.html)
because it compiles with -fPIE but links --shared. 

The errors are caused by taking the address of preemptible code symbols that
are compiled into non-got generating pc-relative references.  

An example of such a reference that can be reproduced on x86, Arm and AArch64
is:

typedef int fptr(void);

int func(void) {
    return 0;
}

fptr* func2(void) {
    return &func;
}
clang t.c --shared -fPIE -ffunction-sections -o t.so -fuse-ld=lld -Wl,-znotext
ld.lld: error: relocation R_X86_64_PC32 cannot be used against symbol func;
recompile with -fPIC

It just so happens that ld.bfd and ld.gold do not error when this occurs and
just resolve the relocation statically. 

I think that this is definitely an error that lld should diagnose and I'm I'm
going to create a bug on hwasan to let them know that they are incompatible
with lld. 

The interesting question is whether lld should allow this with -znotext. It
isn't possible (on AArch64 and Arm at least) to propagate the relocation into
.rel.dyn. Options include resolving the relocation statically as with bfd and
gold (perhaps only when a command line option is used), or indirecting via the
GOT.

I've marked as an enhancement rather than a bug as hwasan seems to using -fPIE
to reduce the number of relocations, at risk of possible symbol preemption
errors.

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