[ELF] Reject local-exec TLS relocations for -shared

[MaskRay]

Summary

For x86-64, D33100 added a diagnostic for local-exec TLS relocations referencing a preemptible symbol.

This patch generalizes it to non-preemptible symbols (see -Bsymbolic in tls.s)
on all targets.

Local-exec TLS relocations resolve to offsets relative to a fixed point within
the static TLS block, which are only meaningful for the executable.

With this change, clang -fpic -shared -fuse-ld=bfd a.c on the following example will be flagged for AArch64/ARM/i386/x86-64/PowerPC/RISC-V

static __attribute__((tls_model("local-exec"))) __thread long TlsVar = 42;
long bump() { return ++TlsVar; }

Note, in GNU ld, at least arm, riscv and x86's ports have the similar
diagnostics, but aarch64 and ppc64 do not error.

[MaskRay]

Improve i386-tls-le.s

lld/test/ELF/ppc64-local-exec-tls.s

17

Note this test is not well organized. All the relocation types have been checked so I'll not add --implicit-check-not=error:

[rprichard]

It looks OK to me.

lld/ELF/Relocations.cpp

1401

I think this should also check for R_NEG_TLS. AFAIK R_NEG_TLS/R_386_TLS_LE_32 is only used on 32-bit x86, and compilers usually prefer R_TLS/R_386_TLS_LE, but it's possible to bypass the new error with assembly:

movl %gs:TlsVar@NTPOFF, %eax ==> R_TLS
movl %gs:TlsVar@TPOFF, %eax ==> R_NEG_TLS

[MaskRay]

Reject R_NEG_TLS

[MaskRay]

lld/ELF/Relocations.cpp

1401

Thanks. R_386_TLS_LE_32 is an obsoleted Solaris TLS relocation type.

[jhenderson]

lld/ELF/Relocations.cpp

1408

handleTlsRelocation has a number of config->shared checks. Can these be dropped given this new diagnostic?

[MaskRay]

lld/ELF/Relocations.cpp

1408

No. The config->shared checks in handleTlsRelocation are about whether GD/LD/IE can be relaxed to IE/LE.

This check is about LE in -shared.

Actually, R_TLS should really be renamed to R_TPOFFSET to be less confusing and I am going to send such a patch.

[MaskRay]

-> R_TPREL/R_TPREL_NEG

[grimar]

X86 part looks good to me.