I noticed that if I use a version script with ld.gold to make a TLS symbol in a DSO local, the symbol remains in the .dynsym table but with local binding. Gold emits DTPMOD/DTPOFF relocations to the local TLS symbol, and when musl tries to load the DSO, it apparently tries to look up the symbol name globally and fails.

$ cat vers
{
global:
get_tls_var;
local:
*;
};

$ cat dso.c
__thread int tlsvar;
int get_tls_var(void) {
return tlsvar;
}

$ cat main.c
int get_tls_var(void);
int main() {
get_tls_var();
return 0;
}

$ musl-gcc dso.c -fpic -shared -fuse-ld=gold -Wl,-version-script=vers -o libdso.so

$ musl-gcc main.c libdso.so -Wl,-rpath,'$ORIGIN' -o main

$ ./main
Error relocating /tmp/local-tls-symbol/libdso.so: tlsvar: symbol not found
Error relocating /tmp/local-tls-symbol/libdso.so: tlsvar: symbol not found

$ readelf -rW --dyn-syms libdso.so
...
0000000000001fd8 0000000100000010 R_X86_64_DTPMOD64 0000000000000000 tlsvar + 0
0000000000001fe0 0000000100000011 R_X86_64_DTPOFF64 0000000000000000 tlsvar + 0
...
1: 0000000000000000 4 TLS LOCAL DEFAULT 13 tlsvar

...

The test program works with ld.bfd, because ld.bfd converts the DTPMOD relocation to 0 and omits the DTPOFF relocation. There was a somewhat similar issue with gold+musl involving a DTPMOD relocation to a local section symbol, https://sourceware.org/bugzilla/show_bug.cgi?id=17699. That issue prompted a thread on the generic-abi group, https://groups.google.com/d/topic/generic-abi/dJ4_Y78aQ2M/discussion.

I'm wondering if this problem is a bug in musl or gold. I also wonder if DTPOFF can reference a TLS section, even though the value of a TLS section symbol isn't suitable for DTPOFF unless it's first adjusted by the segment's p_vaddr.

-Ryan