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Date: Thu, 23 May 2019 17:47:13 -0700
From: Ryan Prichard <rprichard@...gle.com>
To: musl@...ts.openwall.com
Subject: musl can't handle gold's STB_LOCAL TLS symbols
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
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