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add new stage 2b to dynamic linker bootstrap for thread pointer 

commit a603a75a72 removed attribute
const from __errno_location and pthread_self, and the same reasoning
forced arch definitions of __pthread_self to use volatile asm,
significantly impacting code generation and imposing manual caching of
pointers where the impact might be noticable.

reorder the thread pointer setup and place it across a strong barrier
(symbolic function lookup) so that there is no assumed ordering
between the initialization and the accesses to the thread pointer in
stage 3.
This commit is contained in:
Rich Felker 2018-10-16 13:36:51 -04:00
parent 2085378a4f
commit 1c84c99913
1 changed files with 23 additions and 10 deletions
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33
ldso/dynlink.c
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@ -1453,9 +1453,31 @@ hidden void __dls2(unsigned char *base, size_t *sp)
ldso.relocated = 0;
/* Call dynamic linker stage-3, __dls3, looking it up
/* Call dynamic linker stage-2b, __dls2b, looking it up
* symbolically as a barrier against moving the address
* load across the above relocation processing. */
struct symdef dls2b_def = find_sym(&ldso, "__dls2b", 0);
if (DL_FDPIC) ((stage3_func)&ldso.funcdescs[dls2b_def.sym-ldso.syms])(sp);
else ((stage3_func)laddr(&ldso, dls2b_def.sym->st_value))(sp);
}
/* Stage 2b sets up a valid thread pointer, which requires relocations
* completed in stage 2, and on which stage 3 is permitted to depend.
* This is done as a separate stage, with symbolic lookup as a barrier,
* so that loads of the thread pointer and &errno can be pure/const and
* thereby hoistable. */
_Noreturn void __dls2b(size_t *sp)
{
/* Setup early thread pointer in builtin_tls for ldso/libc itself to
* use during dynamic linking. If possible it will also serve as the
* thread pointer at runtime. */
libc.tls_size = sizeof builtin_tls;
libc.tls_align = tls_align;
if (__init_tp(__copy_tls((void *)builtin_tls)) < 0) {
a_crash();
}
struct symdef dls3_def = find_sym(&ldso, "__dls3", 0);
if (DL_FDPIC) ((stage3_func)&ldso.funcdescs[dls3_def.sym-ldso.syms])(sp);
else ((stage3_func)laddr(&ldso, dls3_def.sym->st_value))(sp);
@ -1490,15 +1512,6 @@ _Noreturn void __dls3(size_t *sp)
libc.secure = ((aux[0]&0x7800)!=0x7800 || aux[AT_UID]!=aux[AT_EUID]
|| aux[AT_GID]!=aux[AT_EGID] || aux[AT_SECURE]);
/* Setup early thread pointer in builtin_tls for ldso/libc itself to
* use during dynamic linking. If possible it will also serve as the
* thread pointer at runtime. */
libc.tls_size = sizeof builtin_tls;
libc.tls_align = tls_align;
if (__init_tp(__copy_tls((void *)builtin_tls)) < 0) {
a_crash();
}
/* Only trust user/env if kernel says we're not suid/sgid */
if (!libc.secure) {
env_path = getenv("LD_LIBRARY_PATH");