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

commit a603a75a72bb469c6be4963ed1b55fabe675fe15 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.

diff --git a/ldso/dynlink.c b/ldso/dynlink.c
index c2447bc..ec921df 100644 (file)
--- a/ldso/dynlink.c
+++ b/ldso/dynlink.c
@@ -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");