i#7120: Do not add addend for rela jump slot relocation

Despite "rela" relocations having an explicit addend value and it
being set to non-0 in a new Debian glibc, the addend is assumed to
*not* be added to the symbol value when relocating on x86_64 and
aarch64 (it does seem to be added on RISCV).
This is not obvious and not well documented; we have to just behave like
existing loaders behave from experimentation/examination.
(Yet another reason to possibly invert the private loader and let
the private copy of ld.so do all the loading and relocating: #5437).

Tested on a machine where nearly every client test in our suite was
crashing after a glibc update: now they pass.  Unfortunately it's not
simple to make automated tests for this: we don't have an existing
framework for relocation testing and it would take non-trivial effort
to construct that, beyond the scope of this fix.

Fixes #7120

core/unix/loader.c

@@ -787,8 +787,9 @@ privload_os_finalize(privmod_t *privmod)
         GLRO_dl_tls_static_size_OFFS = last_large_load_offs;
         GLRO_dl_tls_static_align_OFFS = last_large_load_offs + sizeof(void *);
         LOG(GLOBAL, LOG_LOADER, 2,
-            "%s: for glibc 2.34+ workaround found offsets 0x%x 0x%x\n", __FUNCTION__,
-            GLRO_dl_tls_static_size_OFFS, GLRO_dl_tls_static_align_OFFS);
+            "%s: for glibc 2.34+ workaround found offsets 0x%x 0x%x for glro %p\n",
+            __FUNCTION__, GLRO_dl_tls_static_size_OFFS, GLRO_dl_tls_static_align_OFFS,
+            glro);
     }
 #    endif
     if (GLRO_dl_tls_static_size_OFFS == 0) {
@@ -827,7 +828,8 @@ privload_os_finalize(privmod_t *privmod)
         LOG(GLOBAL, LOG_LOADER, 2, "%s: glibc 2.34+ workaround succeeded\n",
             __FUNCTION__);
     }
-    LOG(GLOBAL, LOG_LOADER, 2, "%s: calling %s\n", __FUNCTION__, LIBC_EARLY_INIT_NAME);
+    LOG(GLOBAL, LOG_LOADER, 2, "%s: calling %s @%p\n", __FUNCTION__, LIBC_EARLY_INIT_NAME,
+        libc_early_init);
     (*libc_early_init)(true);
 #endif /* LINUX */
 }

core/unix/module_elf.c

@@ -1,5 +1,5 @@
 /* *******************************************************************************
- * Copyright (c) 2012-2022 Google, Inc.  All rights reserved.
+ * Copyright (c) 2012-2024 Google, Inc.  All rights reserved.
  * Copyright (c) 2011 Massachusetts Institute of Technology  All rights reserved.
  * Copyright (c) 2008-2010 VMware, Inc.  All rights reserved.
  * *******************************************************************************/
@@ -1466,8 +1466,8 @@ module_relocate_symbol(ELF_REL_TYPE *rel, os_privmod_data_t *pd, bool is_rela)
     const char *name;
     bool resolved;
 
-    /* XXX: we assume ELF_REL_TYPE and ELF_RELA_TYPE only differ at the end,
-     * i.e. with or without r_addend.
+    /* ELF_REL_TYPE and ELF_RELA_TYPE differ in where the addend comes from:
+     * stored in the target location, or in rel->r_addend.
      */
     if (is_rela)
         addend = ((ELF_RELA_TYPE *)rel)->r_addend;
@@ -1486,7 +1486,8 @@ module_relocate_symbol(ELF_REL_TYPE *rel, os_privmod_data_t *pd, bool is_rela)
                      ".so has relocation inside PT_DYNAMIC section");
     r_type = (uint)ELF_R_TYPE(rel->r_info);
 
-    LOG(GLOBAL, LOG_LOADER, 5, "%s: reloc @ %p type=%d\n", r_addr, r_type);
+    LOG(GLOBAL, LOG_LOADER, 5, "%s: reloc @ %p type=%d is_rela=%d addend=0x%zx\n",
+        __FUNCTION__, r_addr, r_type, is_rela, addend);
 
     /* handle the most common case, i.e. ELF_R_RELATIVE */
     if (r_type == ELF_R_RELATIVE) {
@@ -1581,7 +1582,18 @@ module_relocate_symbol(ELF_REL_TYPE *rel, os_privmod_data_t *pd, bool is_rela)
 #ifndef RISCV64 /* FIXME i#3544: Check whether ELF_R_DIRECT with !is_rela is OK */
     case ELF_R_GLOB_DAT:
 #endif
-    case ELF_R_JUMP_SLOT: *r_addr = (reg_t)res + addend; break;
+    case ELF_R_JUMP_SLOT:
+        // Neither aarch64 nor x86_64 add the addend for these types, yet riscv does.
+        // This is not obvious and not well documented; we have to just behave like
+        // existing loaders behave from experimentation/examination.
+        // Yet another reason to possibly invert the private loader and let
+        // the private copy of ld.so do all the loading and relocating: i#5437.
+#if defined(AARCH64) || defined(X86)
+        *r_addr = (reg_t)res;
+#else
+        *r_addr = (reg_t)res + addend;
+#endif
+        break;
     case ELF_R_DIRECT: *r_addr = (reg_t)res + (is_rela ? addend : *r_addr); break;
     case ELF_R_COPY:
         if (sym != NULL)