/* On-demand PLT fixup for shared objects. Copyright (C) 1995-2024 Free Software Foundation, Inc. This file is part of the GNU C Library. The GNU C Library is free software; you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation; either version 2.1 of the License, or (at your option) any later version. The GNU C Library is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License for more details. You should have received a copy of the GNU Lesser General Public License along with the GNU C Library; if not, see . */ #include #include #include #include #include #include #include #include "dynamic-link.h" #include #include #include /* This function is called through a special trampoline from the PLT the first time each PLT entry is called. We must perform the relocation specified in the PLT of the given shared object, and return the resolved function address to the trampoline, which will restart the original call to that address. Future calls will bounce directly from the PLT to the function. */ DL_FIXUP_VALUE_TYPE attribute_hidden __attribute ((noinline)) DL_ARCH_FIXUP_ATTRIBUTE _dl_fixup ( # ifdef ELF_MACHINE_RUNTIME_FIXUP_ARGS ELF_MACHINE_RUNTIME_FIXUP_ARGS, # endif struct link_map *l, ElfW(Word) reloc_arg) { const ElfW(Sym) *const symtab = (const void *) D_PTR (l, l_info[DT_SYMTAB]); const char *strtab = (const void *) D_PTR (l, l_info[DT_STRTAB]); const uintptr_t pltgot = (uintptr_t) D_PTR (l, l_info[DT_PLTGOT]); const PLTREL *const reloc = (const void *) (D_PTR (l, l_info[DT_JMPREL]) + reloc_offset (pltgot, reloc_arg)); const ElfW(Sym) *sym = &symtab[ELFW(R_SYM) (reloc->r_info)]; const ElfW(Sym) *refsym = sym; void *const rel_addr = (void *)(l->l_addr + reloc->r_offset); lookup_t result; DL_FIXUP_VALUE_TYPE value; /* Sanity check that we're really looking at a PLT relocation. */ assert (ELFW(R_TYPE)(reloc->r_info) == ELF_MACHINE_JMP_SLOT); /* Look up the target symbol. If the normal lookup rules are not used don't look in the global scope. */ if (__builtin_expect (ELFW(ST_VISIBILITY) (sym->st_other), 0) == 0) { const struct r_found_version *version = NULL; if (l->l_info[VERSYMIDX (DT_VERSYM)] != NULL) { const ElfW(Half) *vernum = (const void *) D_PTR (l, l_info[VERSYMIDX (DT_VERSYM)]); ElfW(Half) ndx = vernum[ELFW(R_SYM) (reloc->r_info)] & 0x7fff; version = &l->l_versions[ndx]; if (version->hash == 0) version = NULL; } /* We need to keep the scope around so do some locking. This is not necessary for objects which cannot be unloaded or when we are not using any threads (yet). */ int flags = DL_LOOKUP_ADD_DEPENDENCY; if (!RTLD_SINGLE_THREAD_P) { THREAD_GSCOPE_SET_FLAG (); flags |= DL_LOOKUP_GSCOPE_LOCK; } #ifdef RTLD_ENABLE_FOREIGN_CALL RTLD_ENABLE_FOREIGN_CALL; #endif result = _dl_lookup_symbol_x (strtab + sym->st_name, l, &sym, l->l_scope, version, ELF_RTYPE_CLASS_PLT, flags, NULL); /* We are done with the global scope. */ if (!RTLD_SINGLE_THREAD_P) THREAD_GSCOPE_RESET_FLAG (); #ifdef RTLD_FINALIZE_FOREIGN_CALL RTLD_FINALIZE_FOREIGN_CALL; #endif /* Currently result contains the base load address (or link map) of the object that defines sym. Now add in the symbol offset. */ value = DL_FIXUP_MAKE_VALUE (result, SYMBOL_ADDRESS (result, sym, false)); } else { /* We already found the symbol. The module (and therefore its load address) is also known. */ value = DL_FIXUP_MAKE_VALUE (l, SYMBOL_ADDRESS (l, sym, true)); result = l; } /* And now perhaps the relocation addend. */ value = elf_machine_plt_value (l, reloc, value); if (sym != NULL && __builtin_expect (ELFW(ST_TYPE) (sym->st_info) == STT_GNU_IFUNC, 0)) value = elf_ifunc_invoke (DL_FIXUP_VALUE_ADDR (value)); #ifdef SHARED /* Auditing checkpoint: we have a new binding. Provide the auditing libraries the possibility to change the value and tell us whether further auditing is wanted. The l_reloc_result is only allocated if there is an audit module which provides a la_symbind. */ if (l->l_reloc_result != NULL) { /* This is the address in the array where we store the result of previous relocations. */ struct reloc_result *reloc_result = &l->l_reloc_result[reloc_index (pltgot, reloc_arg, sizeof (PLTREL))]; unsigned int init = atomic_load_acquire (&reloc_result->init); if (init == 0) { _dl_audit_symbind (l, reloc_result, reloc, sym, &value, result, true); /* Store the result for later runs. */ if (__glibc_likely (! GLRO(dl_bind_not))) { reloc_result->addr = value; /* Guarantee all previous writes complete before init is updated. See CONCURRENCY NOTES below. */ atomic_store_release (&reloc_result->init, 1); } } else value = reloc_result->addr; } #endif /* Finally, fix up the plt itself. */ if (__glibc_unlikely (GLRO(dl_bind_not))) return value; return elf_machine_fixup_plt (l, result, refsym, sym, reloc, rel_addr, value); } #if !defined PROF && defined SHARED DL_FIXUP_VALUE_TYPE __attribute ((noinline)) DL_ARCH_FIXUP_ATTRIBUTE _dl_profile_fixup ( # ifdef ELF_MACHINE_RUNTIME_FIXUP_ARGS ELF_MACHINE_RUNTIME_FIXUP_ARGS, # endif struct link_map *l, ElfW(Word) reloc_arg, ElfW(Addr) retaddr, void *regs, long int *framesizep) { void (*mcount_fct) (ElfW(Addr), ElfW(Addr)) = _dl_mcount; if (l->l_reloc_result == NULL) { /* BZ #14843: ELF_DYNAMIC_RELOCATE is called before l_reloc_result is allocated. We will get here if ELF_DYNAMIC_RELOCATE calls a resolver function to resolve an IRELATIVE relocation and that resolver calls a function that is not yet resolved (lazy). For example, the resolver in x86-64 libm.so calls __get_cpu_features defined in libc.so. Skip audit and resolve the external function in this case. */ *framesizep = -1; return _dl_fixup ( # ifdef ELF_MACHINE_RUNTIME_FIXUP_ARGS # ifndef ELF_MACHINE_RUNTIME_FIXUP_PARAMS # error Please define ELF_MACHINE_RUNTIME_FIXUP_PARAMS. # endif ELF_MACHINE_RUNTIME_FIXUP_PARAMS, # endif l, reloc_arg); } const uintptr_t pltgot = (uintptr_t) D_PTR (l, l_info[DT_PLTGOT]); /* This is the address in the array where we store the result of previous relocations. */ struct reloc_result *reloc_result = &l->l_reloc_result[reloc_index (pltgot, reloc_arg, sizeof (PLTREL))]; /* CONCURRENCY NOTES: Multiple threads may be calling the same PLT sequence and with LD_AUDIT enabled they will be calling into _dl_profile_fixup to update the reloc_result with the result of the lazy resolution. The reloc_result guard variable is reloc_init, and we use acquire/release loads and store to it to ensure that the results of the structure are consistent with the loaded value of the guard. This does not fix all of the data races that occur when two or more threads read reloc_result->reloc_init with a value of zero and read and write to that reloc_result concurrently. The expectation is generally that while this is a data race it works because the threads write the same values. Until the data races are fixed there is a potential for problems to arise from these data races. The reloc result updates should happen in parallel but there should be an atomic RMW which does the final update to the real result entry (see bug 23790). The following code uses reloc_result->init set to 0 to indicate if it is the first time this object is being relocated, otherwise 1 which indicates the object has already been relocated. Reading/Writing from/to reloc_result->reloc_init must not happen before previous writes to reloc_result complete as they could end-up with an incomplete struct. */ DL_FIXUP_VALUE_TYPE value; unsigned int init = atomic_load_acquire (&reloc_result->init); if (init == 0) { /* This is the first time we have to relocate this object. */ const ElfW(Sym) *const symtab = (const void *) D_PTR (l, l_info[DT_SYMTAB]); const char *strtab = (const char *) D_PTR (l, l_info[DT_STRTAB]); const uintptr_t pltgot = (uintptr_t) D_PTR (l, l_info[DT_PLTGOT]); const PLTREL *const reloc = (const void *) (D_PTR (l, l_info[DT_JMPREL]) + reloc_offset (pltgot, reloc_arg)); const ElfW(Sym) *refsym = &symtab[ELFW(R_SYM) (reloc->r_info)]; const ElfW(Sym) *defsym = refsym; lookup_t result; /* Sanity check that we're really looking at a PLT relocation. */ assert (ELFW(R_TYPE)(reloc->r_info) == ELF_MACHINE_JMP_SLOT); /* Look up the target symbol. If the symbol is marked STV_PROTECTED don't look in the global scope. */ if (__builtin_expect (ELFW(ST_VISIBILITY) (refsym->st_other), 0) == 0) { const struct r_found_version *version = NULL; if (l->l_info[VERSYMIDX (DT_VERSYM)] != NULL) { const ElfW(Half) *vernum = (const void *) D_PTR (l, l_info[VERSYMIDX (DT_VERSYM)]); ElfW(Half) ndx = vernum[ELFW(R_SYM) (reloc->r_info)] & 0x7fff; version = &l->l_versions[ndx]; if (version->hash == 0) version = NULL; } /* We need to keep the scope around so do some locking. This is not necessary for objects which cannot be unloaded or when we are not using any threads (yet). */ int flags = DL_LOOKUP_ADD_DEPENDENCY; if (!RTLD_SINGLE_THREAD_P) { THREAD_GSCOPE_SET_FLAG (); flags |= DL_LOOKUP_GSCOPE_LOCK; } result = _dl_lookup_symbol_x (strtab + refsym->st_name, l, &defsym, l->l_scope, version, ELF_RTYPE_CLASS_PLT, flags, NULL); /* We are done with the global scope. */ if (!RTLD_SINGLE_THREAD_P) THREAD_GSCOPE_RESET_FLAG (); /* Currently result contains the base load address (or link map) of the object that defines sym. Now add in the symbol offset. */ value = DL_FIXUP_MAKE_VALUE (result, SYMBOL_ADDRESS (result, defsym, false)); if (defsym != NULL && __builtin_expect (ELFW(ST_TYPE) (defsym->st_info) == STT_GNU_IFUNC, 0)) value = elf_ifunc_invoke (DL_FIXUP_VALUE_ADDR (value)); } else { /* We already found the symbol. The module (and therefore its load address) is also known. */ value = DL_FIXUP_MAKE_VALUE (l, SYMBOL_ADDRESS (l, refsym, true)); if (__builtin_expect (ELFW(ST_TYPE) (refsym->st_info) == STT_GNU_IFUNC, 0)) value = elf_ifunc_invoke (DL_FIXUP_VALUE_ADDR (value)); result = l; } /* And now perhaps the relocation addend. */ value = elf_machine_plt_value (l, reloc, value); /* Auditing checkpoint: we have a new binding. Provide the auditing libraries the possibility to change the value and tell us whether further auditing is wanted. */ if (defsym != NULL && GLRO(dl_naudit) > 0) _dl_audit_symbind (l, reloc_result, reloc, defsym, &value, result, true); /* Store the result for later runs. */ if (__glibc_likely (! GLRO(dl_bind_not))) { reloc_result->addr = value; /* Guarantee all previous writes complete before init is updated. See CONCURRENCY NOTES earlier */ atomic_store_release (&reloc_result->init, 1); } init = 1; } else value = reloc_result->addr; /* By default we do not call the pltexit function. */ long int framesize = -1; /* Auditing checkpoint: report the PLT entering and allow the auditors to change the value. */ _dl_audit_pltenter (l, reloc_result, &value, regs, &framesize); /* Store the frame size information. */ *framesizep = framesize; (*mcount_fct) (retaddr, DL_FIXUP_VALUE_CODE_ADDR (value)); return value; } #endif /* !defined PROF && defined SHARED */