From 2b92982e2369d292560793bee8e730f695f48ff3 Mon Sep 17 00:00:00 2001 From: Michael Jeanson Date: Wed, 3 Jul 2024 12:35:34 -0400 Subject: [PATCH] nptl: fix potential merge of __rseq_* relro symbols While working on a patch to add support for the extensible rseq ABI, we came across an issue where a new 'const' variable would be merged with the existing '__rseq_size' variable. We tracked this to the use of '-fmerge-all-constants' which allows the compiler to merge identical constant variables. This means that all 'const' variables in a compile unit that are of the same size and are initialized to the same value can be merged. In this specific case, on 32 bit systems 'unsigned int' and 'ptrdiff_t' are both 4 bytes and initialized to 0 which should trigger the merge. However for reasons we haven't delved into when the attribute 'section (".data.rel.ro")' is added to the mix, only variables of the same exact types are merged. As far as we know this behavior is not specified anywhere and could change with a new compiler version, hence this patch. Move the definitions of these variables into an assembler file and add hidden writable aliases for internal use. This has the added bonus of removing the asm workaround to set the values on rseq registration. Tested on Debian 12 with GCC 12.2. Signed-off-by: Michael Jeanson Reviewed-by: Mathieu Desnoyers Reviewed-by: Florian Weimer --- elf/Makefile | 1 + elf/dl-rseq-symbols.S | 64 +++++++++++++++++++++++++++++++++++ sysdeps/nptl/dl-tls_init_tp.c | 14 ++++---- 3 files changed, 71 insertions(+), 8 deletions(-) create mode 100644 elf/dl-rseq-symbols.S diff --git a/elf/Makefile b/elf/Makefile index a3475f3fb5..147f1d3437 100644 --- a/elf/Makefile +++ b/elf/Makefile @@ -73,6 +73,7 @@ dl-routines = \ dl-origin \ dl-printf \ dl-reloc \ + dl-rseq-symbols \ dl-runtime \ dl-scope \ dl-setup_hash \ diff --git a/elf/dl-rseq-symbols.S b/elf/dl-rseq-symbols.S new file mode 100644 index 0000000000..b4bba06a99 --- /dev/null +++ b/elf/dl-rseq-symbols.S @@ -0,0 +1,64 @@ +/* Define symbols used by rseq. + Copyright (C) 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 + +#if __WORDSIZE == 64 +#define RSEQ_OFFSET_SIZE 8 +#else +#define RSEQ_OFFSET_SIZE 4 +#endif + +/* Some targets define a macro to denote the zero register. */ +#undef zero + +/* Define 2 symbols: '__rseq_size' is public const and '_rseq_size' (an + alias of '__rseq_size') is hidden and writable for internal use by the + dynamic linker which will initialize the value both symbols point to + before copy relocations take place. */ + + .globl __rseq_size + .type __rseq_size, %object + .size __rseq_size, 4 + .hidden _rseq_size + .globl _rseq_size + .type _rseq_size, %object + .size _rseq_size, 4 + .section .data.rel.ro + .balign 4 +__rseq_size: +_rseq_size: + .zero 4 + +/* Define 2 symbols: '__rseq_offset' is public const and '_rseq_offset' (an + alias of '__rseq_offset') is hidden and writable for internal use by the + dynamic linker which will initialize the value both symbols point to + before copy relocations take place. */ + + .globl __rseq_offset + .type __rseq_offset, %object + .size __rseq_offset, RSEQ_OFFSET_SIZE + .hidden _rseq_offset + .globl _rseq_offset + .type _rseq_offset, %object + .size _rseq_offset, RSEQ_OFFSET_SIZE + .section .data.rel.ro + .balign RSEQ_OFFSET_SIZE +__rseq_offset: +_rseq_offset: + .zero RSEQ_OFFSET_SIZE diff --git a/sysdeps/nptl/dl-tls_init_tp.c b/sysdeps/nptl/dl-tls_init_tp.c index 092c274f36..7eb35fb133 100644 --- a/sysdeps/nptl/dl-tls_init_tp.c +++ b/sysdeps/nptl/dl-tls_init_tp.c @@ -45,8 +45,10 @@ rtld_mutex_dummy (pthread_mutex_t *lock) #endif const unsigned int __rseq_flags; -const unsigned int __rseq_size attribute_relro; -const ptrdiff_t __rseq_offset attribute_relro; + +/* The variables are in .data.relro but are not yet write-protected. */ +extern unsigned int _rseq_size attribute_hidden; +extern ptrdiff_t _rseq_offset attribute_hidden; void __tls_pre_init_tp (void) @@ -105,10 +107,7 @@ __tls_init_tp (void) do_rseq = TUNABLE_GET (rseq, int, NULL); if (rseq_register_current_thread (pd, do_rseq)) { - /* We need a writable view of the variables. They are in - .data.relro and are not yet write-protected. */ - extern unsigned int size __asm__ ("__rseq_size"); - size = sizeof (pd->rseq_area); + _rseq_size = sizeof (pd->rseq_area); } #ifdef RSEQ_SIG @@ -117,8 +116,7 @@ __tls_init_tp (void) all targets support __thread_pointer, so set __rseq_offset only if the rseq registration may have happened because RSEQ_SIG is defined. */ - extern ptrdiff_t offset __asm__ ("__rseq_offset"); - offset = (char *) &pd->rseq_area - (char *) __thread_pointer (); + _rseq_offset = (char *) &pd->rseq_area - (char *) __thread_pointer (); #endif }