From 3a3f4bf76a4790e81ee186ea76731a7f67dba1c8 Mon Sep 17 00:00:00 2001 From: Alan Modra Date: Mon, 14 Oct 2019 13:51:19 +1030 Subject: [PATCH] qsort: elf_link_add_object_symbols weak aliases This particular sort almost certainly does not need to be stable for the ELF linker to work correctly. However it is conceivable that an unstable sort could affect linker output, and thus different output be seen with differing qsort implementations. The argument goes like this: Given more than one strong alias symbol of equal section, value, and size, the aliases will compare equal by elf_sort_symbol and thus which one is chosen as the "real" symbol to be made dynamic depends on qsort. Why would anyone define two symbols at the same address? Well, sometimes the fact that there are more than one strong alias symbol is due to linker script symbols like __bss_start being made dynamic. This will match the first symbol defined in .bss if it doesn't have correct size, and forgetting to properly set size and type of symbols isn't as rare as it should be. This patch adds some more heuristics to elf_sort_symbol. * elflink.c (elf_sort_symbol): Sort on type and name as well. (elf_link_add_object_symbols): Style fix. --- bfd/ChangeLog | 5 +++++ bfd/elflink.c | 56 ++++++++++++++++++++++++++++++++++++++++----------- 2 files changed, 49 insertions(+), 12 deletions(-) diff --git a/bfd/ChangeLog b/bfd/ChangeLog index 6a5f673dea..bb6c907ac7 100644 --- a/bfd/ChangeLog +++ b/bfd/ChangeLog @@ -1,3 +1,8 @@ +2019-10-14 Alan Modra + + * elflink.c (elf_sort_symbol): Sort on type and name as well. + (elf_link_add_object_symbols): Style fix. + 2019-10-14 Alan Modra * elf.c (_bfd_elf_map_sections_to_segments): Init target_index diff --git a/bfd/elflink.c b/bfd/elflink.c index bfd0f019aa..9d7f69afda 100644 --- a/bfd/elflink.c +++ b/bfd/elflink.c @@ -3587,27 +3587,60 @@ on_needed_list (const char *soname, return FALSE; } -/* Sort symbol by value, section, and size. */ +/* Sort symbol by value, section, size, and type. */ static int elf_sort_symbol (const void *arg1, const void *arg2) { const struct elf_link_hash_entry *h1; const struct elf_link_hash_entry *h2; bfd_signed_vma vdiff; + int sdiff; + const char *n1; + const char *n2; h1 = *(const struct elf_link_hash_entry **) arg1; h2 = *(const struct elf_link_hash_entry **) arg2; vdiff = h1->root.u.def.value - h2->root.u.def.value; if (vdiff != 0) return vdiff > 0 ? 1 : -1; - else - { - int sdiff = h1->root.u.def.section->id - h2->root.u.def.section->id; - if (sdiff != 0) - return sdiff > 0 ? 1 : -1; - } + + sdiff = h1->root.u.def.section->id - h2->root.u.def.section->id; + if (sdiff != 0) + return sdiff; + + /* Sort so that sized symbols are selected over zero size symbols. */ vdiff = h1->size - h2->size; - return vdiff == 0 ? 0 : vdiff > 0 ? 1 : -1; + if (vdiff != 0) + return vdiff > 0 ? 1 : -1; + + /* Sort so that STT_OBJECT is selected over STT_NOTYPE. */ + if (h1->type != h2->type) + return h1->type - h2->type; + + /* If symbols are properly sized and typed, and multiple strong + aliases are not defined in a shared library by the user we + shouldn't get here. Unfortunately linker script symbols like + __bss_start sometimes match a user symbol defined at the start of + .bss without proper size and type. We'd like to preference the + user symbol over reserved system symbols. Sort on leading + underscores. */ + n1 = h1->root.root.string; + n2 = h2->root.root.string; + while (*n1 == *n2) + { + if (*n1 == 0) + break; + ++n1; + ++n2; + } + if (*n1 == '_') + return -1; + if (*n2 == '_') + return 1; + + /* Final sort on name selects user symbols like '_u' over reserved + system symbols like '_Z' and also will avoid qsort instability. */ + return *n1 - *n2; } /* This function is used to adjust offsets into .dynstr for @@ -5345,8 +5378,8 @@ error_free_dyn: defined symbol, search time for N weak defined symbols will be O(N^2). Binary search will cut it down to O(NlogN). */ amt = extsymcount; - amt *= sizeof (struct elf_link_hash_entry *); - sorted_sym_hash = (struct elf_link_hash_entry **) bfd_malloc (amt); + amt *= sizeof (*sorted_sym_hash); + sorted_sym_hash = bfd_malloc (amt); if (sorted_sym_hash == NULL) goto error_return; sym_hash = sorted_sym_hash; @@ -5366,8 +5399,7 @@ error_free_dyn: } } - qsort (sorted_sym_hash, sym_count, - sizeof (struct elf_link_hash_entry *), + qsort (sorted_sym_hash, sym_count, sizeof (*sorted_sym_hash), elf_sort_symbol); while (weaks != NULL) -- 2.34.1