* [PATCH 0/2] objtool: Fix code reloc vs weak symbols
@ 2022-04-19 20:32 Peter Zijlstra
2022-04-19 20:32 ` [PATCH 1/2] objtool: Fix type of reloc::addend Peter Zijlstra
` (2 more replies)
0 siblings, 3 replies; 8+ messages in thread
From: Peter Zijlstra @ 2022-04-19 20:32 UTC (permalink / raw)
To: x86, jpoimboe; +Cc: linux-kernel, peterz, mbenes
Two fun bugs that make can cause other fun fail if you have a fresh enough
toolchain that strips unused section symbols.
The symptoms can vary between a faulty unwind to a kernel splat for trying to
patch the wrong text.
I found it while playing with text patching and Josh confirmed it very much
affects ORC data too.
^ permalink raw reply [flat|nested] 8+ messages in thread* [PATCH 1/2] objtool: Fix type of reloc::addend 2022-04-19 20:32 [PATCH 0/2] objtool: Fix code reloc vs weak symbols Peter Zijlstra @ 2022-04-19 20:32 ` Peter Zijlstra 2022-04-22 10:27 ` [tip: objtool/urgent] " tip-bot2 for Peter Zijlstra 2022-04-19 20:32 ` [PATCH 2/2] objtool: Fix code relocs vs weak symbols Peter Zijlstra 2022-04-21 15:48 ` [PATCH 0/2] objtool: Fix code reloc " Josh Poimboeuf 2 siblings, 1 reply; 8+ messages in thread From: Peter Zijlstra @ 2022-04-19 20:32 UTC (permalink / raw) To: x86, jpoimboe; +Cc: linux-kernel, peterz, mbenes Elf{32,64}_Rela::r_addend is of type: Elf{32,64}_Sword, that means that our reloc::addend needs to be long or face tuncation issues when we do elf_rebuild_reloc_section(): - 107: 48 b8 00 00 00 00 00 00 00 00 movabs $0x0,%rax 109: R_X86_64_64 level4_kernel_pgt+0x80000067 + 107: 48 b8 00 00 00 00 00 00 00 00 movabs $0x0,%rax 109: R_X86_64_64 level4_kernel_pgt-0x7fffff99 Fixes: 627fce14809b ("objtool: Add ORC unwind table generation") Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> --- tools/objtool/check.c | 8 ++++---- tools/objtool/elf.c | 2 +- tools/objtool/include/objtool/elf.h | 4 ++-- 3 files changed, 7 insertions(+), 7 deletions(-) --- a/tools/objtool/check.c +++ b/tools/objtool/check.c @@ -560,12 +560,12 @@ static int add_dead_ends(struct objtool_ else if (reloc->addend == reloc->sym->sec->sh.sh_size) { insn = find_last_insn(file, reloc->sym->sec); if (!insn) { - WARN("can't find unreachable insn at %s+0x%x", + WARN("can't find unreachable insn at %s+0x%lx", reloc->sym->sec->name, reloc->addend); return -1; } } else { - WARN("can't find unreachable insn at %s+0x%x", + WARN("can't find unreachable insn at %s+0x%lx", reloc->sym->sec->name, reloc->addend); return -1; } @@ -595,12 +595,12 @@ static int add_dead_ends(struct objtool_ else if (reloc->addend == reloc->sym->sec->sh.sh_size) { insn = find_last_insn(file, reloc->sym->sec); if (!insn) { - WARN("can't find reachable insn at %s+0x%x", + WARN("can't find reachable insn at %s+0x%lx", reloc->sym->sec->name, reloc->addend); return -1; } } else { - WARN("can't find reachable insn at %s+0x%x", + WARN("can't find reachable insn at %s+0x%lx", reloc->sym->sec->name, reloc->addend); return -1; } --- a/tools/objtool/elf.c +++ b/tools/objtool/elf.c @@ -546,7 +546,7 @@ static struct section *elf_create_reloc_ int reltype); int elf_add_reloc(struct elf *elf, struct section *sec, unsigned long offset, - unsigned int type, struct symbol *sym, int addend) + unsigned int type, struct symbol *sym, long addend) { struct reloc *reloc; --- a/tools/objtool/include/objtool/elf.h +++ b/tools/objtool/include/objtool/elf.h @@ -73,7 +73,7 @@ struct reloc { struct symbol *sym; unsigned long offset; unsigned int type; - int addend; + long addend; int idx; bool jump_table_start; }; @@ -135,7 +135,7 @@ struct elf *elf_open_read(const char *na struct section *elf_create_section(struct elf *elf, const char *name, unsigned int sh_flags, size_t entsize, int nr); int elf_add_reloc(struct elf *elf, struct section *sec, unsigned long offset, - unsigned int type, struct symbol *sym, int addend); + unsigned int type, struct symbol *sym, long addend); int elf_add_reloc_to_insn(struct elf *elf, struct section *sec, unsigned long offset, unsigned int type, struct section *insn_sec, unsigned long insn_off); ^ permalink raw reply [flat|nested] 8+ messages in thread
* [tip: objtool/urgent] objtool: Fix type of reloc::addend 2022-04-19 20:32 ` [PATCH 1/2] objtool: Fix type of reloc::addend Peter Zijlstra @ 2022-04-22 10:27 ` tip-bot2 for Peter Zijlstra 0 siblings, 0 replies; 8+ messages in thread From: tip-bot2 for Peter Zijlstra @ 2022-04-22 10:27 UTC (permalink / raw) To: linux-tip-commits Cc: Peter Zijlstra (Intel), Josh Poimboeuf, x86, linux-kernel The following commit has been merged into the objtool/urgent branch of tip: Commit-ID: c087c6e7b551b7f208c0b852304f044954cf2bb3 Gitweb: https://git.kernel.org/tip/c087c6e7b551b7f208c0b852304f044954cf2bb3 Author: Peter Zijlstra <peterz@infradead.org> AuthorDate: Sun, 17 Apr 2022 17:03:40 +02:00 Committer: Peter Zijlstra <peterz@infradead.org> CommitterDate: Fri, 22 Apr 2022 12:13:55 +02:00 objtool: Fix type of reloc::addend Elf{32,64}_Rela::r_addend is of type: Elf{32,64}_Sword, that means that our reloc::addend needs to be long or face tuncation issues when we do elf_rebuild_reloc_section(): - 107: 48 b8 00 00 00 00 00 00 00 00 movabs $0x0,%rax 109: R_X86_64_64 level4_kernel_pgt+0x80000067 + 107: 48 b8 00 00 00 00 00 00 00 00 movabs $0x0,%rax 109: R_X86_64_64 level4_kernel_pgt-0x7fffff99 Fixes: 627fce14809b ("objtool: Add ORC unwind table generation") Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Acked-by: Josh Poimboeuf <jpoimboe@redhat.com> Link: https://lkml.kernel.org/r/20220419203807.596871927@infradead.org --- tools/objtool/check.c | 8 ++++---- tools/objtool/elf.c | 2 +- tools/objtool/include/objtool/elf.h | 4 ++-- 3 files changed, 7 insertions(+), 7 deletions(-) diff --git a/tools/objtool/check.c b/tools/objtool/check.c index 5f10653..3f67854 100644 --- a/tools/objtool/check.c +++ b/tools/objtool/check.c @@ -559,12 +559,12 @@ static int add_dead_ends(struct objtool_file *file) else if (reloc->addend == reloc->sym->sec->sh.sh_size) { insn = find_last_insn(file, reloc->sym->sec); if (!insn) { - WARN("can't find unreachable insn at %s+0x%x", + WARN("can't find unreachable insn at %s+0x%lx", reloc->sym->sec->name, reloc->addend); return -1; } } else { - WARN("can't find unreachable insn at %s+0x%x", + WARN("can't find unreachable insn at %s+0x%lx", reloc->sym->sec->name, reloc->addend); return -1; } @@ -594,12 +594,12 @@ reachable: else if (reloc->addend == reloc->sym->sec->sh.sh_size) { insn = find_last_insn(file, reloc->sym->sec); if (!insn) { - WARN("can't find reachable insn at %s+0x%x", + WARN("can't find reachable insn at %s+0x%lx", reloc->sym->sec->name, reloc->addend); return -1; } } else { - WARN("can't find reachable insn at %s+0x%x", + WARN("can't find reachable insn at %s+0x%lx", reloc->sym->sec->name, reloc->addend); return -1; } diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c index d7b99a7..0cfe84a 100644 --- a/tools/objtool/elf.c +++ b/tools/objtool/elf.c @@ -546,7 +546,7 @@ static struct section *elf_create_reloc_section(struct elf *elf, int reltype); int elf_add_reloc(struct elf *elf, struct section *sec, unsigned long offset, - unsigned int type, struct symbol *sym, int addend) + unsigned int type, struct symbol *sym, long addend) { struct reloc *reloc; diff --git a/tools/objtool/include/objtool/elf.h b/tools/objtool/include/objtool/elf.h index 22ba7e2..9b36802 100644 --- a/tools/objtool/include/objtool/elf.h +++ b/tools/objtool/include/objtool/elf.h @@ -73,7 +73,7 @@ struct reloc { struct symbol *sym; unsigned long offset; unsigned int type; - int addend; + long addend; int idx; bool jump_table_start; }; @@ -135,7 +135,7 @@ struct elf *elf_open_read(const char *name, int flags); struct section *elf_create_section(struct elf *elf, const char *name, unsigned int sh_flags, size_t entsize, int nr); int elf_add_reloc(struct elf *elf, struct section *sec, unsigned long offset, - unsigned int type, struct symbol *sym, int addend); + unsigned int type, struct symbol *sym, long addend); int elf_add_reloc_to_insn(struct elf *elf, struct section *sec, unsigned long offset, unsigned int type, struct section *insn_sec, unsigned long insn_off); ^ permalink raw reply related [flat|nested] 8+ messages in thread
* [PATCH 2/2] objtool: Fix code relocs vs weak symbols 2022-04-19 20:32 [PATCH 0/2] objtool: Fix code reloc vs weak symbols Peter Zijlstra 2022-04-19 20:32 ` [PATCH 1/2] objtool: Fix type of reloc::addend Peter Zijlstra @ 2022-04-19 20:32 ` Peter Zijlstra 2022-04-20 2:16 ` Josh Poimboeuf 2022-04-22 10:27 ` [tip: objtool/urgent] " tip-bot2 for Peter Zijlstra 2022-04-21 15:48 ` [PATCH 0/2] objtool: Fix code reloc " Josh Poimboeuf 2 siblings, 2 replies; 8+ messages in thread From: Peter Zijlstra @ 2022-04-19 20:32 UTC (permalink / raw) To: x86, jpoimboe; +Cc: linux-kernel, peterz, mbenes Occasionally objtool driven code patching (think .static_call_sites .retpoline_sites etc..) goes sideways and it tries to patch an instruction that doesn't match. Much head-scatching and cursing later the problem is as outlined below and affects every section that objtool generates for us, very much including the ORC data. The below uses .static_call_sites because it's convenient for demonstration purposes, but as mentioned the ORC sections, .retpoline_sites and __mount_loc are all similarly affected. Consider: foo-weak.c: extern void __SCT__foo(void); __attribute__((weak)) void foo(void) { return __SCT__foo(); } foo.c: extern void __SCT__foo(void); extern void my_foo(void); void foo(void) { my_foo(); return __SCT__foo(); } These generate the obvious code (gcc -O2 -fcf-protection=none -fno-asynchronous-unwind-tables -c foo*.c): foo-weak.o: 0000000000000000 <foo>: 0: e9 00 00 00 00 jmpq 5 <foo+0x5> 1: R_X86_64_PLT32 __SCT__foo-0x4 foo.o: 0000000000000000 <foo>: 0: 48 83 ec 08 sub $0x8,%rsp 4: e8 00 00 00 00 callq 9 <foo+0x9> 5: R_X86_64_PLT32 my_foo-0x4 9: 48 83 c4 08 add $0x8,%rsp d: e9 00 00 00 00 jmpq 12 <foo+0x12> e: R_X86_64_PLT32 __SCT__foo-0x4 Now, when we link these two files together, you get something like (ld -r -o foos.o foo-weak.o foo.o): foos.o: 0000000000000000 <foo-0x10>: 0: e9 00 00 00 00 jmpq 5 <foo-0xb> 1: R_X86_64_PLT32 __SCT__foo-0x4 5: 66 2e 0f 1f 84 00 00 00 00 00 nopw %cs:0x0(%rax,%rax,1) f: 90 nop 0000000000000010 <foo>: 10: 48 83 ec 08 sub $0x8,%rsp 14: e8 00 00 00 00 callq 19 <foo+0x9> 15: R_X86_64_PLT32 my_foo-0x4 19: 48 83 c4 08 add $0x8,%rsp 1d: e9 00 00 00 00 jmpq 22 <foo+0x12> 1e: R_X86_64_PLT32 __SCT__foo-0x4 Noting that ld preserves the weak function text, but strips the symbol off of it (hence objdump doing that funny negative offset thing). This does lead to 'interesting' unused code issues with objtool when ran on linked objects, but that seems to be working (fingers crossed). So far so good.. Now lets consider the objtool static_call output section (readelf output, old binutils): foo-weak.o: Relocation section '.rela.static_call_sites' at offset 0x2c8 contains 1 entry: Offset Info Type Symbol's Value Symbol's Name + Addend 0000000000000000 0000000200000002 R_X86_64_PC32 0000000000000000 .text + 0 0000000000000004 0000000d00000002 R_X86_64_PC32 0000000000000000 __SCT__foo + 1 foo.o: Relocation section '.rela.static_call_sites' at offset 0x310 contains 2 entries: Offset Info Type Symbol's Value Symbol's Name + Addend 0000000000000000 0000000200000002 R_X86_64_PC32 0000000000000000 .text + d 0000000000000004 0000000d00000002 R_X86_64_PC32 0000000000000000 __SCT__foo + 1 foos.o: Relocation section '.rela.static_call_sites' at offset 0x430 contains 4 entries: Offset Info Type Symbol's Value Symbol's Name + Addend 0000000000000000 0000000100000002 R_X86_64_PC32 0000000000000000 .text + 0 0000000000000004 0000000d00000002 R_X86_64_PC32 0000000000000000 __SCT__foo + 1 0000000000000008 0000000100000002 R_X86_64_PC32 0000000000000000 .text + 1d 000000000000000c 0000000d00000002 R_X86_64_PC32 0000000000000000 __SCT__foo + 1 So we have two patch sites, one in the dead code of the weak foo and one in the real foo. All is well. *HOWEVER*, when the toolchain strips unused section symbols it generates things like this (using new enough binutils): foo-weak.o: Relocation section '.rela.static_call_sites' at offset 0x2c8 contains 1 entry: Offset Info Type Symbol's Value Symbol's Name + Addend 0000000000000000 0000000200000002 R_X86_64_PC32 0000000000000000 foo + 0 0000000000000004 0000000d00000002 R_X86_64_PC32 0000000000000000 __SCT__foo + 1 foo.o: Relocation section '.rela.static_call_sites' at offset 0x310 contains 2 entries: Offset Info Type Symbol's Value Symbol's Name + Addend 0000000000000000 0000000200000002 R_X86_64_PC32 0000000000000000 foo + d 0000000000000004 0000000d00000002 R_X86_64_PC32 0000000000000000 __SCT__foo + 1 foos.o: Relocation section '.rela.static_call_sites' at offset 0x430 contains 4 entries: Offset Info Type Symbol's Value Symbol's Name + Addend 0000000000000000 0000000100000002 R_X86_64_PC32 0000000000000000 foo + 0 0000000000000004 0000000d00000002 R_X86_64_PC32 0000000000000000 __SCT__foo + 1 0000000000000008 0000000100000002 R_X86_64_PC32 0000000000000000 foo + d 000000000000000c 0000000d00000002 R_X86_64_PC32 0000000000000000 __SCT__foo + 1 And now we can see how that foos.o .static_call_sites goes side-ways, we now have _two_ patch sites in foo. One for the weak symbol at foo+0 (which is no longer a static_call site!) and one at foo+d which is in fact the right location. This seems to happen when objtool cannot find a section symbol, in which case it falls back to any other symbol to key off of, however in this case that goes terribly wrong! As such, teach objtool to create a section symbol when there isn't one. Fixes: 44f6a7c0755d ("objtool: Fix seg fault with Clang non-section symbols") Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> --- tools/objtool/elf.c | 188 +++++++++++++++++++++++++++++++++++++++++++++------- 1 file changed, 166 insertions(+), 22 deletions(-) --- a/tools/objtool/elf.c +++ b/tools/objtool/elf.c @@ -575,37 +575,181 @@ int elf_add_reloc(struct elf *elf, struc return 0; } -int elf_add_reloc_to_insn(struct elf *elf, struct section *sec, - unsigned long offset, unsigned int type, - struct section *insn_sec, unsigned long insn_off) +/* + * Ensure that any reloc section containing references to @sym is marked + * changed such that it will get re-generated in elf_rebuild_reloc_sections() + * with the new symbol index. + */ +static void elf_dirty_reloc_sym(struct elf *elf, struct symbol *sym) +{ + struct section *sec; + + list_for_each_entry(sec, &elf->sections, list) { + struct reloc *reloc; + + if (sec->changed) + continue; + + list_for_each_entry(reloc, &sec->reloc_list, list) { + if (reloc->sym == sym) { + sec->changed = true; + break; + } + } + } +} + +/* + * Move the first global symbol, as per sh_info, into a new, higher symbol + * index. This fees up the shndx for a new local symbol. + */ +static int elf_move_global_symbol(struct elf *elf, struct section *symtab, + struct section *symtab_shndx) { + Elf_Data *data, *shndx_data = NULL; + Elf32_Word first_non_local; struct symbol *sym; - int addend; + Elf_Scn *s; - if (insn_sec->sym) { - sym = insn_sec->sym; - addend = insn_off; + first_non_local = symtab->sh.sh_info; - } else { - /* - * The Clang assembler strips section symbols, so we have to - * reference the function symbol instead: - */ - sym = find_symbol_containing(insn_sec, insn_off); - if (!sym) { - /* - * Hack alert. This happens when we need to reference - * the NOP pad insn immediately after the function. - */ - sym = find_symbol_containing(insn_sec, insn_off - 1); + sym = find_symbol_by_index(elf, first_non_local); + if (!sym) { + WARN("no non-local symbols !?"); + return first_non_local; + } + + s = elf_getscn(elf->elf, symtab->idx); + if (!s) { + WARN_ELF("elf_getscn"); + return -1; + } + + data = elf_newdata(s); + if (!data) { + WARN_ELF("elf_newdata"); + return -1; + } + + data->d_buf = &sym->sym; + data->d_size = sizeof(sym->sym); + data->d_align = 1; + data->d_type = ELF_T_SYM; + + sym->idx = symtab->sh.sh_size / sizeof(sym->sym); + elf_dirty_reloc_sym(elf, sym); + + symtab->sh.sh_info += 1; + symtab->sh.sh_size += data->d_size; + symtab->changed = true; + + /* XXX create ? */ + if (symtab_shndx) { + s = elf_getscn(elf->elf, symtab_shndx->idx); + if (!s) { + WARN_ELF("elf_getscn"); + return -1; } - if (!sym) { - WARN("can't find symbol containing %s+0x%lx", insn_sec->name, insn_off); + shndx_data = elf_newdata(s); + if (!shndx_data) { + WARN_ELF("elf_newshndx_data"); return -1; } - addend = insn_off - sym->offset; + shndx_data->d_buf = &sym->sec->idx; + shndx_data->d_size = sizeof(Elf32_Word); + shndx_data->d_align = 4; + shndx_data->d_type = ELF_T_WORD; + + symtab_shndx->sh.sh_size += 4; + symtab_shndx->changed = true; + } + + return first_non_local; +} + +static struct symbol * +elf_create_section_symbol(struct elf *elf, struct section *sec) +{ + struct section *symtab, *symtab_shndx; + Elf_Data *shndx_data = NULL; + struct symbol *sym; + Elf32_Word shndx; + + symtab = find_section_by_name(elf, ".symtab"); + if (symtab) { + symtab_shndx = find_section_by_name(elf, ".symtab_shndx"); + if (symtab_shndx) + shndx_data = symtab_shndx->data; + } else { + WARN("no .symtab"); + return NULL; + } + + sym = malloc(sizeof(*sym)); + if (!sym) { + perror("malloc"); + return NULL; + } + memset(sym, 0, sizeof(*sym)); + + sym->idx = elf_move_global_symbol(elf, symtab, symtab_shndx); + if (sym->idx < 0) { + WARN("elf_move_global_symbol"); + return NULL; + } + + sym->name = sec->name; + sym->sec = sec; + + // st_name 0 + sym->sym.st_info = GELF_ST_INFO(STB_LOCAL, STT_SECTION); + // st_other 0 + // st_value 0 + // st_size 0 + shndx = sec->idx; + if (shndx >= SHN_UNDEF && shndx < SHN_LORESERVE) { + sym->sym.st_shndx = shndx; + if (!shndx_data) + shndx = 0; + } else { + sym->sym.st_shndx = SHN_XINDEX; + if (!shndx_data) { + WARN("no .symtab_shndx"); + return NULL; + } + } + + if (!gelf_update_symshndx(symtab->data, shndx_data, sym->idx, &sym->sym, shndx)) { + WARN_ELF("gelf_update_symshndx"); + return NULL; + } + + elf_add_symbol(elf, sym); + + return sym; +} + +int elf_add_reloc_to_insn(struct elf *elf, struct section *sec, + unsigned long offset, unsigned int type, + struct section *insn_sec, unsigned long insn_off) +{ + struct symbol *sym = insn_sec->sym; + int addend = insn_off; + + if (!sym) { + /* + * Due to how weak functions work, we must use section based + * relocations. Symbol based relocations would result in the + * weak and non-weak function annotations being overlaid on the + * non-weak function after linking. + */ + sym = elf_create_section_symbol(elf, insn_sec); + if (!sym) + return -1; + + insn_sec->sym = sym; } return elf_add_reloc(elf, sec, offset, type, sym, addend); ^ permalink raw reply [flat|nested] 8+ messages in thread
* Re: [PATCH 2/2] objtool: Fix code relocs vs weak symbols 2022-04-19 20:32 ` [PATCH 2/2] objtool: Fix code relocs vs weak symbols Peter Zijlstra @ 2022-04-20 2:16 ` Josh Poimboeuf 2022-04-20 6:57 ` Peter Zijlstra 2022-04-22 10:27 ` [tip: objtool/urgent] " tip-bot2 for Peter Zijlstra 1 sibling, 1 reply; 8+ messages in thread From: Josh Poimboeuf @ 2022-04-20 2:16 UTC (permalink / raw) To: Peter Zijlstra; +Cc: x86, linux-kernel, mbenes On Tue, Apr 19, 2022 at 10:32:56PM +0200, Peter Zijlstra wrote: > + /* XXX create ? */ > + if (symtab_shndx) { Not sure what this comment means? Otherwise both patches look good to me. Let me run it through some tests. -- Josh ^ permalink raw reply [flat|nested] 8+ messages in thread
* Re: [PATCH 2/2] objtool: Fix code relocs vs weak symbols 2022-04-20 2:16 ` Josh Poimboeuf @ 2022-04-20 6:57 ` Peter Zijlstra 0 siblings, 0 replies; 8+ messages in thread From: Peter Zijlstra @ 2022-04-20 6:57 UTC (permalink / raw) To: Josh Poimboeuf; +Cc: x86, linux-kernel, mbenes On Tue, Apr 19, 2022 at 07:16:01PM -0700, Josh Poimboeuf wrote: > On Tue, Apr 19, 2022 at 10:32:56PM +0200, Peter Zijlstra wrote: > > + /* XXX create ? */ > > + if (symtab_shndx) { > > Not sure what this comment means? I'm not sure if we will trip the case where the input ELF will not have/need the extended table, but our output file would need it. That said, upon thinking about it this morning, it wouldn't be this code path triggering it I think. Specifically, we only create section symbols for text sections and objtool doesn't generate text sections (you've talked me out of that). So the number of text sections doesn't change, so section index number shouldn't increase due to all this. ^ permalink raw reply [flat|nested] 8+ messages in thread
* [tip: objtool/urgent] objtool: Fix code relocs vs weak symbols 2022-04-19 20:32 ` [PATCH 2/2] objtool: Fix code relocs vs weak symbols Peter Zijlstra 2022-04-20 2:16 ` Josh Poimboeuf @ 2022-04-22 10:27 ` tip-bot2 for Peter Zijlstra 1 sibling, 0 replies; 8+ messages in thread From: tip-bot2 for Peter Zijlstra @ 2022-04-22 10:27 UTC (permalink / raw) To: linux-tip-commits Cc: Peter Zijlstra (Intel), Josh Poimboeuf, x86, linux-kernel The following commit has been merged into the objtool/urgent branch of tip: Commit-ID: 4abff6d48dbcea8200c7ea35ba70c242d128ebf3 Gitweb: https://git.kernel.org/tip/4abff6d48dbcea8200c7ea35ba70c242d128ebf3 Author: Peter Zijlstra <peterz@infradead.org> AuthorDate: Sun, 17 Apr 2022 17:03:36 +02:00 Committer: Peter Zijlstra <peterz@infradead.org> CommitterDate: Fri, 22 Apr 2022 12:13:55 +02:00 objtool: Fix code relocs vs weak symbols Occasionally objtool driven code patching (think .static_call_sites .retpoline_sites etc..) goes sideways and it tries to patch an instruction that doesn't match. Much head-scatching and cursing later the problem is as outlined below and affects every section that objtool generates for us, very much including the ORC data. The below uses .static_call_sites because it's convenient for demonstration purposes, but as mentioned the ORC sections, .retpoline_sites and __mount_loc are all similarly affected. Consider: foo-weak.c: extern void __SCT__foo(void); __attribute__((weak)) void foo(void) { return __SCT__foo(); } foo.c: extern void __SCT__foo(void); extern void my_foo(void); void foo(void) { my_foo(); return __SCT__foo(); } These generate the obvious code (gcc -O2 -fcf-protection=none -fno-asynchronous-unwind-tables -c foo*.c): foo-weak.o: 0000000000000000 <foo>: 0: e9 00 00 00 00 jmpq 5 <foo+0x5> 1: R_X86_64_PLT32 __SCT__foo-0x4 foo.o: 0000000000000000 <foo>: 0: 48 83 ec 08 sub $0x8,%rsp 4: e8 00 00 00 00 callq 9 <foo+0x9> 5: R_X86_64_PLT32 my_foo-0x4 9: 48 83 c4 08 add $0x8,%rsp d: e9 00 00 00 00 jmpq 12 <foo+0x12> e: R_X86_64_PLT32 __SCT__foo-0x4 Now, when we link these two files together, you get something like (ld -r -o foos.o foo-weak.o foo.o): foos.o: 0000000000000000 <foo-0x10>: 0: e9 00 00 00 00 jmpq 5 <foo-0xb> 1: R_X86_64_PLT32 __SCT__foo-0x4 5: 66 2e 0f 1f 84 00 00 00 00 00 nopw %cs:0x0(%rax,%rax,1) f: 90 nop 0000000000000010 <foo>: 10: 48 83 ec 08 sub $0x8,%rsp 14: e8 00 00 00 00 callq 19 <foo+0x9> 15: R_X86_64_PLT32 my_foo-0x4 19: 48 83 c4 08 add $0x8,%rsp 1d: e9 00 00 00 00 jmpq 22 <foo+0x12> 1e: R_X86_64_PLT32 __SCT__foo-0x4 Noting that ld preserves the weak function text, but strips the symbol off of it (hence objdump doing that funny negative offset thing). This does lead to 'interesting' unused code issues with objtool when ran on linked objects, but that seems to be working (fingers crossed). So far so good.. Now lets consider the objtool static_call output section (readelf output, old binutils): foo-weak.o: Relocation section '.rela.static_call_sites' at offset 0x2c8 contains 1 entry: Offset Info Type Symbol's Value Symbol's Name + Addend 0000000000000000 0000000200000002 R_X86_64_PC32 0000000000000000 .text + 0 0000000000000004 0000000d00000002 R_X86_64_PC32 0000000000000000 __SCT__foo + 1 foo.o: Relocation section '.rela.static_call_sites' at offset 0x310 contains 2 entries: Offset Info Type Symbol's Value Symbol's Name + Addend 0000000000000000 0000000200000002 R_X86_64_PC32 0000000000000000 .text + d 0000000000000004 0000000d00000002 R_X86_64_PC32 0000000000000000 __SCT__foo + 1 foos.o: Relocation section '.rela.static_call_sites' at offset 0x430 contains 4 entries: Offset Info Type Symbol's Value Symbol's Name + Addend 0000000000000000 0000000100000002 R_X86_64_PC32 0000000000000000 .text + 0 0000000000000004 0000000d00000002 R_X86_64_PC32 0000000000000000 __SCT__foo + 1 0000000000000008 0000000100000002 R_X86_64_PC32 0000000000000000 .text + 1d 000000000000000c 0000000d00000002 R_X86_64_PC32 0000000000000000 __SCT__foo + 1 So we have two patch sites, one in the dead code of the weak foo and one in the real foo. All is well. *HOWEVER*, when the toolchain strips unused section symbols it generates things like this (using new enough binutils): foo-weak.o: Relocation section '.rela.static_call_sites' at offset 0x2c8 contains 1 entry: Offset Info Type Symbol's Value Symbol's Name + Addend 0000000000000000 0000000200000002 R_X86_64_PC32 0000000000000000 foo + 0 0000000000000004 0000000d00000002 R_X86_64_PC32 0000000000000000 __SCT__foo + 1 foo.o: Relocation section '.rela.static_call_sites' at offset 0x310 contains 2 entries: Offset Info Type Symbol's Value Symbol's Name + Addend 0000000000000000 0000000200000002 R_X86_64_PC32 0000000000000000 foo + d 0000000000000004 0000000d00000002 R_X86_64_PC32 0000000000000000 __SCT__foo + 1 foos.o: Relocation section '.rela.static_call_sites' at offset 0x430 contains 4 entries: Offset Info Type Symbol's Value Symbol's Name + Addend 0000000000000000 0000000100000002 R_X86_64_PC32 0000000000000000 foo + 0 0000000000000004 0000000d00000002 R_X86_64_PC32 0000000000000000 __SCT__foo + 1 0000000000000008 0000000100000002 R_X86_64_PC32 0000000000000000 foo + d 000000000000000c 0000000d00000002 R_X86_64_PC32 0000000000000000 __SCT__foo + 1 And now we can see how that foos.o .static_call_sites goes side-ways, we now have _two_ patch sites in foo. One for the weak symbol at foo+0 (which is no longer a static_call site!) and one at foo+d which is in fact the right location. This seems to happen when objtool cannot find a section symbol, in which case it falls back to any other symbol to key off of, however in this case that goes terribly wrong! As such, teach objtool to create a section symbol when there isn't one. Fixes: 44f6a7c0755d ("objtool: Fix seg fault with Clang non-section symbols") Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Acked-by: Josh Poimboeuf <jpoimboe@redhat.com> Link: https://lkml.kernel.org/r/20220419203807.655552918@infradead.org --- tools/objtool/elf.c | 187 +++++++++++++++++++++++++++++++++++++------ 1 file changed, 165 insertions(+), 22 deletions(-) diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c index 0cfe84a..ebf2ba5 100644 --- a/tools/objtool/elf.c +++ b/tools/objtool/elf.c @@ -575,37 +575,180 @@ int elf_add_reloc(struct elf *elf, struct section *sec, unsigned long offset, return 0; } -int elf_add_reloc_to_insn(struct elf *elf, struct section *sec, - unsigned long offset, unsigned int type, - struct section *insn_sec, unsigned long insn_off) +/* + * Ensure that any reloc section containing references to @sym is marked + * changed such that it will get re-generated in elf_rebuild_reloc_sections() + * with the new symbol index. + */ +static void elf_dirty_reloc_sym(struct elf *elf, struct symbol *sym) +{ + struct section *sec; + + list_for_each_entry(sec, &elf->sections, list) { + struct reloc *reloc; + + if (sec->changed) + continue; + + list_for_each_entry(reloc, &sec->reloc_list, list) { + if (reloc->sym == sym) { + sec->changed = true; + break; + } + } + } +} + +/* + * Move the first global symbol, as per sh_info, into a new, higher symbol + * index. This fees up the shndx for a new local symbol. + */ +static int elf_move_global_symbol(struct elf *elf, struct section *symtab, + struct section *symtab_shndx) { + Elf_Data *data, *shndx_data = NULL; + Elf32_Word first_non_local; struct symbol *sym; - int addend; + Elf_Scn *s; - if (insn_sec->sym) { - sym = insn_sec->sym; - addend = insn_off; + first_non_local = symtab->sh.sh_info; - } else { - /* - * The Clang assembler strips section symbols, so we have to - * reference the function symbol instead: - */ - sym = find_symbol_containing(insn_sec, insn_off); - if (!sym) { - /* - * Hack alert. This happens when we need to reference - * the NOP pad insn immediately after the function. - */ - sym = find_symbol_containing(insn_sec, insn_off - 1); + sym = find_symbol_by_index(elf, first_non_local); + if (!sym) { + WARN("no non-local symbols !?"); + return first_non_local; + } + + s = elf_getscn(elf->elf, symtab->idx); + if (!s) { + WARN_ELF("elf_getscn"); + return -1; + } + + data = elf_newdata(s); + if (!data) { + WARN_ELF("elf_newdata"); + return -1; + } + + data->d_buf = &sym->sym; + data->d_size = sizeof(sym->sym); + data->d_align = 1; + data->d_type = ELF_T_SYM; + + sym->idx = symtab->sh.sh_size / sizeof(sym->sym); + elf_dirty_reloc_sym(elf, sym); + + symtab->sh.sh_info += 1; + symtab->sh.sh_size += data->d_size; + symtab->changed = true; + + if (symtab_shndx) { + s = elf_getscn(elf->elf, symtab_shndx->idx); + if (!s) { + WARN_ELF("elf_getscn"); + return -1; } - if (!sym) { - WARN("can't find symbol containing %s+0x%lx", insn_sec->name, insn_off); + shndx_data = elf_newdata(s); + if (!shndx_data) { + WARN_ELF("elf_newshndx_data"); return -1; } - addend = insn_off - sym->offset; + shndx_data->d_buf = &sym->sec->idx; + shndx_data->d_size = sizeof(Elf32_Word); + shndx_data->d_align = 4; + shndx_data->d_type = ELF_T_WORD; + + symtab_shndx->sh.sh_size += 4; + symtab_shndx->changed = true; + } + + return first_non_local; +} + +static struct symbol * +elf_create_section_symbol(struct elf *elf, struct section *sec) +{ + struct section *symtab, *symtab_shndx; + Elf_Data *shndx_data = NULL; + struct symbol *sym; + Elf32_Word shndx; + + symtab = find_section_by_name(elf, ".symtab"); + if (symtab) { + symtab_shndx = find_section_by_name(elf, ".symtab_shndx"); + if (symtab_shndx) + shndx_data = symtab_shndx->data; + } else { + WARN("no .symtab"); + return NULL; + } + + sym = malloc(sizeof(*sym)); + if (!sym) { + perror("malloc"); + return NULL; + } + memset(sym, 0, sizeof(*sym)); + + sym->idx = elf_move_global_symbol(elf, symtab, symtab_shndx); + if (sym->idx < 0) { + WARN("elf_move_global_symbol"); + return NULL; + } + + sym->name = sec->name; + sym->sec = sec; + + // st_name 0 + sym->sym.st_info = GELF_ST_INFO(STB_LOCAL, STT_SECTION); + // st_other 0 + // st_value 0 + // st_size 0 + shndx = sec->idx; + if (shndx >= SHN_UNDEF && shndx < SHN_LORESERVE) { + sym->sym.st_shndx = shndx; + if (!shndx_data) + shndx = 0; + } else { + sym->sym.st_shndx = SHN_XINDEX; + if (!shndx_data) { + WARN("no .symtab_shndx"); + return NULL; + } + } + + if (!gelf_update_symshndx(symtab->data, shndx_data, sym->idx, &sym->sym, shndx)) { + WARN_ELF("gelf_update_symshndx"); + return NULL; + } + + elf_add_symbol(elf, sym); + + return sym; +} + +int elf_add_reloc_to_insn(struct elf *elf, struct section *sec, + unsigned long offset, unsigned int type, + struct section *insn_sec, unsigned long insn_off) +{ + struct symbol *sym = insn_sec->sym; + int addend = insn_off; + + if (!sym) { + /* + * Due to how weak functions work, we must use section based + * relocations. Symbol based relocations would result in the + * weak and non-weak function annotations being overlaid on the + * non-weak function after linking. + */ + sym = elf_create_section_symbol(elf, insn_sec); + if (!sym) + return -1; + + insn_sec->sym = sym; } return elf_add_reloc(elf, sec, offset, type, sym, addend); ^ permalink raw reply related [flat|nested] 8+ messages in thread
* Re: [PATCH 0/2] objtool: Fix code reloc vs weak symbols 2022-04-19 20:32 [PATCH 0/2] objtool: Fix code reloc vs weak symbols Peter Zijlstra 2022-04-19 20:32 ` [PATCH 1/2] objtool: Fix type of reloc::addend Peter Zijlstra 2022-04-19 20:32 ` [PATCH 2/2] objtool: Fix code relocs vs weak symbols Peter Zijlstra @ 2022-04-21 15:48 ` Josh Poimboeuf 2 siblings, 0 replies; 8+ messages in thread From: Josh Poimboeuf @ 2022-04-21 15:48 UTC (permalink / raw) To: Peter Zijlstra; +Cc: x86, linux-kernel, mbenes On Tue, Apr 19, 2022 at 10:32:54PM +0200, Peter Zijlstra wrote: > Two fun bugs that make can cause other fun fail if you have a fresh enough > toolchain that strips unused section symbols. > > The symptoms can vary between a faulty unwind to a kernel splat for trying to > patch the wrong text. > > I found it while playing with text patching and Josh confirmed it very much > affects ORC data too. Seems to work ok with testing. With the XXX comment removed or fixed: Acked-by: Josh Poimboeuf <jpoimboe@redhat.com> -- Josh ^ permalink raw reply [flat|nested] 8+ messages in thread
end of thread, other threads:[~2022-04-22 10:28 UTC | newest] Thread overview: 8+ messages (download: mbox.gz / follow: Atom feed) -- links below jump to the message on this page -- 2022-04-19 20:32 [PATCH 0/2] objtool: Fix code reloc vs weak symbols Peter Zijlstra 2022-04-19 20:32 ` [PATCH 1/2] objtool: Fix type of reloc::addend Peter Zijlstra 2022-04-22 10:27 ` [tip: objtool/urgent] " tip-bot2 for Peter Zijlstra 2022-04-19 20:32 ` [PATCH 2/2] objtool: Fix code relocs vs weak symbols Peter Zijlstra 2022-04-20 2:16 ` Josh Poimboeuf 2022-04-20 6:57 ` Peter Zijlstra 2022-04-22 10:27 ` [tip: objtool/urgent] " tip-bot2 for Peter Zijlstra 2022-04-21 15:48 ` [PATCH 0/2] objtool: Fix code reloc " Josh Poimboeuf
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