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Date: Thu, 30 Sep 2021 11:40:32 -0700 From: Nick Desaulniers <ndesaulniers@...gle.com> To: Sami Tolvanen <samitolvanen@...gle.com> Cc: x86@...nel.org, Kees Cook <keescook@...omium.org>, Josh Poimboeuf <jpoimboe@...hat.com>, Peter Zijlstra <peterz@...radead.org>, Nathan Chancellor <nathan@...nel.org>, Sedat Dilek <sedat.dilek@...il.com>, linux-hardening@...r.kernel.org, linux-kernel@...r.kernel.org, llvm@...ts.linux.dev Subject: Re: [PATCH v4 01/15] objtool: Add CONFIG_CFI_CLANG support On Thu, Sep 30, 2021 at 11:05 AM Sami Tolvanen <samitolvanen@...gle.com> wrote: > > With CONFIG_CFI_CLANG, the compiler replaces function references with > references to the CFI jump table, which confuses objtool. This change, > based on Josh's initial patch [1], goes through the list of relocations > and replaces jump table symbols with the actual function symbols. > > [1] https://lore.kernel.org/r/d743f4b36e120c06506567a9f87a062ae03da47f.1611263462.git.jpoimboe@redhat.com/ > > Reported-by: Sedat Dilek <sedat.dilek@...il.com> > Suggested-by: Josh Poimboeuf <jpoimboe@...hat.com> > Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com> > --- > tools/objtool/arch/x86/decode.c | 17 ++++++++++ > tools/objtool/elf.c | 51 ++++++++++++++++++++++++++++ > tools/objtool/include/objtool/arch.h | 3 ++ > tools/objtool/include/objtool/elf.h | 2 +- > 4 files changed, 72 insertions(+), 1 deletion(-) > > diff --git a/tools/objtool/arch/x86/decode.c b/tools/objtool/arch/x86/decode.c > index 3172983bf808..9b043220b0af 100644 > --- a/tools/objtool/arch/x86/decode.c > +++ b/tools/objtool/arch/x86/decode.c > @@ -63,6 +63,23 @@ bool arch_callee_saved_reg(unsigned char reg) > } > } > > +unsigned long arch_cfi_section_reloc_offset(struct reloc *reloc) > +{ > + if (!reloc->addend) > + return 0; > + > + if (reloc->type == R_X86_64_PC32 || reloc->type == R_X86_64_PLT32) > + return reloc->addend + 4; > + > + return reloc->addend; > +} > + > +unsigned long arch_cfi_jump_reloc_offset(unsigned long offset) > +{ > + /* offset to the relocation in a jmp instruction */ Thanks for the added comment. ;) Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com> > + return offset + 1; > +} > + > unsigned long arch_dest_reloc_offset(int addend) > { > return addend + 4; > diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c > index 8676c7598728..05a5f51aad2c 100644 > --- a/tools/objtool/elf.c > +++ b/tools/objtool/elf.c > @@ -18,6 +18,7 @@ > #include <errno.h> > #include <objtool/builtin.h> > > +#include <objtool/arch.h> > #include <objtool/elf.h> > #include <objtool/warn.h> > > @@ -291,6 +292,10 @@ static int read_sections(struct elf *elf) > if (sec->sh.sh_flags & SHF_EXECINSTR) > elf->text_size += sec->len; > > + /* Detect -fsanitize=cfi jump table sections */ > + if (!strncmp(sec->name, ".text..L.cfi.jumptable", 22)) > + sec->cfi_jt = true; > + > list_add_tail(&sec->list, &elf->sections); > elf_hash_add(section, &sec->hash, sec->idx); > elf_hash_add(section_name, &sec->name_hash, str_hash(sec->name)); > @@ -576,6 +581,49 @@ static int read_rela_reloc(struct section *sec, int i, struct reloc *reloc, unsi > return 0; > } > > +/* > + * CONFIG_CFI_CLANG replaces function relocations to refer to an intermediate > + * jump table. Undo the conversion so objtool can make sense of things. > + */ > +static int fix_cfi_relocs(const struct elf *elf) > +{ > + struct section *sec; > + struct reloc *reloc; > + > + list_for_each_entry(sec, &elf->sections, list) { > + list_for_each_entry(reloc, &sec->reloc_list, list) { > + struct reloc *cfi_reloc; > + unsigned long offset; > + > + if (!reloc->sym->sec->cfi_jt) > + continue; > + > + if (reloc->sym->type == STT_SECTION) > + offset = arch_cfi_section_reloc_offset(reloc); > + else > + offset = reloc->sym->offset; > + > + /* > + * The jump table immediately jumps to the actual function, > + * so look up the relocation there. > + */ > + offset = arch_cfi_jump_reloc_offset(offset); > + cfi_reloc = find_reloc_by_dest(elf, reloc->sym->sec, offset); > + > + if (!cfi_reloc || !cfi_reloc->sym) { > + WARN("can't find a CFI jump table relocation at %s+0x%lx", > + reloc->sym->sec->name, offset); > + return -1; > + } > + > + reloc->sym = cfi_reloc->sym; > + reloc->addend = 0; > + } > + } > + > + return 0; > +} > + > static int read_relocs(struct elf *elf) > { > struct section *sec; > @@ -639,6 +687,9 @@ static int read_relocs(struct elf *elf) > tot_reloc += nr_reloc; > } > > + if (fix_cfi_relocs(elf)) > + return -1; > + > if (stats) { > printf("max_reloc: %lu\n", max_reloc); > printf("tot_reloc: %lu\n", tot_reloc); > diff --git a/tools/objtool/include/objtool/arch.h b/tools/objtool/include/objtool/arch.h > index 589ff58426ab..93bde8aaf2e3 100644 > --- a/tools/objtool/include/objtool/arch.h > +++ b/tools/objtool/include/objtool/arch.h > @@ -81,6 +81,9 @@ unsigned long arch_jump_destination(struct instruction *insn); > > unsigned long arch_dest_reloc_offset(int addend); > > +unsigned long arch_cfi_section_reloc_offset(struct reloc *reloc); > +unsigned long arch_cfi_jump_reloc_offset(unsigned long offset); > + > const char *arch_nop_insn(int len); > const char *arch_ret_insn(int len); > > diff --git a/tools/objtool/include/objtool/elf.h b/tools/objtool/include/objtool/elf.h > index c3857fadee7a..e8d838217c77 100644 > --- a/tools/objtool/include/objtool/elf.h > +++ b/tools/objtool/include/objtool/elf.h > @@ -39,7 +39,7 @@ struct section { > char *name; > int idx; > unsigned int len; > - bool changed, text, rodata, noinstr; > + bool changed, text, rodata, noinstr, cfi_jt; > }; > > struct symbol { > -- > 2.33.0.800.g4c38ced690-goog > -- Thanks, ~Nick Desaulniers
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