On Thu, Apr 23, 2026 at 08:38:02PM -0700, Josh Poimboeuf wrote: > I discovered it's not just FineIBT, it's basically any CALL_PADDING+CFI, > like so:
Indeed. This looks good, thanks! > From: Josh Poimboeuf <[email protected]> > Subject: [PATCH] objtool: Grow __cfi_* symbols for all kCFI+CALL_PADDING > > For all CONFIG_CFI+CONFIG_CALL_PADDING configs, the __cfi_ symbols only > cover the 5-byte kCFI type hash. After that there also N bytes of NOP > padding between the hash and the function entry which aren't associated > with any symbol. > > The NOPs can be replaced with actual code at runtime. Without a symbol, > unwinders and tooling have no way of knowing where those bytes belong. > > Grow the existing __cfi_* symbols to fill that gap. > > Also, CONFIG_PREFIX_SYMBOLS has no reason to exist: CONFIG_CALL_PADDING > is what causes the compiler to emit NOP padding before function entry > (via -fpatchable-function-entry), so it's the right condition for > creating prefix symbols. > > Remove CONFIG_PREFIX_SYMBOLS, as it's no longer needed. Simplify the > LONGEST_SYM_KUNIT_TEST dependency accordingly. > > Update the --cfi and --prefix usage strings to reflect their current > scope. > > Signed-off-by: Josh Poimboeuf <[email protected]> > --- > arch/x86/Kconfig | 4 ---- > lib/Kconfig.debug | 2 +- > scripts/Makefile.lib | 5 ++++- > tools/objtool/builtin-check.c | 9 +++++++-- > tools/objtool/check.c | 13 ++++++++++++- > tools/objtool/elf.c | 20 ++++++++++++++++++++ > tools/objtool/include/objtool/elf.h | 1 + > 7 files changed, 45 insertions(+), 9 deletions(-) > > diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig > index f3f7cb01d69d..3eb3c48d764a 100644 > --- a/arch/x86/Kconfig > +++ b/arch/x86/Kconfig > @@ -2437,10 +2437,6 @@ config CALL_THUNKS > def_bool n > select CALL_PADDING > > -config PREFIX_SYMBOLS > - def_bool y > - depends on CALL_PADDING && !CFI > - > menuconfig CPU_MITIGATIONS > bool "Mitigations for CPU vulnerabilities" > default y > diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug > index 77c3774c1c49..8b41720069b3 100644 > --- a/lib/Kconfig.debug > +++ b/lib/Kconfig.debug > @@ -3059,7 +3059,7 @@ config FORTIFY_KUNIT_TEST > config LONGEST_SYM_KUNIT_TEST > tristate "Test the longest symbol possible" if !KUNIT_ALL_TESTS > depends on KUNIT && KPROBES > - depends on !PREFIX_SYMBOLS && !CFI && !GCOV_KERNEL > + depends on !CALL_PADDING && !GCOV_KERNEL > default KUNIT_ALL_TESTS > help > Tests the longest symbol possible > diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib > index 0718e39cedda..562d89f051f0 100644 > --- a/scripts/Makefile.lib > +++ b/scripts/Makefile.lib > @@ -187,7 +187,10 @@ objtool-args-$(CONFIG_HAVE_JUMP_LABEL_HACK) > += --hacks=jump_label > objtool-args-$(CONFIG_HAVE_NOINSTR_HACK) += --hacks=noinstr > objtool-args-$(CONFIG_MITIGATION_CALL_DEPTH_TRACKING) += > --hacks=skylake > objtool-args-$(CONFIG_X86_KERNEL_IBT) += --ibt > -objtool-args-$(CONFIG_FINEIBT) += --cfi > +objtool-args-$(CONFIG_CALL_PADDING) += > --prefix=$(CONFIG_FUNCTION_PADDING_BYTES) > +ifdef CONFIG_CFI > +objtool-args-$(CONFIG_CALL_PADDING) += --cfi > +endif > objtool-args-$(CONFIG_FTRACE_MCOUNT_USE_OBJTOOL) += --mcount > ifdef CONFIG_FTRACE_MCOUNT_USE_OBJTOOL > objtool-args-$(CONFIG_HAVE_OBJTOOL_NOP_MCOUNT) += --mnop > diff --git a/tools/objtool/builtin-check.c b/tools/objtool/builtin-check.c > index ec7f10a5ef19..254ceb6b0e2c 100644 > --- a/tools/objtool/builtin-check.c > +++ b/tools/objtool/builtin-check.c > @@ -73,7 +73,6 @@ static int parse_hacks(const struct option *opt, const char > *str, int unset) > > static const struct option check_options[] = { > OPT_GROUP("Actions:"), > - OPT_BOOLEAN(0, "cfi", &opts.cfi, "annotate kernel control > flow integrity (kCFI) function preambles"), > OPT_STRING_OPTARG('d', "disas", &opts.disas, "function-pattern", > "disassemble functions", "*"), > OPT_CALLBACK_OPTARG('h', "hacks", NULL, NULL, > "jump_label,noinstr,skylake", "patch toolchain bugs/limitations", > parse_hacks), > OPT_BOOLEAN('i', "ibt", &opts.ibt, "validate and annotate IBT"), > @@ -84,7 +83,7 @@ static const struct option check_options[] = { > OPT_BOOLEAN('r', "retpoline", &opts.retpoline, "validate and > annotate retpoline usage"), > OPT_BOOLEAN(0, "rethunk", &opts.rethunk, "validate and > annotate rethunk usage"), > OPT_BOOLEAN(0, "unret", &opts.unret, "validate entry unret > placement"), > - OPT_INTEGER(0, "prefix", &opts.prefix, "generate prefix > symbols"), > + OPT_INTEGER(0, "prefix", &opts.prefix, "generate or grow > prefix symbols for N-byte function padding"), > OPT_BOOLEAN('l', "sls", &opts.sls, "validate > straight-line-speculation mitigations"), > OPT_BOOLEAN('s', "stackval", &opts.stackval, "validate frame > pointer rules"), > OPT_BOOLEAN('t', "static-call", &opts.static_call, "annotate > static calls"), > @@ -92,6 +91,7 @@ static const struct option check_options[] = { > OPT_CALLBACK_OPTARG(0, "dump", NULL, NULL, "orc", "dump metadata", > parse_dump), > > OPT_GROUP("Options:"), > + OPT_BOOLEAN(0, "cfi", &opts.cfi, "annotate and grow kCFI > preamble symbols (use with --prefix)"), > OPT_BOOLEAN(0, "backtrace", &opts.backtrace, "unwind on > error"), > OPT_BOOLEAN(0, "backup", &opts.backup, "create backup (.orig) > file on warning/error"), > OPT_BOOLEAN(0, "dry-run", &opts.dryrun, "don't write > modifications"), > @@ -163,6 +163,11 @@ static bool opts_valid(void) > return false; > } > > + if (opts.cfi && !opts.prefix) { > + ERROR("--cfi requires --prefix"); > + return false; > + } > + > if (opts.disas || > opts.hack_jump_label || > opts.hack_noinstr || > diff --git a/tools/objtool/check.c b/tools/objtool/check.c > index 410061aeed26..fb24fd284e09 100644 > --- a/tools/objtool/check.c > +++ b/tools/objtool/check.c > @@ -923,6 +923,17 @@ static int create_cfi_sections(struct objtool_file *file) > return -1; > > idx++; > + > + /* > + * Grow the __cfi_ symbol to fill the NOP gap between the > + * 'mov <hash>, %rax' and the start of the function. > + */ > + if (sym->len == 5) { > + sym->len += opts.prefix; > + sym->sym.st_size = sym->len; > + if (elf_write_symbol(file->elf, sym)) > + return -1; > + } > } > > return 0; > @@ -4927,7 +4938,7 @@ int check(struct objtool_file *file) > goto out; > } > > - if (opts.prefix) { > + if (opts.prefix && !opts.cfi) { > ret = create_prefix_symbols(file); > if (ret) > goto out; > diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c > index 2ca1151de815..ede87dd9644c 100644 > --- a/tools/objtool/elf.c > +++ b/tools/objtool/elf.c > @@ -983,6 +983,26 @@ struct symbol *elf_create_symbol(struct elf *elf, const > char *name, > return sym; > } > > +int elf_write_symbol(struct elf *elf, struct symbol *sym) > +{ > + struct section *symtab, *symtab_shndx; > + > + symtab = find_section_by_name(elf, ".symtab"); > + if (!symtab) { > + ERROR("no .symtab"); > + return -1; > + } > + > + symtab_shndx = find_section_by_name(elf, ".symtab_shndx"); > + > + if (elf_update_symbol(elf, symtab, symtab_shndx, sym)) > + return -1; > + > + mark_sec_changed(elf, symtab, true); > + > + return 0; > +} > + > struct symbol *elf_create_section_symbol(struct elf *elf, struct section > *sec) > { > struct symbol *sym = calloc(1, sizeof(*sym)); > diff --git a/tools/objtool/include/objtool/elf.h > b/tools/objtool/include/objtool/elf.h > index 0fd1a9b563e9..4c8a67a68063 100644 > --- a/tools/objtool/include/objtool/elf.h > +++ b/tools/objtool/include/objtool/elf.h > @@ -199,6 +199,7 @@ struct reloc *elf_init_reloc_data_sym(struct elf *elf, > struct section *sec, > struct symbol *sym, > s64 addend); > > +int elf_write_symbol(struct elf *elf, struct symbol *sym); > int elf_write_insn(struct elf *elf, struct section *sec, unsigned long > offset, > unsigned int len, const char *insn); > > -- > 2.53.0 >
