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objtool: Improve and simplify prefix symbol detection

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Only create prefix symbols for functions that have
__patchable_function_entries entries, since those are the only C
functions where prefix NOPs are intentional.

This both simplifies the detection and makes it more accurate.

Note that assembly functions using SYM_TYPED_FUNC_START() can also have
prefixed NOPs, but that macro already creates their __cfi_ symbols.

Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
This commit is contained in:
Josh Poimboeuf
2026-04-07 20:56:48 -07:00
parent f7ceffd21a
commit fe6a87e0ab
1 changed files with 21 additions and 69 deletions
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90
tools/objtool/check.c
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@@ -4296,17 +4296,6 @@ static bool ignore_unreachable_insn(struct objtool_file *file, struct instructio
* For FineIBT or kCFI, a certain number of bytes preceding the function may be
* NOPs. Those NOPs may be rewritten at runtime and executed, so give them a
* proper function name: __pfx_<func>.
*
* The NOPs may not exist for the following cases:
*
* - compiler cloned functions (*.cold, *.part0, etc)
* - asm functions created with inline asm or without SYM_FUNC_START()
*
* Also, the function may already have a prefix from a previous objtool run
* (livepatch extracted functions, or manually running objtool multiple times).
*
* So return 0 if the NOPs are missing or the function already has a prefix
* symbol.
*/
static int create_prefix_symbol(struct objtool_file *file, struct symbol *func)
{
@@ -4314,10 +4303,6 @@ static int create_prefix_symbol(struct objtool_file *file, struct symbol *func)
char name[SYM_NAME_LEN];
struct cfi_state *cfi;
if (!is_func_sym(func) || is_prefix_func(func) || is_cold_func(func) ||
func->static_call_tramp)
return 0;
if ((strlen(func->name) + sizeof("__pfx_") > SYM_NAME_LEN)) {
WARN("%s: symbol name too long, can't create __pfx_ symbol",
func->name);
@@ -4327,59 +4312,21 @@ static int create_prefix_symbol(struct objtool_file *file, struct symbol *func)
if (snprintf_check(name, SYM_NAME_LEN, "__pfx_%s", func->name))
return -1;
if (file->klp) {
struct symbol *pfx;
pfx = find_symbol_by_offset(func->sec, func->offset - opts.prefix);
if (pfx && is_prefix_func(pfx) && !strcmp(pfx->name, name))
return 0;
}
insn = find_insn(file, func->sec, func->offset);
if (!insn) {
WARN("%s: can't find starting instruction", func->name);
if (!elf_create_symbol(file->elf, name, func->sec,
GELF_ST_BIND(func->sym.st_info),
GELF_ST_TYPE(func->sym.st_info),
func->offset - opts.prefix, opts.prefix))
return -1;
}
for (prev = prev_insn_same_sec(file, insn);
prev;
prev = prev_insn_same_sec(file, prev)) {
u64 offset;
if (prev->type != INSN_NOP)
return 0;
offset = func->offset - prev->offset;
if (offset > opts.prefix)
return 0;
if (offset < opts.prefix)
continue;
if (!elf_create_symbol(file->elf, name, func->sec,
GELF_ST_BIND(func->sym.st_info),
GELF_ST_TYPE(func->sym.st_info),
prev->offset, opts.prefix))
return -1;
break;
}
if (!prev)
return 0;
if (!insn->cfi) {
/*
* This can happen if stack validation isn't enabled or the
* function is annotated with STACK_FRAME_NON_STANDARD.
*/
return 0;
}
/* Propagate insn->cfi to the prefix code */
insn = find_insn(file, func->sec, func->offset);
if (!insn || !insn->cfi)
return 0;
cfi = cfi_hash_find_or_add(insn->cfi);
for (; prev != insn; prev = next_insn_same_sec(file, prev))
for (prev = find_insn(file, func->sec, func->offset - opts.prefix);
prev && prev != insn;
prev = next_insn_same_sec(file, prev))
prev->cfi = cfi;
return 0;
@@ -4387,15 +4334,20 @@ static int create_prefix_symbol(struct objtool_file *file, struct symbol *func)
static int create_prefix_symbols(struct objtool_file *file)
{
struct section *sec;
struct section *pfe_sec;
struct symbol *func;
struct reloc *reloc;
for_each_sec(file->elf, sec) {
if (!is_text_sec(sec))
for_each_sec(file->elf, pfe_sec) {
if (strcmp(pfe_sec->name, "__patchable_function_entries"))
continue;
if (!pfe_sec->rsec)
continue;
sec_for_each_sym(sec, func) {
if (create_prefix_symbol(file, func))
for_each_reloc(pfe_sec->rsec, reloc) {
func = find_func_by_offset(reloc->sym->sec,
reloc->sym->offset + reloc_addend(reloc) + opts.prefix);
if (func && create_prefix_symbol(file, func))
return -1;
}
}