objtool: Add option to generate prefix symbols

When code is compiled with:

  -fpatchable-function-entry=${PADDING_BYTES},${PADDING_BYTES}

functions will have PADDING_BYTES of NOP in front of them. Unwinders
and other things that symbolize code locations will typically
attribute these bytes to the preceding function.

Given that these bytes nominally belong to the following symbol this
mis-attribution is confusing.

Inspired by the fact that CFI_CLANG emits __cfi_##name symbols to
claim these bytes, allow objtool to emit __pfx_##name symbols to do
the same.

Therefore add the objtool --prefix=N argument, to conditionally place
a __pfx_##name symbol at N bytes ahead of symbol 'name' when: all
these preceding bytes are NOP and name-N is an instruction boundary.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Yujie Liu <yujie.liu@intel.com>
Link: https://lkml.kernel.org/r/20221028194453.526899822@infradead.org

Author: Peter Zijlstra

tools/objtool/builtin-check.c

@@ -75,6 +75,7 @@ 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_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"),

tools/objtool/check.c

@@ -3417,7 +3417,8 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
 
 		if (func && insn_func(insn) && func != insn_func(insn)->pfunc) {
 			/* Ignore KCFI type preambles, which always fall through */
-			if (!strncmp(func->name, "__cfi_", 6))
+			if (!strncmp(func->name, "__cfi_", 6) ||
+			    !strncmp(func->name, "__pfx_", 6))
 				return 0;
 
 			WARN("%s() falls through to next function %s()",
@@ -3972,6 +3973,34 @@ static bool ignore_unreachable_insn(struct objtool_file *file, struct instructio
 	return false;
 }
 
+static int add_prefix_symbol(struct objtool_file *file, struct symbol *func,
+			     struct instruction *insn)
+{
+	if (!opts.prefix)
+		return 0;
+
+	for (;;) {
+		struct instruction *prev = list_prev_entry(insn, list);
+		u64 offset;
+
+		if (&prev->list == &file->insn_list)
+			break;
+
+		if (prev->type != INSN_NOP)
+			break;
+
+		offset = func->offset - prev->offset;
+		if (offset >= opts.prefix) {
+			if (offset == opts.prefix)
+				elf_create_prefix_symbol(file->elf, func, opts.prefix);
+			break;
+		}
+		insn = prev;
+	}
+
+	return 0;
+}
+
 static int validate_symbol(struct objtool_file *file, struct section *sec,
 			   struct symbol *sym, struct insn_state *state)
 {
@@ -3990,6 +4019,8 @@ static int validate_symbol(struct objtool_file *file, struct section *sec,
 	if (!insn || insn->ignore || insn->visited)
 		return 0;
 
+	add_prefix_symbol(file, sym, insn);
+
 	state->uaccess = sym->uaccess_safe;
 
 	ret = validate_branch(file, insn_func(insn), insn, *state);

tools/objtool/elf.c

@@ -819,6 +819,37 @@ elf_create_section_symbol(struct elf *elf, struct section *sec)
 	return sym;
 }
 
+static int elf_add_string(struct elf *elf, struct section *strtab, char *str);
+
+struct symbol *
+elf_create_prefix_symbol(struct elf *elf, struct symbol *orig, long size)
+{
+	struct symbol *sym = calloc(1, sizeof(*sym));
+	size_t namelen = strlen(orig->name) + sizeof("__pfx_");
+	char *name = malloc(namelen);
+
+	if (!sym || !name) {
+		perror("malloc");
+		return NULL;
+	}
+
+	snprintf(name, namelen, "__pfx_%s", orig->name);
+
+	sym->name = name;
+	sym->sec = orig->sec;
+
+	sym->sym.st_name = elf_add_string(elf, NULL, name);
+	sym->sym.st_info = orig->sym.st_info;
+	sym->sym.st_value = orig->sym.st_value - size;
+	sym->sym.st_size = size;
+
+	sym = __elf_create_symbol(elf, sym);
+	if (sym)
+		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)

tools/objtool/include/objtool/builtin.h

@@ -26,6 +26,7 @@ struct opts {
 	bool stackval;
 	bool static_call;
 	bool uaccess;
+	int prefix;
 
 	/* options: */
 	bool backtrace;

tools/objtool/include/objtool/elf.h

@@ -146,6 +146,8 @@ static inline bool has_multiple_files(struct elf *elf)
 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);
 
+struct symbol *elf_create_prefix_symbol(struct elf *elf, struct symbol *orig, long size);
+
 int elf_add_reloc(struct elf *elf, struct section *sec, unsigned long offset,
 		  unsigned int type, struct symbol *sym, s64 addend);
 int elf_add_reloc_to_insn(struct elf *elf, struct section *sec,