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

1 files changed, 32 insertions, 1 deletions

diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 7936312e10c7..27f35f5f831a 100644
--- a/tools/objtool/check.c
+++ b/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);