objtool: Grow __cfi_* prefix symbols for all CFI+CALL_PADDING

For all CONFIG_CFI+CONFIG_CALL_PADDING configs, for C functions, 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.

Note that assembly functions with SYM_TYPED_FUNC_START() aren't affected
by this issue, their __cfi_ symbols also cover the padding.

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.  Rework objtool's
arguments a bit to handle the variety of prefix/cfi-related cases.

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>

1 files changed, 40 insertions, 9 deletions

diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 4242582e941a..73e99bdfaaf6 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -881,6 +881,31 @@ static int create_ibt_endbr_seal_sections(struct objtool_file *file)
 	return 0;
 }
 
+/*
+* Grow __cfi_ symbols to fill the NOP gap between the 'mov <hash>, %rax' and
+* the start of the function.
+*/
+static int grow_cfi_symbols(struct objtool_file *file)
+{
+	struct symbol *sym;
+
+	for_each_sym(file->elf, sym) {
+		if (!is_func_sym(sym) || !strstarts(sym->name, "__cfi_") ||
+		    sym->len != 5)
+			continue;
+
+		if (!find_func_by_offset(sym->sec, sym->offset + sym->len + opts.prefix))
+			continue;
+
+		sym->len += opts.prefix;
+		sym->sym.st_size = sym->len;
+		if (elf_write_symbol(file->elf, sym))
+			return -1;
+	}
+
+	return 0;
+}
+
 static int create_cfi_sections(struct objtool_file *file)
 {
 	struct section *sec;
@@ -4903,12 +4928,6 @@ int check(struct objtool_file *file)
 			goto out;
 	}
 
-	if (opts.cfi) {
-		ret = create_cfi_sections(file);
-		if (ret)
-			goto out;
-	}
-
 	if (opts.rethunk) {
 		ret = create_return_sites_sections(file);
 		if (ret)
@@ -4928,9 +4947,21 @@ int check(struct objtool_file *file)
 	}
 
 	if (opts.prefix) {
-		ret = create_prefix_symbols(file);
-		if (ret)
-			goto out;
+		if (!opts.cfi) {
+			ret = create_prefix_symbols(file);
+			if (ret)
+				goto out;
+		} else {
+			ret = grow_cfi_symbols(file);
+			if (ret)
+				goto out;
+
+			if (opts.fineibt) {
+				ret = create_cfi_sections(file);
+				if (ret)
+					goto out;
+			}
+		}
 	}
 
 	if (opts.ibt) {