[PATCH 21/27] objtool: Fix noreturn detection for non-sibling jumps to SYM_CODE

[Josh Poimboeuf <jpoimboe@kernel.org>]

Objtool doesn't consider a jump to be a sibling call if the destination
isn't a function, so srso_alias_untrain_ret()'s jump to
srso_alias_return_thunk() is ignored by objtool and the former is
falsely classified as noreturn.

Change the noreturn detection to consider such cases as effectively
sibling calls, except for one known exception: jumping to .altinstr_aux.

Currently the misclassification is harmless: dead ends aren't marked
when calling a noreturn from an alternative replacement
(CALL_UNTRAIN_RET), they're ignored by rethunk validation, and ORC
generation relies on unwind hints in the SRSO code regardless.

Fix it in the name of robustness and in preparation for a subsequent
patch which generates a list of exported noreturns.

Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
---
 tools/objtool/check.c | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index c55851ec389cd..d27303220c29d 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -264,8 +264,22 @@ static bool might_return(struct objtool_file *file, struct symbol *func)
 		if (insn->type == INSN_RETURN)
 			return true;
 
-		if (!is_sibling_call(insn))
+		if (!is_sibling_call(insn)) {
+			/*
+			 * Assume a jump into a non-function eventually returns
+			 * to the original caller one way or another, e.g., the
+			 * jump in srso_alias_untrain_ret().
+			 *
+			 * .altinstr_aux is an exception, cpu_feature_enabled()
+			 * jumps there and then right back.
+			 */
+			if (is_static_jump(insn) && insn->jump_dest &&
+			    !insn_func(insn->jump_dest) &&
+			    strcmp(insn->jump_dest->sec->name, ".altinstr_aux"))
+				return true;
+
 			continue;
+		}
 
 		dest = insn_call_dest(insn);
 		if (!dest || !is_noreturn(dest))
-- 
2.55.0