linux-kernel - [PATCH 1/6] objtool: Support Clang AUTOFDO .cold functions

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Message-ID: <20a67326f04b2a361c031b56d58e8a803b3c5893.1763671318.git.jpoimboe@kernel.org>
Date: Thu, 20 Nov 2025 12:52:15 -0800
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: x86@...nel.org
Cc: linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Rong Xu <xur@...gle.com>,
	Han Shen <shenhan@...gle.com>,
	Sriraman Tallam <tmsriram@...gle.com>,
	Krzysztof Pszeniczny <kpszeniczny@...gle.com>
Subject: [PATCH 1/6] objtool: Support Clang AUTOFDO .cold functions

AutoFDO enables -fsplit-machine-functions which can move the cold parts
of a function to a <func>.cold symbol in a .text.split.<func> section.

Unlike GCC, the Clang <func>.cold symbols are not marked STT_FUNC.  This
confuses objtool in several ways, resulting in warnings like the
following:

  vmlinux.o: warning: objtool: apply_retpolines.cold+0xfc: unsupported instruction in callable function
  vmlinux.o: warning: objtool: machine_check_poll.cold+0x2e: unsupported instruction in callable function
  vmlinux.o: warning: objtool: free_deferred_objects.cold+0x1f: relocation to !ENDBR: free_deferred_objects.cold+0x26
  vmlinux.o: warning: objtool: rpm_idle.cold+0xe0: relocation to !ENDBR: rpm_idle.cold+0xe7
  vmlinux.o: warning: objtool: tcp_rcv_state_process.cold+0x1c: relocation to !ENDBR: tcp_rcv_state_process.cold+0x23

Fix it by marking the .cold symbols as STT_FUNC.

Fixes: 2fd65f7afd5a ("AutoFDO: Enable machine function split optimization for AutoFDO")
Reported-by: Rong Xu <xur@...gle.com>
Closes: https://lore.kernel.org/20251103215244.2080638-2-xur@google.com
Signed-off-by: Josh Poimboeuf <jpoimboe@...nel.org>
---
 tools/objtool/elf.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c
index 3f20b257ab25..7895f65aca2a 100644
--- a/tools/objtool/elf.c
+++ b/tools/objtool/elf.c
@@ -502,8 +502,16 @@ static int elf_add_symbol(struct elf *elf, struct symbol *sym)
 	if (strstarts(sym->name, ".klp.sym"))
 		sym->klp = 1;
 
-	if (!sym->klp && is_func_sym(sym) && strstr(sym->name, ".cold"))
+	if (!sym->klp && !is_sec_sym(sym) && strstr(sym->name, ".cold")) {
 		sym->cold = 1;
+
+		/*
+		 * Clang doesn't mark cold subfunctions as STT_FUNC, which
+		 * breaks several objtool assumptions.  Fake it.
+		 */
+		sym->type = STT_FUNC;
+	}
+
 	sym->pfunc = sym->cfunc = sym;
 
 	sym->demangled_name = demangle_name(sym);
-- 
2.51.1