linux-kernel - [tip: objtool/core] objtool: Support Clang AUTOFDO .cold functions

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Message-ID: <176371908079.498.4706819198639272769.tip-bot2@tip-bot2>
Date: Fri, 21 Nov 2025 09:58:00 -0000
From: "tip-bot2 for Josh Poimboeuf" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Rong Xu <xur@...gle.com>, Josh Poimboeuf <jpoimboe@...nel.org>,
 "Peter Zijlstra (Intel)" <peterz@...radead.org>, x86@...nel.org,
 linux-kernel@...r.kernel.org
Subject: [tip: objtool/core] objtool: Support Clang AUTOFDO .cold functions

The following commit has been merged into the objtool/core branch of tip:

Commit-ID:     024020e2b6adb4e568fb80f624b5e20d8943f107
Gitweb:        https://git.kernel.org/tip/024020e2b6adb4e568fb80f624b5e20d8943f107
Author:        Josh Poimboeuf <jpoimboe@...nel.org>
AuthorDate:    Thu, 20 Nov 2025 12:52:15 -08:00
Committer:     Peter Zijlstra <peterz@...radead.org>
CommitterDate: Fri, 21 Nov 2025 10:04:07 +01:00

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")
Closes: https://lore.kernel.org/20251103215244.2080638-2-xur@google.com
Reported-by: Rong Xu <xur@...gle.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@...nel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Reviewed-by: xur@...gle.com
Tested-by: xur@...gle.com
Link: https://patch.msgid.link/20a67326f04b2a361c031b56d58e8a803b3c5893.1763671318.git.jpoimboe@kernel.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 3f20b25..7895f65 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);