From 024020e2b6adb4e568fb80f624b5e20d8943f107 Mon Sep 17 00:00:00 2001 From: Josh Poimboeuf Date: Thu, 20 Nov 2025 12:52:15 -0800 Subject: [PATCH] objtool: Support Clang AUTOFDO .cold functions AutoFDO enables -fsplit-machine-functions which can move the cold parts of a function to a .cold symbol in a .text.split. section. Unlike GCC, the Clang .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 Signed-off-by: Josh Poimboeuf Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: xur@google.com Tested-by: xur@google.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 3f20b257ab25c..7895f65aca2a3 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); -- 2.47.3