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@google.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: xur@google.com
Tested-by: xur@google.com
Link: https://patch.msgid.link/20a67326f04b2a361c031b56d58e8a803b3c5893.1763671318.git.jpoimboe@kernel.org

diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c
index 3f20b257ab25cb7b70cb4e3b8cbf7a69a54634fa..7895f65aca2a3d1dc68b1d5b004c3858832c1114 100644 (file)
--- 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);