Commit 024020e2 authored by Josh Poimboeuf's avatar Josh Poimboeuf Committed by Peter Zijlstra
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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: 2fd65f7a ("AutoFDO: Enable machine function split optimization for AutoFDO")
Closes: https://lore.kernel.org/20251103215244.2080638-2-xur@google.com


Reported-by: default avatarRong Xu <xur@google.com>
Signed-off-by: default avatarJosh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: default avatar <xur@google.com>
Tested-by: default avatar <xur@google.com>
Link: https://patch.msgid.link/20a67326f04b2a361c031b56d58e8a803b3c5893.1763671318.git.jpoimboe@kernel.org
parent 2092007a

tools/objtool/elf.c

+9 −1
Original line number Diff line number Diff line
@@ -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);