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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: 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

authored by

Josh Poimboeuf and committed by
Peter Zijlstra 024020e2 2092007a

+9 -1
+9 -1
tools/objtool/elf.c
··· 502 502 if (strstarts(sym->name, ".klp.sym")) 503 503 sym->klp = 1; 504 504 505 - if (!sym->klp && is_func_sym(sym) && strstr(sym->name, ".cold")) 505 + if (!sym->klp && !is_sec_sym(sym) && strstr(sym->name, ".cold")) { 506 506 sym->cold = 1; 507 + 508 + /* 509 + * Clang doesn't mark cold subfunctions as STT_FUNC, which 510 + * breaks several objtool assumptions. Fake it. 511 + */ 512 + sym->type = STT_FUNC; 513 + } 514 + 507 515 sym->pfunc = sym->cfunc = sym; 508 516 509 517 sym->demangled_name = demangle_name(sym);