From cd77849a69cf5d81a86dd88dac7a594a67cb5c87 Mon Sep 17 00:00:00 2001 From: Josh Poimboeuf Date: Fri, 1 Jun 2018 07:23:51 -0500 Subject: [PATCH] objtool: Fix GCC 8 cold subfunction detection for aliased functions The kbuild test robot reported the following issue: kernel/time/posix-stubs.o: warning: objtool: sys_ni_posix_timers.cold.1()+0x0: unreachable instruction This file creates symbol aliases for the sys_ni_posix_timers() function. So there are multiple ELF function symbols for the same function: 23: 0000000000000150 26 FUNC GLOBAL DEFAULT 1 __x64_sys_timer_create 24: 0000000000000150 26 FUNC GLOBAL DEFAULT 1 sys_ni_posix_timers 25: 0000000000000150 26 FUNC GLOBAL DEFAULT 1 __ia32_sys_timer_create 26: 0000000000000150 26 FUNC GLOBAL DEFAULT 1 __x64_sys_timer_gettime Here's the corresponding cold subfunction: 11: 0000000000000000 45 FUNC LOCAL DEFAULT 6 sys_ni_posix_timers.cold.1 When analyzing overlapping functions, objtool only looks at the first one in the symbol list. The rest of the functions are basically ignored because they point to instructions which have already been analyzed. So in this case it analyzes the __x64_sys_timer_create() function, but then it fails to recognize that its cold subfunction is sys_ni_posix_timers.cold.1(), because the names are different. Make the subfunction detection a little smarter by associating each subfunction with the first function which jumps to it, since that's the one which will be analyzed. Unfortunately we still have to leave the original subfunction detection code in place, thanks to GCC switch tables. (See the comment for more details.) Fixes: 13810435b9a7 ("objtool: Support GCC 8's cold subfunctions") Reported-by: kbuild test robot Signed-off-by: Josh Poimboeuf Signed-off-by: Thomas Gleixner Cc: Peter Zijlstra Link: https://lkml.kernel.org/r/d3ba52662cbc8e3a64a3b64d44b4efc5674fd9ab.1527855808.git.jpoimboe@redhat.com --- tools/objtool/check.c | 22 ++++++++++++++++++++++ 1 file changed, 22 insertions(+) diff --git a/tools/objtool/check.c b/tools/objtool/check.c index 3a31b23..38047c6 100644 --- a/tools/objtool/check.c +++ b/tools/objtool/check.c @@ -543,6 +543,28 @@ static int add_jump_destinations(struct objtool_file *file) dest_off); return -1; } + + /* + * For GCC 8+, create parent/child links for any cold + * subfunctions. This is _mostly_ redundant with a similar + * initialization in read_symbols(). + * + * If a function has aliases, we want the *first* such function + * in the symbol table to be the subfunction's parent. In that + * case we overwrite the initialization done in read_symbols(). + * + * However this code can't completely replace the + * read_symbols() code because this doesn't detect the case + * where the parent function's only reference to a subfunction + * is through a switch table. + */ + if (insn->func && insn->jump_dest->func && + insn->func != insn->jump_dest->func && + !strstr(insn->func->name, ".cold.") && + strstr(insn->jump_dest->func->name, ".cold.")) { + insn->func->cfunc = insn->jump_dest->func; + insn->jump_dest->func->pfunc = insn->func; + } } return 0; -- 2.7.4