From cd77849a69cf5d81a86dd88dac7a594a67cb5c87 Mon Sep 17 00:00:00 2001 From: Josh Poimboeuf Date: Fri, 1 Jun 2018 07:23:51 -0500 Subject: 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(+) (limited to 'tools/objtool') diff --git a/tools/objtool/check.c b/tools/objtool/check.c index 3a31b238f885..38047c6aa575 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; -- cgit v1.3-8-gc7d7 From 684fb246578b9e81fc7b4ca5c71eae22edb650b2 Mon Sep 17 00:00:00 2001 From: Josh Poimboeuf Date: Tue, 19 Jun 2018 10:47:50 -0500 Subject: objtool: Add machine_real_restart() to the noreturn list machine_real_restart() is annotated as '__noreturn", so add it to the objtool noreturn list. This fixes the following warning with clang and CONFIG_CC_OPTIMIZE_FOR_SIZE=y: arch/x86/kernel/reboot.o: warning: objtool: native_machine_emergency_restart() falls through to next function machine_power_off() Reported-by: Matthias Kaehlcke Signed-off-by: Josh Poimboeuf Signed-off-by: Thomas Gleixner Tested-by: Matthias Kaehlcke Reviewed-by: Matthias Kaehlcke Link: https://lkml.kernel.org/r/791712792aa4431bdd55bf1beb33a169ddf3b4a2.1529423255.git.jpoimboe@redhat.com --- tools/objtool/check.c | 1 + 1 file changed, 1 insertion(+) (limited to 'tools/objtool') diff --git a/tools/objtool/check.c b/tools/objtool/check.c index 38047c6aa575..f4a25bd1871f 100644 --- a/tools/objtool/check.c +++ b/tools/objtool/check.c @@ -164,6 +164,7 @@ static int __dead_end_function(struct objtool_file *file, struct symbol *func, "lbug_with_loc", "fortify_panic", "usercopy_abort", + "machine_real_restart", }; if (func->bind == STB_WEAK) -- cgit v1.3-8-gc7d7 From 08b393d01c88aff27347ed2b1b354eb4db2f1532 Mon Sep 17 00:00:00 2001 From: Josh Poimboeuf Date: Wed, 27 Jun 2018 17:03:45 -0500 Subject: objtool: Support GCC 8 '-fnoreorder-functions' Since the following commit: cd77849a69cf ("objtool: Fix GCC 8 cold subfunction detection for aliased functions") ... if the kernel is built with EXTRA_CFLAGS='-fno-reorder-functions', objtool can get stuck in an infinite loop. That flag causes the new GCC 8 cold subfunctions to be placed in .text instead of .text.unlikely. But it also has an unfortunate quirk: in the symbol table, the subfunction (e.g., nmi_panic.cold.7) is nested inside the parent (nmi_panic). That function overlap confuses objtool, and causes it to get into an infinite loop in next_insn_same_func(). Here's Allan's description of the loop: "Objtool iterates through the instructions in nmi_panic using next_insn_same_func. Once it reaches the end of nmi_panic at 0x534 it jumps to 0x528 as that's the start of nmi_panic.cold.7. However, since the instructions starting at 0x528 are still associated with nmi_panic objtool will get stuck in a loop, continually jumping back to 0x528 after reaching 0x534." Fix it by shortening the length of the parent function so that the functions no longer overlap. Reported-and-analyzed-by: Allan Xavier Signed-off-by: Josh Poimboeuf Cc: Allan Xavier Cc: Andy Lutomirski Cc: Borislav Petkov Cc: Brian Gerst Cc: Denys Vlasenko Cc: H. Peter Anvin Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Thomas Gleixner Link: http://lkml.kernel.org/r/9e704c52bee651129b036be14feda317ae5606ae.1530136978.git.jpoimboe@redhat.com Signed-off-by: Ingo Molnar --- tools/objtool/elf.c | 41 ++++++++++++++++++++++++++++------------- 1 file changed, 28 insertions(+), 13 deletions(-) (limited to 'tools/objtool') diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c index 4e60e105583e..0d1acb704f64 100644 --- a/tools/objtool/elf.c +++ b/tools/objtool/elf.c @@ -302,19 +302,34 @@ static int read_symbols(struct elf *elf) continue; sym->pfunc = sym->cfunc = sym; coldstr = strstr(sym->name, ".cold."); - if (coldstr) { - coldstr[0] = '\0'; - pfunc = find_symbol_by_name(elf, sym->name); - coldstr[0] = '.'; - - if (!pfunc) { - WARN("%s(): can't find parent function", - sym->name); - goto err; - } - - sym->pfunc = pfunc; - pfunc->cfunc = sym; + if (!coldstr) + continue; + + coldstr[0] = '\0'; + pfunc = find_symbol_by_name(elf, sym->name); + coldstr[0] = '.'; + + if (!pfunc) { + WARN("%s(): can't find parent function", + sym->name); + goto err; + } + + sym->pfunc = pfunc; + pfunc->cfunc = sym; + + /* + * Unfortunately, -fnoreorder-functions puts the child + * inside the parent. Remove the overlap so we can + * have sane assumptions. + * + * Note that pfunc->len now no longer matches + * pfunc->sym.st_size. + */ + if (sym->sec == pfunc->sec && + sym->offset >= pfunc->offset && + sym->offset + sym->len == pfunc->offset + pfunc->len) { + pfunc->len -= sym->len; } } } -- cgit v1.3-8-gc7d7