Messages in this thread Patch in this message | | | Date | Mon, 25 May 2020 12:10:33 +0200 (CEST) | From | Miroslav Benes <> | Subject | Re: linux-next: Tree for May 21 (objtool warnings) |
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On Thu, 21 May 2020, Randy Dunlap wrote:
> On 5/21/20 7:12 AM, Stephen Rothwell wrote: > > Hi all, > > > > Changes since 20200519: > >
These are indeed caused by -flive-patching
> on x86_64: > > fs/open.o: warning: objtool: chmod_common()+0x104: unreachable instruction > fs/namei.o: warning: objtool: do_renameat2()+0x482: unreachable instruction
Two "funny" ones. When CONFIG_FILE_LOCKING is not set, break_deleg_wait() is just BUG(). GCC decides to isra optimize it to break_deleg_wait.isra.0 and call it in chmod_common() and do_renameat2() instead of just inline it.
chmod_common() with -flive-patching:
981: e8 f7 fe ff ff callq 87d <break_deleg_wait.isra.0> 986: 41 89 c4 mov %eax,%r12d 989: 85 c0 test %eax,%eax 98b: 0f 84 76 ff ff ff je 907 <chmod_common+0x7b> 991: 48 89 df mov %rbx,%rdi 994: e8 00 00 00 00 callq 999 <chmod_common+0x10d> 995: R_X86_64_PLT32 __tsan_read8-0x4
without:
985: e8 f3 fe ff ff callq 87d <break_deleg_wait.isra.0> 98a: 48 89 df mov %rbx,%rdi 98d: e8 00 00 00 00 callq 992 <chmod_common+0x106> 98e: R_X86_64_PLT32 __tsan_read8-0x4
The error checking of break_deleg_wait() is correctly compiled out, because it is unreachable.
I wondered how come objtool (correctly) says the instructions are unreachable. It means it knows that break_deleg_wait.isra.0 is a dead end (dead_end_function() returns true while checking INSN_CALL). And of course "return 0;" in break_deleg_wait() is compiled out.
000000000000087d <break_deleg_wait.isra.0>: 87d: 55 push %rbp 87e: 48 89 e5 mov %rsp,%rbp 881: 48 8b 7d 08 mov 0x8(%rbp),%rdi 885: e8 00 00 00 00 callq 88a <break_deleg_wait.isra.0+0xd> 886: R_X86_64_PLT32 __tsan_func_entry-0x4 88a: 0f 0b ud2
One way to fix it is to mark break_deleg_wait() as __always_inline. Then it all works.
Note: there are more functions calling break_deleg_wait() with this pattern.
I'll try to find out which optimization does this, because it is a slightly different scenario than hiding __noreturn from the callees. Probably -fno-ipa-pure-const again.
> kernel/exit.o: warning: objtool: __ia32_sys_exit_group()+0x2e: unreachable instruction
Easy one. do_group_exit() is noreturn and should be marked as such.
> Full randconfig file is attached. > > > -- > ~Randy > Reported-by: Randy Dunlap <rdunlap@infradead.org>
Thanks for reporting, Randy.
Miroslav
--- diff --git a/include/linux/fs.h b/include/linux/fs.h index ac8bd95b5fe8..2b79932e924d 100644 --- a/include/linux/fs.h +++ b/include/linux/fs.h @@ -2546,7 +2546,7 @@ static inline int try_break_deleg(struct inode *inode, struct inode **delegated_ return 0; } -static inline int break_deleg_wait(struct inode **delegated_inode) +static __always_inline int break_deleg_wait(struct inode **delegated_inode) { BUG(); return 0; diff --git a/kernel/exit.c b/kernel/exit.c index ed56917d50e3..9437c1ca37c5 100644 --- a/kernel/exit.c +++ b/kernel/exit.c @@ -876,7 +876,7 @@ SYSCALL_DEFINE1(exit, int, error_code) * Take down every thread in the group. This is called by fatal signals * as well as by sys_exit_group (below). */ -void +void __noreturn do_group_exit(int exit_code) { struct signal_struct *sig = current->signal;
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