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SubjectRe: linux-next: Tree for May 21 (objtool warnings)
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On 5/25/20 3:10 AM, Miroslav Benes wrote:
> 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

Acked-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>

Thanks for the patch.


> ---
> 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;
>


--
~Randy

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