linux-kernel - Re: in_compat_syscall() returns from kernel thread for X86

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Message-ID: <871s8ndg6a.fsf@notabene.neil.brown.name>
Date:   Thu, 18 Oct 2018 15:36:45 +1100
From:   NeilBrown <neilb@...e.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Dmitry Safonov <dsafonov@...tuozzo.com>,
        Andrew Lutomirski <luto@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...nel.org>,
        Brian Gerst <brgerst@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-tip-commits@...r.kernel.org, jsimmons@...radead.org
Subject: Re: in_compat_syscall() returns from kernel thread for X86_32.

On Wed, Oct 17 2018, Andy Lutomirski wrote:

> On Wed, Oct 17, 2018 at 6:48 PM NeilBrown <neilb@...e.com> wrote:
>>
>>
>> Was: Re: [tip:x86/asm] x86/entry: Rename is_{ia32,x32}_task() to in_{ia32,x32}_syscall()
>> On Tue, Apr 19 2016, tip-bot for Dmitry Safonov wrote:
>>
>> > Commit-ID:  abfb9498ee1327f534df92a7ecaea81a85913bae
>> > Gitweb:     http://git.kernel.org/tip/abfb9498ee1327f534df92a7ecaea81a85913bae
>> > Author:     Dmitry Safonov <dsafonov@...tuozzo.com>
>> > AuthorDate: Mon, 18 Apr 2016 16:43:43 +0300
>> > Committer:  Ingo Molnar <mingo@...nel.org>
>> > CommitDate: Tue, 19 Apr 2016 10:44:52 +0200
>> >
>> > x86/entry: Rename is_{ia32,x32}_task() to in_{ia32,x32}_syscall()
>> >
>> ...
>> > @@ -318,7 +318,7 @@ static inline bool is_x32_task(void)
>> >
>> >  static inline bool in_compat_syscall(void)
>> >  {
>> > -     return is_ia32_task() || is_x32_task();
>> > +     return in_ia32_syscall() || in_x32_syscall();
>> >  }
>>
>> Hi,
>>  I'm reply to this patch largely to make sure I get the right people
>>  .....
>>
>> This test is always true when CONFIG_X86_32 is set, as that forces
>> in_ia32_syscall() to true.
>> However we might not be in a syscall at all - we might be running a
>> kernel thread which is always in 64 mode.
>> Every other implementation of in_compat_syscall() that I found is
>> dependant on a thread flag or syscall register flag, and so returns
>> "false" in a kernel thread.
>>
>> Might something like this be appropriate?
>>
>> diff --git a/arch/x86/include/asm/thread_info.h b/arch/x86/include/asm/thread_info.h
>> index 2ff2a30a264f..c265b40a78f2 100644
>> --- a/arch/x86/include/asm/thread_info.h
>> +++ b/arch/x86/include/asm/thread_info.h
>> @@ -219,7 +219,7 @@ static inline int arch_within_stack_frames(const void * const stack,
>>  #ifndef __ASSEMBLY__
>>
>>  #ifdef CONFIG_X86_32
>> -#define in_ia32_syscall() true
>> +#define in_ia32_syscall() (!(current->flags & PF_KTHREAD))
>>  #else
>>  #define in_ia32_syscall() (IS_ENABLED(CONFIG_IA32_EMULATION) && \
>>                            current_thread_info()->status & TS_COMPAT)
>>
>> This came up in the (no out-of-tree) lustre filesystem where some code
>> needs to assume 32-bit mode in X86_32 syscalls, and 64-bit mode in kernel
>> threads.
>>
>
> I could get on board with:
>
> ({WARN_ON_ONCE(current->flags & PF_KTHREAD); true})
>
> The point of these accessors is to be used *in a syscall*.
>
> What on Earth is Lustre doing that makes it have this problem?

Lustre uses it in the ->getattr method to make sure ->ino, ->dev and
->rdev are appropriately sized.  This isn't very different from the
usage in ext4 to ensure the seek offset for directories is suitable.

These interfaces can be used both from systemcalls and from kernel
threads, such as via nfsd.

I don't *know* if nfsd is the particular kthread that causes problems
for lustre.  All I know is that ->getattr returns 32bit squashed inode
numbers in kthread context where 64 bit numbers would be expected.

Thanks,
NeilBrown


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