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

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Message-ID: <CALCETrXB8PE4Ue0BeD0wUS9psLLjpv_pt+nj=SEaj+Nb_dZzUA@mail.gmail.com>
Date:   Wed, 17 Oct 2018 19:37:42 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     NeilBrown <neilb@...e.com>
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 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?