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

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Date:   Thu, 18 Oct 2018 12:47:50 +1100
From:   NeilBrown <neilb@...e.com>
To:     peterz@...radead.org, dsafonov@...tuozzo.com, luto@...nel.org,
        hpa@...or.com, dvlasenk@...hat.com, luto@...capital.net,
        torvalds@...ux-foundation.org, bp@...en8.de, mingo@...nel.org,
        brgerst@...il.com, linux-kernel@...r.kernel.org,
        tglx@...utronix.de, linux-tip-commits@...r.kernel.org
Cc:     James Simmons <jsimmons@...radead.org>
Subject: in_compat_syscall() returns from kernel thread for X86_32.

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.

Thanks,
NeilBrown

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