Subject: Re: [PATCH 1/9] kernel: add a PF_FORCE_COMPAT flag

From: Al Viro <[email-masked]>
Date: Fri, 18 Sep 2020 14:40:12 +0100

On Fri, Sep 18, 2020 at 02:45:25PM +0200, Christoph Hellwig wrote:
> Add a flag to force processing a syscall as a compat syscall.  This is
> required so that in_compat_syscall() works for I/O submitted by io_uring
> helper threads on behalf of compat syscalls.
> 
> Signed-off-by: Christoph Hellwig <[email-masked]>
> ---
>  arch/sparc/include/asm/compat.h | 3 ++-
>  arch/x86/include/asm/compat.h   | 2 +-
>  fs/io_uring.c                   | 9 +++++++++
>  include/linux/compat.h          | 5 ++++-
>  include/linux/sched.h           | 1 +
>  5 files changed, 17 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/sparc/include/asm/compat.h b/arch/sparc/include/asm/compat.h
> index 40a267b3bd5208..fee6c51d36e869 100644
> --- a/arch/sparc/include/asm/compat.h
> +++ b/arch/sparc/include/asm/compat.h
> @@ -211,7 +211,8 @@ static inline int is_compat_task(void)
>  static inline bool in_compat_syscall(void)
>  {
>  	/* Vector 0x110 is LINUX_32BIT_SYSCALL_TRAP */
> -	return pt_regs_trap_type(current_pt_regs()) == 0x110;
> +	return pt_regs_trap_type(current_pt_regs()) == 0x110 ||
> +		(current->flags & PF_FORCE_COMPAT);

Can't say I like that approach ;-/  Reasoning about the behaviour is much
harder when it's controlled like that - witness set_fs() shite...