linux-kernel - Re: [PATCH] powerpc/ptrace: cleanup do_syscall_trace

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Message-ID: <87y38otn5x.fsf@concordia.ellerman.id.au>
Date:   Mon, 17 Dec 2018 22:20:26 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     "Dmitry V. Levin" <ldv@...linux.org>,
        Oleg Nesterov <oleg@...hat.com>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] powerpc/ptrace: cleanup do_syscall_trace_enter

"Dmitry V. Levin" <ldv@...linux.org> writes:
> Invoke tracehook_report_syscall_entry once.

Thanks.

> Signed-off-by: Dmitry V. Levin <ldv@...linux.org>
> ---
>  arch/powerpc/kernel/ptrace.c | 54 +++++++++++++++++++++---------------
>  1 file changed, 31 insertions(+), 23 deletions(-)
>
> diff --git a/arch/powerpc/kernel/ptrace.c b/arch/powerpc/kernel/ptrace.c
> index 714c3480c52d..8794d32c2d9e 100644
> --- a/arch/powerpc/kernel/ptrace.c
> +++ b/arch/powerpc/kernel/ptrace.c
> @@ -3263,32 +3263,40 @@ static inline int do_seccomp(struct pt_regs *regs) { return 0; }
>   */
>  long do_syscall_trace_enter(struct pt_regs *regs)
>  {
> +	u32 cached_flags;
> +

Do you mind if I just call it "flags", I find "cached_flags" a bit
unwieldy for some reason.

I'm happy to fix it up when applying.

cheers

>  	user_exit();
>  
> -	if (test_thread_flag(TIF_SYSCALL_EMU)) {
> -		/*
> -		 * A nonzero return code from tracehook_report_syscall_entry()
> -		 * tells us to prevent the syscall execution, but we are not
> -		 * going to execute it anyway.
> -		 *
> -		 * Returning -1 will skip the syscall execution. We want to
> -		 * avoid clobbering any register also, thus, not 'gotoing'
> -		 * skip label.
> -		 */
> -		if (tracehook_report_syscall_entry(regs))
> -			;
> -		return -1;
> -	}
> +	cached_flags = READ_ONCE(current_thread_info()->flags) &
> +		       (_TIF_SYSCALL_EMU | _TIF_SYSCALL_TRACE);
>  
> -	/*
> -	 * The tracer may decide to abort the syscall, if so tracehook
> -	 * will return !0. Note that the tracer may also just change
> -	 * regs->gpr[0] to an invalid syscall number, that is handled
> -	 * below on the exit path.
> -	 */
> -	if (test_thread_flag(TIF_SYSCALL_TRACE) &&
> -	    tracehook_report_syscall_entry(regs))
> -		goto skip;
> +	if (cached_flags) {
> +		int rc = tracehook_report_syscall_entry(regs);
> +
> +		if (unlikely(cached_flags & _TIF_SYSCALL_EMU)) {
> +			/*
> +			 * A nonzero return code from
> +			 * tracehook_report_syscall_entry() tells us
> +			 * to prevent the syscall execution, but
> +			 * we are not going to execute it anyway.
> +			 *
> +			 * Returning -1 will skip the syscall execution.
> +			 * We want to avoid clobbering any register also,
> +			 * thus, not 'gotoing' skip label.
> +			 */
> +			return -1;
> +		}
> +
> +		if (rc) {
> +			/*
> +			 * The tracer decided to abort the syscall.
> +			 * Note that the tracer may also just change
> +			 * regs->gpr[0] to an invalid syscall number,
> +			 * that is handled below on the exit path.
> +			 */
> +			goto skip;
> +		}
> +	}
>  
>  	/* Run seccomp after ptrace; allow it to set gpr[3]. */
>  	if (do_seccomp(regs))
> -- 
> ldv