linux-kernel - Re: [PATCH] bpf: test_bpf: turn of preemption in function __run

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Date:   Thu, 21 Feb 2019 16:37:57 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Anders Roxell <anders.roxell@...aro.org>, ast@...nel.org
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] bpf: test_bpf: turn of preemption in function __run_once

On 02/21/2019 09:44 AM, Anders Roxell wrote:
> When running test seccomp_bpf the following splat occurs:
> 
> [ RUN      ] global.secseccomp_bpf.c:2136:global.detect_seccomp_filter_flags:Expected 22 (22) == (*__errno_location ()) (14)
> seccomp_bpf.c:2138:global.detect_seccomp_filter_flags:Failed to detect that an unknown
>   filter flag (0x8) is unsupported! Does a new flag need to be added to this test?
> [ 2155.677841] BUG: assuming atomic context at kernel/seccomp.c:271
> [ 2155.689351] in_atomic(): 0, irqs_disabled(): 0, pid: 28540, name: seccomp_bpf
> [ 2155.696597] INFO: lockdep is turned off.
> [ 2155.700605] CPU: 5 PID: 28540 Comm: seccomp_bpf Tainted: G        W         5.0.0-rc7-next-20190220 #1
> [ 2155.709972] Hardware name: HiKey Development Board (DT)
> [ 2155.715232] Call trace:
> [ 2155.717710]  dump_backtrace+0x0/0x160
> [ 2155.721399]  show_stack+0x24/0x30
> [ 2155.724742]  dump_stack+0xc8/0x114
> [ 2155.728172]  __cant_sleep+0xf0/0x108
> [ 2155.731777]  __seccomp_filter+0x8c/0x5c8
> [ 2155.735727]  __secure_computing+0x4c/0xe8
> [ 2155.739767]  syscall_trace_enter+0xf8/0x2b8
> [ 2155.743982]  el0_svc_common+0xf0/0x130
> [ 2155.747758]  el0_svc_handler+0x38/0x78
> [ 2155.751534]  el0_svc+0x8/0xc
> 
> Rework so that preemption is disabled when we loop over function
> 'BPF_PROG_RUN(...)'.
> Commit 568f196756ad ("bpf: check that BPF programs run with preemption disabled")
> highlighted the issue.
> 
> Suggested-by: Arnd Bergmann <arnd@...db.de>
> Signed-off-by: Anders Roxell <anders.roxell@...aro.org>

Hmm, wrong commit description? Below code is not related to seccomp
but rather BPF test suite. Could you fix it up and resubmit? Rest
looks okay to me.

> ---
>  lib/test_bpf.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/lib/test_bpf.c b/lib/test_bpf.c
> index f3e570722a7e..0845f635f404 100644
> --- a/lib/test_bpf.c
> +++ b/lib/test_bpf.c
> @@ -6668,12 +6668,14 @@ static int __run_one(const struct bpf_prog *fp, const void *data,
>  	u64 start, finish;
>  	int ret = 0, i;
>  
> +	preempt_disable();
>  	start = ktime_get_ns();
>  
>  	for (i = 0; i < runs; i++)
>  		ret = BPF_PROG_RUN(fp, data);
>  
>  	finish = ktime_get_ns();
> +	preempt_enable();
>  
>  	*duration = finish - start;
>  	do_div(*duration, runs);
>