netdev - Re: [PATCH bpf-next] bpf: fix raw_tp test run in preempt kernel

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Date:   Tue, 29 Sep 2020 17:00:07 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Song Liu <songliubraving@...com>
Cc:     netdev@...r.kernel.org, bpf@...r.kernel.org, kernel-team@...com,
        ast@...nel.org, daniel@...earbox.net, john.fastabend@...il.com,
        kpsingh@...omium.org
Subject: Re: [PATCH bpf-next] bpf: fix raw_tp test run in preempt kernel

On Tue, Sep 29, 2020 at 03:50:13PM -0700, Song Liu wrote:
> In preempt kernel, BPF_PROG_TEST_RUN on raw_tp triggers:
> 
> [   35.874974] BUG: using smp_processor_id() in preemptible [00000000]
> code: new_name/87
> [   35.893983] caller is bpf_prog_test_run_raw_tp+0xd4/0x1b0
> [   35.900124] CPU: 1 PID: 87 Comm: new_name Not tainted 5.9.0-rc6-g615bd02bf #1
> [   35.907358] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
> BIOS 1.10.2-1ubuntu1 04/01/2014
> [   35.916941] Call Trace:
> [   35.919660]  dump_stack+0x77/0x9b
> [   35.923273]  check_preemption_disabled+0xb4/0xc0
> [   35.928376]  bpf_prog_test_run_raw_tp+0xd4/0x1b0
> [   35.933872]  ? selinux_bpf+0xd/0x70
> [   35.937532]  __do_sys_bpf+0x6bb/0x21e0
> [   35.941570]  ? find_held_lock+0x2d/0x90
> [   35.945687]  ? vfs_write+0x150/0x220
> [   35.949586]  do_syscall_64+0x2d/0x40
> [   35.953443]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> 
> Fix this by calling migrate_disable() before smp_processor_id().
> 
> Fixes: 1b4d60ec162f ("bpf: Enable BPF_PROG_TEST_RUN for raw_tracepoint")
> Reported-by: Alexei Starovoitov <ast@...nel.org>
> Signed-off-by: Song Liu <songliubraving@...com>
> ---
>  net/bpf/test_run.c | 20 ++++++++------------
>  1 file changed, 8 insertions(+), 12 deletions(-)
> 
> diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
> index fde5db93507c4..3ea05a5daf544 100644
> --- a/net/bpf/test_run.c
> +++ b/net/bpf/test_run.c
> @@ -251,11 +251,7 @@ __bpf_prog_test_run_raw_tp(void *data)
>  {
>  	struct bpf_raw_tp_test_run_info *info = data;
>  
> -	rcu_read_lock();
> -	migrate_disable();
>  	info->retval = BPF_PROG_RUN(info->prog, info->ctx);
> -	migrate_enable();
> -	rcu_read_unlock();

I would keep rcu_read_lock here, since there is no need to expand its scope.

>  }
>  
>  int bpf_prog_test_run_raw_tp(struct bpf_prog *prog,
> @@ -293,27 +289,27 @@ int bpf_prog_test_run_raw_tp(struct bpf_prog *prog,
>  
>  	info.prog = prog;
>  
> +	rcu_read_lock();
> +	migrate_disable();
>  	if ((kattr->test.flags & BPF_F_TEST_RUN_ON_CPU) == 0 ||
>  	    cpu == smp_processor_id()) {

may be use get_cpu/put_cpu instead?

>  		__bpf_prog_test_run_raw_tp(&info);
> -	} else {
> +	} else if (cpu >= nr_cpu_ids || !cpu_online(cpu)) {
>  		/* smp_call_function_single() also checks cpu_online()
>  		 * after csd_lock(). However, since cpu is from user
>  		 * space, let's do an extra quick check to filter out
>  		 * invalid value before smp_call_function_single().
>  		 */
> -		if (cpu >= nr_cpu_ids || !cpu_online(cpu)) {
>  		err = -ENXIO;
> -			goto out;
> -		}
> -
> +	} else {
>  		err = smp_call_function_single(cpu, __bpf_prog_test_run_raw_tp,
>  					       &info, 1);
> -		if (err)
> -			goto out;
>  	}
> +	migrate_enable();
> +	rcu_read_unlock();
>  
> -	if (copy_to_user(&uattr->test.retval, &info.retval, sizeof(u32)))
> +	if (err == 0 &&

!err would be canonical.

> +	    copy_to_user(&uattr->test.retval, &info.retval, sizeof(u32)))
>  		err = -EFAULT;
>  
>  out:
> -- 
> 2.24.1
>