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Message-ID: <d0fdced7-a9a5-473e-991f-4f5e4c13f616@linux.dev>
Date: Mon, 6 Oct 2025 22:15:25 -0700
From: Yonghong Song <yonghong.song@...ux.dev>
To: Sahil Chandna <chandna.linuxkernel@...il.com>, ast@...nel.org,
daniel@...earbox.net, andrii@...nel.org, martin.lau@...ux.dev,
song@...nel.org, john.fastabend@...il.com, haoluo@...gle.com,
jolsa@...nel.org, bpf@...r.kernel.org, netdev@...r.kernel.org
Cc: david.hunter.linux@...il.com, skhan@...uxfoundation.org,
khalid@...nel.org, syzbot+1f1fbecb9413cdbfbef8@...kaller.appspotmail.com
Subject: Re: [PATCH] bpf: test_run: Fix timer mode initialization to
NO_MIGRATE mode
On 10/5/25 10:43 PM, Sahil Chandna wrote:
> By default, the timer mode is being initialized to `NO_PREEMPT`.
> This disables preemption and forces execution in atomic context.
> This can cause issue with PREEMPT_RT when calling spin_lock_bh() due
> to sleeping nature of the lock.
> ...
> BUG: sleeping function called from invalid context at kernel/locking/spinlock_rt.c:48
> in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 6107, name: syz.0.17
> preempt_count: 1, expected: 0
> RCU nest depth: 1, expected: 1
> Preemption disabled at:
> [<ffffffff891fce58>] bpf_test_timer_enter+0xf8/0x140 net/bpf/test_run.c:42
> Call Trace:
> <TASK>
> dump_stack_lvl+0x189/0x250 lib/dump_stack.c:120
> __might_resched+0x44b/0x5d0 kernel/sched/core.c:8957
> __rt_spin_lock kernel/locking/spinlock_rt.c:48 [inline]
> rt_spin_lock+0xc7/0x2c0 kernel/locking/spinlock_rt.c:57
> spin_lock_bh include/linux/spinlock_rt.h:88 [inline]
> __sock_map_delete net/core/sock_map.c:421 [inline]
> sock_map_delete_elem+0xb7/0x170 net/core/sock_map.c:452
> bpf_prog_2c29ac5cdc6b1842+0x43/0x4b
> bpf_dispatcher_nop_func include/linux/bpf.h:1332 [inline]
> ...
> Change initialization to NO_MIGRATE mode to prevent this.
>
> Reported-by: syzbot+1f1fbecb9413cdbfbef8@...kaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=1f1fbecb9413cdbfbef8
> Tested-by: syzbot+1f1fbecb9413cdbfbef8@...kaller.appspotmail.com
> Signed-off-by: Sahil Chandna <chandna.linuxkernel@...il.com>
> ---
> net/bpf/test_run.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
> index 4a862d605386..daf966dfed69 100644
> --- a/net/bpf/test_run.c
> +++ b/net/bpf/test_run.c
> @@ -1368,7 +1368,7 @@ int bpf_prog_test_run_flow_dissector(struct bpf_prog *prog,
> const union bpf_attr *kattr,
> union bpf_attr __user *uattr)
> {
> - struct bpf_test_timer t = { NO_PREEMPT };
> + struct bpf_test_timer t = { NO_MIGRATE };
I checked the original reproducer. And changing from
NO_PREEMPT to NO_MIGRATE is needed only with
CONFIG_PREEMPT_RT enabled.
> u32 size = kattr->test.data_size_in;
> struct bpf_flow_dissector ctx = {};
> u32 repeat = kattr->test.repeat;
> @@ -1436,7 +1436,7 @@ int bpf_prog_test_run_flow_dissector(struct bpf_prog *prog,
> int bpf_prog_test_run_sk_lookup(struct bpf_prog *prog, const union bpf_attr *kattr,
> union bpf_attr __user *uattr)
> {
> - struct bpf_test_timer t = { NO_PREEMPT };
> + struct bpf_test_timer t = { NO_MIGRATE };
This change is not needed for the above particular BUG.
> struct bpf_prog_array *progs = NULL;
> struct bpf_sk_lookup_kern ctx = {};
> u32 repeat = kattr->test.repeat;
Checking the git history. I found the earliest NO_PREEMPT usage can be traced
back to this commit:
commit a439184d515fbf4805f57d11fa5dfd4524d2c0eb
Author: Stanislav Fomichev <sdf@...gle.com>
Date: Tue Feb 19 10:54:17 2019 -0800
bpf/test_run: fix unkillable BPF_PROG_TEST_RUN for flow dissector
At that time, migrate_disable/enable() are not used yet.
So I suspect that we can remove NO_PREEMPT/NO_MIGRATE in test_run.c
and use migrate_disable()/migrate_enable() universally.
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