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Tue, 09 Sep 2025 12:27:06 -0700 (PDT) Message-ID: <4918e983-9964-46a2-aee0-59c91a9c6b35@kernel.dk> Date: Tue, 9 Sep 2025 13:27:05 -0600 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [syzbot] [kernel?] general protection fault in try_to_wake_up (3) To: Sebastian Andrzej Siewior Cc: Peter Zijlstra , syzbot , andrealmeid@igalia.com, dave@stgolabs.net, dvhart@infradead.org, linux-kernel@vger.kernel.org, mingo@redhat.com, syzkaller-bugs@googlegroups.com, tglx@linutronix.de References: <68b75989.050a0220.3db4df.01dd.GAE@google.com> <20250902214628.GL4067720@noisy.programming.kicks-ass.net> <20250903130712.br0G6lOq@linutronix.de> <20250904162820.NS1U-oZp@linutronix.de> From: Jens Axboe Content-Language: en-US In-Reply-To: <20250904162820.NS1U-oZp@linutronix.de> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 9/4/25 10:28 AM, Sebastian Andrzej Siewior wrote: > On 2025-09-03 12:51:09 [-0600], Jens Axboe wrote: >>> The syz-reproducer lists only: >>> | timer_create(0x0, &(0x7f0000000080)={0x0, 0x11, 0x0, @thr={0x0, 0x0}}, &(0x7f0000000000)) >>> | timer_settime(0x0, 0x0, &(0x7f0000000240)={{0x0, 0x8}, {0x0, 0x9}}, 0x0) >>> | futex(&(0x7f000000cffc), 0x80000000000b, 0x0, 0x0, &(0x7f0000048000), 0x0) >>> | futex(&(0x7f000000cffc), 0xc, 0x1, 0x0, &(0x7f0000048000), 0x0) >>> >>> and that is probably why it can't come up with C-reproducer. >>> The whole log has (filtered) the following lines: >>> >>> | io_uring_setup(0x85a, &(0x7f0000000180)={0x0, 0x58b9, 0x1, 0x2, 0x383}) >>> | syz_io_uring_setup(0x88f, &(0x7f0000000300)={0x0, 0xaedf, 0x0, 0x0, 0x25d}, &(0x7f0000000140)=0x0, &(0x7f0000000280)=0x0) >>> | syz_memcpy_off$IO_URING_METADATA_GENERIC(r0, 0x4, &(0x7f0000000080)=0xfffffffc, 0x0, 0x4) >>> | syz_io_uring_submit(r0, r1, &(0x7f00000001c0)=@IORING_OP_RECVMSG={0xa, 0x8, 0x1, r2, 0x0, &(0x7f0000000440)={0x0, 0x0, 0x0}, 0x0, 0x40000020, 0x1, {0x2}}) >>> >>> This should explain the how the waiter got NULL. There is no private >>> flag so that is how they interact with each other. >>> Do we want this: >>> >>> diff --git a/kernel/futex/requeue.c b/kernel/futex/requeue.c >>> index c716a66f86929..0c98256ebdcb7 100644 >>> --- a/kernel/futex/requeue.c >>> +++ b/kernel/futex/requeue.c >>> @@ -312,6 +312,8 @@ futex_proxy_trylock_atomic(u32 __user *pifutex, struct futex_hash_bucket *hb1, >>> if (!top_waiter->rt_waiter || top_waiter->pi_state) >>> return -EINVAL; > > I've been poking at this today and I have one problem with my > explanation: > The io_uring code initializes its futex_q with futex_q_init. At this > point futex_q::rt_waiter is set to NULL and never set to something else. > We should bail out here instead of going further. > Only the PI bits set rt_waiter. Only io_uring sets task to NULL. > I'm hopeless, this makes no sense. Was on the road, and now back at least for a day or two... So I took a gander at this one too. One thing that puzzles me is the io_uring traces in that syzbot log - if they are to be trusted, it only ever submits an io_uring RECVMSG request? IOW, no futex usage on the io_uring side at all? Going to try and download the disk image and kernel and see if I can actually run this locally. -- Jens Axboe