linux-kernel - Re: [syzbot] WARNING in mntput_no

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Date:   Sun, 4 Apr 2021 02:38:30 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     Jens Axboe <axboe@...nel.dk>,
        syzbot <syzbot+c88a7030da47945a3cc3@...kaller.appspotmail.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        syzkaller-bugs@...glegroups.com, io-uring@...r.kernel.org
Subject: Re: [syzbot] WARNING in mntput_no_expire (2)

On Sun, Apr 04, 2021 at 02:34:08AM +0000, Al Viro wrote:

> I really wonder what mount is it happening to.  BTW, how painful would
> it be to teach syzcaller to turn those cascades of
> 	NONFAILING(*(uint8_t*)0x20000080 = 0x12);
> 	NONFAILING(*(uint8_t*)0x20000081 = 0);
> 	NONFAILING(*(uint16_t*)0x20000082 = 0);
> 	NONFAILING(*(uint32_t*)0x20000084 = 0xffffff9c);
> 	NONFAILING(*(uint64_t*)0x20000088 = 0);
> 	NONFAILING(*(uint64_t*)0x20000090 = 0x20000180);
> 	NONFAILING(memcpy((void*)0x20000180, "./file0\000", 8));
> 	NONFAILING(*(uint32_t*)0x20000098 = 0);
> 	NONFAILING(*(uint32_t*)0x2000009c = 0x80);
> 	NONFAILING(*(uint64_t*)0x200000a0 = 0x23456);
> 	....
> 	NONFAILING(syz_io_uring_submit(r[1], r[2], 0x20000080, 0));
> into something more readable?  Bloody annoyance every time...  Sure, I can
> manually translate it into
> 	struct io_uring_sqe *sqe = (void *)0x20000080;
> 	char *s = (void *)0x20000180;
> 	memset(sqe, '\0', sizeof(*sqe));
> 	sqe->opcode = 0x12; // IORING_OP_OPENAT?
> 	sqe->fd = -100;	// AT_FDCWD?
> 	sqe->addr = s;
> 	strcpy(s, "./file0");
> 	sqe->open_flags = 0x80;	// O_EXCL???
> 	sqe->user_data = 0x23456;	// random tag?
> 	syz_io_uring_submit(r[1], r[2], (unsigned long)p, 0);
> but it's really annoying as hell, especially since syz_io_uring_submit()
> comes from syzcaller and the damn thing _knows_ that the third argument
> is sodding io_uring_sqe, and never passed to anything other than
> memcpy() in there, at that, so the exact address can't matter.

... especially since the native syzcaller reproducer clearly *does* have
that information.  Simply putting that into comments side-by-side with
what gets put into C reproducer would be nice, especially if it goes with
field names...