linux-kernel - Re: [syzbot] [overlayfs?] possible deadlock in seq_read

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Date: Sun, 28 Jan 2024 21:43:35 +0000
From: Al Viro <viro@...iv.linux.org.uk>
To: Hillf Danton <hdanton@...a.com>
Cc: syzbot <syzbot+da4f9f61f96525c62cc7@...kaller.appspotmail.com>,
	linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [overlayfs?] possible deadlock in seq_read_iter (2)

On Sat, Jan 27, 2024 at 07:46:10PM +0800, Hillf Danton wrote:
> On Tue, 19 Dec 2023 11:43:27 -0800
> > syzbot has found a reproducer for the following issue on:
> > 
> > HEAD commit:    2cf4f94d8e86 Merge tag 'scsi-fixes' of git://git.kernel.or..
> > git tree:       upstream
> > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=154aa8d6e80000
> 
> #syz test https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git  2cf4f94d8e86
> 
> --- x/fs/namei.c
> +++ y/fs/namei.c
> @@ -3533,6 +3533,8 @@ static const char *open_last_lookups(str
>  
>  	if (open_flag & (O_CREAT | O_TRUNC | O_WRONLY | O_RDWR)) {
>  		got_write = !mnt_want_write(nd->path.mnt);
> +		if (!got_write && (open_flag & O_CREAT))
> +			return ERR_PTR(-EISDIR);

NAK.

Please, RTFComment just below your addition.  Besides, EISDIR is
obviously bogus in a lot of cases, starting with attempting to
create a new file on a read-only filesystem.  Surely
echo bugger > /mnt/cdrom/no_such_file_there
should *not* fail with "no_such_file_there: Is a directory"?