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Date: Wed, 12 Apr 2023 14:13:48 +0100
From: Matthew Wilcox <willy@infradead.org>
To: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc: syzbot <syzbot+96cee7d33ca3f87eee86@syzkaller.appspotmail.com>,
	ntfs3@lists.linux.dev, syzkaller-bugs@googlegroups.com,
	Konstantin Komarov <almaz.alexandrovich@paragon-software.com>,
	Hillf Danton <hdanton@sina.com>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	linux-mm <linux-mm@kvack.org>, trix@redhat.com,
	ndesaulniers@google.com, nathan@kernel.org
Subject: Re: [PATCH] fs/ntfs3: disable page fault during ntfs_fiemap()
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On Wed, Apr 12, 2023 at 10:11:08PM +0900, Tetsuo Handa wrote:
> syzbot is reporting circular locking dependency between ntfs_file_mmap()
> (which has mm->mmap_lock => ni->ni_lock dependency) and ntfs_fiemap()
> (which has ni->ni_lock => mm->mmap_lock dependency).
> 
> Since ni_fiemap() is called by ioctl(FS_IOC_FIEMAP) via optional
> "struct inode_operations"->fiemap callback, I assume that importance of
> ni_fiemap() is lower than ntfs_file_mmap().
> 
> Also, since Documentation/filesystems/fiemap.rst says that "If an error
> is encountered while copying the extent to user memory, -EFAULT will be
> returned.", I assume that ioctl(FS_IOC_FIEMAP) users can handle -EFAULT
> error.

What?  No, that doesn't mean "You can return -EFAULT because random luck".
That means "If you pass it an invalid address, you'll get -EFAULT back".

NACK.