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Date: Wed, 12 Apr 2023 15:24:12 +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 10:29:37PM +0900, Tetsuo Handa wrote:
> On 2023/04/12 22:13, Matthew Wilcox wrote:
> >> 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.
> 
> Then, why does fiemap.rst say "If an error is encountered" rather than
> "If an invalid address is passed" ?

Because people are bad at writing.

> Does the definition of -EFAULT limited to "the caller does not have permission
> to access this address" ?

Or the address isn't mapped.

> If copy_to_user() must not fail other than "the caller does not have
> permission to access this address", what should we do for now?
> Just remove ntfs_fiemap() and return -EOPNOTSUPP ?

No, fix it properly.  Or don't fix it at all and let somebody else fix it.