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Date: Fri, 6 Apr 2018 12:35:45 +0100
From: Alan Cox <gnomes@lxorguk.ukuu.org.uk>
Subject: Re: [PATCH] gup: return -EFAULT on access_ok failure
Message-ID: <20180406123545.24953eb4@alans-desktop>
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To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>, Al Viro <viro@zeniv.linux.org.uk>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, stable <stable@vger.kernel.org>, syzbot+6304bf97ef436580fede@syzkaller.appspotmail.com, linux-mm <linux-mm@kvack.org>, "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>, Andrew Morton <akpm@linux-foundation.org>, Huang Ying <ying.huang@intel.com>, Jonathan Corbet <corbet@lwn.net>, Peter Zijlstra <peterz@infradead.org>, Thomas Gleixner <tglx@linutronix.de>, Thorsten Leemhuis <regressions@leemhuis.info>

> so an error on the 1st page gets propagated to the caller,
> and that get_user_pages_unlocked eventually calls __get_user_pages
> so it does return an error sometimes.
> 
> Would it be correct to apply the second part of the patch then
> (pasted below for reference) or should get_user_pages_fast
> and all its callers be changed to return 0 on error instead?

0 isn't an error. As SuS sees it (ie from the userspace end of the pile)

returning the number you asked for means it worked

returning a smaller number means it worked partially and that much was
consumed (or in some cases more and the rest if so was lost - depends
what you are reading/writing)

returning 0 means you read nothing as you were at the end of file

returning an error code means it broke, or you should try again
(EAGAIN/EWOULDBLOCK)

The ugly bit there is the try-again semantics needs to exactly match the
attached poll() behaviour or you get busy loops.

Alan