linux-kernel - Re: [PATCH] gup: return -EFAULT on access

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

> 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