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Date:   Fri, 6 Apr 2018 00:08:08 +0300
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     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>
Subject: Re: [PATCH] gup: return -EFAULT on access_ok failure
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On Thu, Apr 05, 2018 at 11:43:27AM -0700, Linus Torvalds wrote:
> On Thu, Apr 5, 2018 at 11:28 AM, Michael S. Tsirkin <mst@redhat.com> wrote:
> >
> > to repeat what you are saying IIUC __get_user_pages_fast returns 0 if it can't
> > pin any pages and that is by design.  Returning 0 on error isn't usual I think
> > so I guess this behaviour should we well documented.
> 
> Arguably it happens elsewhere too, and not just in the kernel.
> "read()" at past the end of a file is not an error, you'll just get 0
> for EOF.
> 
> So it's not really "returning 0 on error".
> 
> It really is simply returning the number of pages it got. End of
> story. That number of pages can be smaller than the requested number
> of pages, and _that_ is due to some error, but note how it can return
> "5" on error too - you asked for 10 pages, but the error happened in
> the middle!
> 
> So the right way to check for error is to bverify that you get the
> number of pages that you asked for. If you don't, something bad
> happened.
> 
> Of course, many users don't actually care about "I didn't get
> everything". They only care about "did I get _something_". Then that 0
> ends up being the error case, but note how it depends on the caller.
> 
> > What about get_user_pages_fast though?
> 
> We do seem to special-case the first page there. I'm not sure it's a
> good idea. But like the __get_user_pages_fast(), we seem to have users
> that know about the particular semantics and depend on it.
> 
> It's all ugly, I agree.
> 
> End result: we can't just change semantics of either of them.
> 
> At least not without going through every single user and checking that
> they are ok with it.
> 
> Which I guess I could be ok with. Maybe changing the semantics of
> __get_user_pages_fast() is acceptable, if you just change it
> *everywhere* (which includes not just he users, but also the couple of
> architecture-specific versions of that same function that we have.
> 
>                     Linus

For now I sent a patchset
1. documenting current behaviour for __get_user_pages_fast.
2. fixing get_user_pages_fast for consistency.

-- 
MST