From: Michal Hocko <mhocko@kernel.org>
To: Peter Xu <peterx@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Linux-MM <linux-mm@kvack.org>,
Andrew Morton <akpm@linux-foundation.org>,
syzbot+693dc11fcb53120b5559@syzkaller.appspotmail.com
Subject: Re: [PATCH 1/2] mm/mempolicy: Allow lookup_node() to handle fatal signal
Date: Tue, 14 Apr 2020 16:18:59 +0200 [thread overview]
Message-ID: <20200414141859.GM4629@dhcp22.suse.cz> (raw)
In-Reply-To: <20200414134906.GF38470@xz-x1>
On Tue 14-04-20 09:49:06, Peter Xu wrote:
> On Tue, Apr 14, 2020 at 01:04:29PM +0200, Michal Hocko wrote:
>
> [...]
>
> > @@ -1247,6 +1248,10 @@ int fixup_user_fault(struct task_struct *tsk, struct mm_struct *mm,
> > }
> > EXPORT_SYMBOL_GPL(fixup_user_fault);
> >
> > +/*
> > + * Please note that this function, unlike __get_user_pages will not
> > + * return 0 for nr_pages > 0 without FOLL_NOWAIT
> > + */
> > static __always_inline long __get_user_pages_locked(struct task_struct *tsk,
> > struct mm_struct *mm,
> > unsigned long start,
> > diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> > index 48ba9729062e..1965e2681877 100644
> > --- a/mm/mempolicy.c
> > +++ b/mm/mempolicy.c
> > @@ -927,10 +927,7 @@ static int lookup_node(struct mm_struct *mm, unsigned long addr)
> >
> > int locked = 1;
> > err = get_user_pages_locked(addr & PAGE_MASK, 1, 0, &p, &locked);
> > - if (err == 0) {
> > - /* E.g. GUP interrupted by fatal signal */
> > - err = -EFAULT;
> > - } else if (err > 0) {
> > + if (err > 0) {
> > err = page_to_nid(p);
> > put_page(p);
> > }
>
> Hi, Michal,
>
> IIUC this is not the only place that we check against ret==0 for gup.
> For example, the other direct caller of the same function,
> get_vaddr_frames(), which will set -EFAULT too if ret==0. So do we
> want to change all the places and don't check against zero explicitly?
This would require to analyze each such a call. For example
get_vaddr_frames has to handle get_user_pages_locked returning 0 because
it allows callers to specify FOLL_NOWAIT. Whether EFAULT is a proper
return value for that case is a question I didn't really get to analyze.
> I'm now thinking whether this would be good even if we refactored gup
> and only allow it to return either >0 as number of page pinned, or <0
> for all the rest. I'm not sure how others will see this, but the
> answer is probably the same at least to me as before for this issue.
I would consider a semantic without that special case for FOLL_NOWAIT
much more clear but I do not really understand the historical background
for it TBH so I do not dare to touch that.
> As a caller, I'll see gup as a black box. Even if the gup function
> guarantees that the retcode won't be zero and documented it, I (as a
> caller) will be using that to index page array so I'd still better to
> check that value before I do anything (because it's meaningless to
> index an array with zero size), and a convertion of "ret==0" -->
> "-EFAULT" (or some other failures) in this case still makes sense.
> While removing that doesn't help a lot, imho, but instead make it
> slightly unsafer.
Well, my experience tells me that people really love to copy&paste code
and error handling and if the error handling is bogus it just spreads
all over the place until it really defines a new standard which is close
to impossible to get rid of. So if the error handling can be done
properly then I would really prefer it. In the above case it is clearly
misleading, because fatal signal should be never reflected by err==0.
--
Michal Hocko
SUSE Labs
next prev parent reply other threads:[~2020-04-14 14:19 UTC|newest]
Thread overview: 56+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-04-08 1:40 [PATCH 0/2] mm: Two small fixes for recent syzbot reports Peter Xu
2020-04-08 1:40 ` [PATCH 1/2] mm/mempolicy: Allow lookup_node() to handle fatal signal Peter Xu
2020-04-08 10:21 ` Michal Hocko
2020-04-08 14:20 ` Peter Xu
2020-04-08 14:30 ` Michal Hocko
2020-04-08 15:24 ` Peter Xu
2020-04-08 15:26 ` Michal Hocko
2020-04-09 7:02 ` Michal Hocko
2020-04-09 12:52 ` Peter Xu
2020-04-09 13:00 ` Peter Xu
2020-04-09 13:53 ` Michal Hocko
2020-04-09 16:42 ` Linus Torvalds
2020-04-14 11:04 ` Michal Hocko
2020-04-14 13:49 ` Peter Xu
2020-04-14 14:18 ` Michal Hocko [this message]
2020-04-20 12:47 ` Michal Hocko
2020-04-20 17:31 ` Linus Torvalds
2020-04-21 7:09 ` Michal Hocko
2020-04-08 1:40 ` [PATCH 2/2] mm/gup: Mark lock taken only after a successful retake Peter Xu
2020-04-09 0:47 ` [PATCH 0/2] mm: Two small fixes for recent syzbot reports Andrew Morton
2020-04-09 11:49 ` Matthew Wilcox
2020-04-09 13:00 ` Dmitry Vyukov
2020-04-09 18:16 ` Andrew Morton
2020-04-09 18:53 ` Linus Torvalds
2020-04-09 19:12 ` Andrew Morton
2020-04-09 19:46 ` Linus Torvalds
2020-04-09 19:56 ` Matthew Wilcox
2020-04-09 19:58 ` Linus Torvalds
2020-04-09 20:27 ` Eric Biggers
2020-04-09 20:34 ` Linus Torvalds
2020-04-09 23:34 ` Stephen Rothwell
2020-04-10 1:11 ` Theodore Y. Ts'o
2020-04-09 12:55 ` Dmitry Vyukov
2020-04-09 16:32 ` Linus Torvalds
2020-04-09 16:58 ` Qian Cai
2020-04-09 17:05 ` Linus Torvalds
2020-04-09 17:58 ` Qian Cai
2020-04-09 18:06 ` Linus Torvalds
2020-04-09 21:14 ` Qian Cai
2020-04-10 13:12 ` Tetsuo Handa
2020-04-10 14:26 ` Qian Cai
2020-04-10 17:26 ` Andrew Morton
2020-04-10 19:46 ` Qian Cai
2020-04-09 23:29 ` Stephen Rothwell
2020-04-13 22:06 ` Qian Cai
2020-04-13 23:05 ` Jens Axboe
2020-04-14 11:12 ` Dmitry Vyukov
2020-04-14 11:59 ` Qian Cai
2020-04-14 12:05 ` Dmitry Vyukov
2020-04-14 19:28 ` Dan Rue
2020-04-15 11:09 ` Dmitry Vyukov
2020-04-15 16:23 ` Dan Rue
2020-04-16 0:34 ` Stephen Rothwell
2020-05-11 15:29 ` Dmitry Vyukov
2020-04-14 4:07 ` Hillf Danton
2020-04-14 4:31 ` Jens Axboe
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