linux-kernel - Re: [PATCH v2 1/7] mm/gup: fix wrongly calculated returned value in fault_in_safe

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Message-ID: <Z+v0IghIBLwha/oX@MiWiFi-R3L-srv>
Date: Tue, 1 Apr 2025 22:11:46 +0800
From: Baoquan He <bhe@...hat.com>
To: David Hildenbrand <david@...hat.com>
Cc: akpm@...ux-foundation.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, yanjun.zhu@...ux.dev
Subject: Re: [PATCH v2 1/7] mm/gup: fix wrongly calculated returned value in
 fault_in_safe_writeable()

On 04/01/25 at 10:10am, David Hildenbrand wrote:
> On 31.03.25 10:13, Baoquan He wrote:
> > Not like fault_in_readable() or fault_in_writeable(), in
> > fault_in_safe_writeable() local variable 'start' is increased page
> > by page to loop till the whole address range is handled. However,
> > it mistakenly calcalates the size of handled range with 'uaddr - start'.
> > 
> > Fix it here.
> 
> Fixes: ? Do we know of user-visible effects?

I believe it should impact, while I didn't hear of any complaint.
Yeah, it's worth to have one Fixes.

> 
> > 
> > Signed-off-by: Baoquan He <bhe@...hat.com>
> > ---
> > v1->v2:
> > - Fix a patch log typo caused by copy-and-paste error. Thanks
> >    to Yanjun.
> > 
> >   mm/gup.c | 4 ++--
> >   1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/mm/gup.c b/mm/gup.c
> > index 855ab860f88b..73777b1de679 100644
> > --- a/mm/gup.c
> > +++ b/mm/gup.c
> > @@ -2207,8 +2207,8 @@ size_t fault_in_safe_writeable(const char __user *uaddr, size_t size)
> >   	} while (start != end);
> >   	mmap_read_unlock(mm);
> > -	if (size > (unsigned long)uaddr - start)
> > -		return size - ((unsigned long)uaddr - start);
> > +	if (size > start - (unsigned long)uaddr)
> > +		return size - (start - (unsigned long)uaddr);
> >   	return 0;
> >   }
> >   EXPORT_SYMBOL(fault_in_safe_writeable);
> 
> Can we instead just use the uaddr and start variables like in
> fault_in_readable?
> 
> That is, turn "start" into a const and adjust uaddr instead.

Sounds good to me, that makes these similar blocks own consistent code
style. Will change in v3. Thanks for reviewing.

> 
> (maybe we should also handle the types of these variables similar to as in
> fault_in_readable)
> 
> -- 
> Cheers,
> 
> David / dhildenb
>