linux-kernel - Re: [PATCH 08/11] fuse: use req->page

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Date:	Thu, 25 Oct 2012 19:38:30 +0400
From:	"Maxim V. Patlasov" <mpatlasov@...allels.com>
To:	Miklos Szeredi <miklos@...redi.hu>
CC:	"fuse-devel@...ts.sourceforge.net" <fuse-devel@...ts.sourceforge.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devel@...nvz.org" <devel@...nvz.org>
Subject: Re: [PATCH 08/11] fuse: use req->page_descs[] for argpages cases

Miklos,

10/25/2012 06:05 PM, Miklos Szeredi пишет:
> Maxim Patlasov <mpatlasov@...allels.com> writes:
>
>> @@ -888,11 +888,11 @@ static int fuse_copy_pages(struct fuse_copy_state *cs, unsigned nbytes,
>>   {
>>   	unsigned i;
>>   	struct fuse_req *req = cs->req;
>> -	unsigned offset = req->page_descs[0].offset;
>> -	unsigned count = min(nbytes, (unsigned) PAGE_SIZE - offset);
>>   
>>   	for (i = 0; i < req->num_pages && (nbytes || zeroing); i++) {
>>   		int err;
>> +		unsigned offset = req->page_descs[i].offset;
>> +		unsigned count = min(nbytes, req->page_descs[i].length);
> Wouldn't it be cleaner if callers calculated the last page's .length
> value from the total number of bytes?  So this would just be
>
> 		unsigned count = req->page_descs[i].length;
>
> And at the end of the function we can assert that nbytes went to exactly
> zero with a WARN_ON().
>
> But this is a change that needs careful testing, so maybe we're better
> off having that as a separate incremental patch later...

It cannot be as simple as 'unsigned count = req->page_descs[i].length' 
because in case of short reads 'nbytes' (coming from userspace) can be 
unpredictably small. Modulo you share my opinion that a caller of 
fuse_copy_pages() shouldn't modify req->page_descs[i].length.

As for WARN_ON(), we could probably guarantee that 'nbytes' <= 
capacity(req->pages[]) in WRITEs, but in READs, 'nbytes' comes from 
userspace and I'm not sure it's OK to clutter logs due to misbehaved 
userspace fuse (if we get 'nbytes' unexpectedly large).

Thanks,
Maxim
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