From: Al Viro <viro@ZenIV.linux.org.uk>
To: Kumar Gaurav <kumargauravgupta3@gmail.com>
Cc: kernel-janitors@vger.kernel.org,
Dan Carpenter <dan.carpenter@oracle.com>,
Greg KH <gregkh@linuxfoundation.org>,
linux-kernel@vger.kernel.org
Subject: Re: Clarification needed on use of put_user inside a loop
Date: Fri, 25 Apr 2014 21:34:14 +0000 [thread overview]
Message-ID: <20140425213414.GC18016@ZenIV.linux.org.uk> (raw)
In-Reply-To: <535A88D5.3030008@gmail.com>
On Fri, Apr 25, 2014 at 09:39:57PM +0530, Kumar Gaurav wrote:
> I have found some codes in the driver which use put_user() in loop.
> Can we avoid the overhead of checking the same memory area( where
> put_user() writes) again n again using __put_user() in side loop and
> checking permission using access_ok before entering the loop?
> if (put_user(type, dst) ||
> put_user(chs_bytes, dst + 1))
> return -EFAULT;
> dst += 2;
^^^^^^^^^
Note that increment. It's *not* "the same memory area" next time
around. Sure, you can check the whole range once before the loop
and switch the stuff inside to __put_user()/__copy_to_user(), but
it's not guaranteed to buy you any speedup.
BTW, you might be a bit confused about the work done by access_ok() - e.g.
on an architectures with separate kernel and userland MMU contexts it might
very well be a no-op (always return true). It's *not* checking if user has
permissions of some sort.
WARNING: multiple messages have this Message-ID (diff)
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Kumar Gaurav <kumargauravgupta3@gmail.com>
Cc: kernel-janitors@vger.kernel.org,
Dan Carpenter <dan.carpenter@oracle.com>,
Greg KH <gregkh@linuxfoundation.org>,
linux-kernel@vger.kernel.org
Subject: Re: Clarification needed on use of put_user inside a loop
Date: Fri, 25 Apr 2014 22:34:14 +0100 [thread overview]
Message-ID: <20140425213414.GC18016@ZenIV.linux.org.uk> (raw)
In-Reply-To: <535A88D5.3030008@gmail.com>
On Fri, Apr 25, 2014 at 09:39:57PM +0530, Kumar Gaurav wrote:
> I have found some codes in the driver which use put_user() in loop.
> Can we avoid the overhead of checking the same memory area( where
> put_user() writes) again n again using __put_user() in side loop and
> checking permission using access_ok before entering the loop?
> if (put_user(type, dst) ||
> put_user(chs_bytes, dst + 1))
> return -EFAULT;
> dst += 2;
^^^^^^^^^
Note that increment. It's *not* "the same memory area" next time
around. Sure, you can check the whole range once before the loop
and switch the stuff inside to __put_user()/__copy_to_user(), but
it's not guaranteed to buy you any speedup.
BTW, you might be a bit confused about the work done by access_ok() - e.g.
on an architectures with separate kernel and userland MMU contexts it might
very well be a no-op (always return true). It's *not* checking if user has
permissions of some sort.
next prev parent reply other threads:[~2014-04-25 21:34 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-04-25 16:09 Clarification needed on use of put_user inside a loop Kumar Gaurav
2014-04-25 16:21 ` Kumar Gaurav
2014-04-25 19:05 ` Mateusz Guzik
2014-04-25 19:05 ` Mateusz Guzik
2014-04-25 21:34 ` Al Viro [this message]
2014-04-25 21:34 ` Al Viro
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