linux-kernel - RE: [PATCH] Convert filldir[64]() from __put_user() to unsafe_put

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Message-ID: <c58c2a8a5366409abd4169d10a58196a@AcuMS.aculab.com>
Date:   Mon, 7 Oct 2019 15:40:21 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Linus Torvalds' <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>
CC:     Guenter Roeck <linux@...ck-us.net>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: RE: [PATCH] Convert filldir[64]() from __put_user() to
 unsafe_put_user()

> From: Linus Torvalds
> Sent: 07 October 2019 04:12
...
> In this case, I think it's done a few callers up in i915_gem_pread_ioctl():
> 
>         if (!access_ok(u64_to_user_ptr(args->data_ptr),
>                        args->size))
>                 return -EFAULT;
> 
> but honestly, trying to optimize away another "access_ok()" is just
> not worth it. I'd rather have an extra one than miss one.

You don't really want an extra access_ok() for every 'word' of a copy.
Some copies have to be done a word at a time.

And the checks someone added to copy_to/from_user() to detect kernel
buffer overruns must kill performance when the buffers are way down the stack
or in kmalloc()ed space.

	David

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