Re: [RFC PATCH 2/4] uaccess: Add non-pagefault user-space read functions

[Masami Hiramatsu <mhiramat@kernel.org>]

Hi Linus,

On Mon, 25 Feb 2019 09:00:57 -0800
Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Mon, Feb 25, 2019 at 7:06 AM Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > Would something like so work for people?
> 
> Looks reasonable to me.
> 
> > Why not keep it simple:
> >
> >         mm_segment_t old_fs = get_fs();
> >
> >         set_fs(USER_DS);
> >         ret = __strncpy...();
> >         set_fs(old_fd);
> >
> >         return ret;
> 
> So none of this code looks sane. First odd, there's no real reason to
> use __get_user(). The thing should never be used. It does the whole
> stac/clac for every byte.

Ah, I got it. I just followed the commit bd28b14591b9 ("x86: remove more
uaccess_32.h complexity") as same as strnlen_from_unsafe(). No special
reason.

> 
> In the copy_from_user() case, I suggested re-doing it as one common
> routine without the set_fs() dance for the "already there" case to
> simplify error handling. Here it doesn't do that.
> 
> But honestly, I think for the strncpy case, we could just do
> 
>   long strncpy_from_unsafe_user(char *dst, const void __user *src, long count)
>   {
>       long ret;
>       mm_segment_t old_fs = get_fs();
> 
>       set_fs(USER_DS);
>       pagefault_disable();
>       ret = strncpy_from_user(dst, src, count);
>       pagefault_enable();
>       set_fs(old_fs);
>       return ret;
>   }
> 
> and be done with it. Efficient and simple.

Yes, it looks good to me :)

> 
> Note: the above will *only* work for actual user addresses, because
> strncpy_from_user() does that proper range check.

I think we can reuse do_strncpy_from_user() for strncpy_from_unsafe().
(so maybe we should move it from mm/maccess.c to lib/strncpy_from_user.c?)

As Kees pointed out, I think it is a good chance to sort the behavior of
these strXcpy APIs to match their names.

Thank you,

-- 
Masami Hiramatsu <mhiramat@kernel.org>