Re: [PATCH v12 01/12] lib: introduce copy_struct_{to,from}_user helpers

Al Viro
2019-09-06T00:14:31Z

On Fri, Sep 06, 2019 at 12:49:44AM +0100, Al Viro wrote:
> On Fri, Sep 06, 2019 at 09:00:03AM +1000, Aleksa Sarai wrote:
> > > > + return -EFAULT;
> > > > + }
> > > > + /* Copy the interoperable parts of the struct. */
> > > > + if (__copy_to_user(dst, src, size))
> > > > + return -EFAULT;
> > >
> > > Why not simply clear_user() and copy_to_user()?
> >
> > I'm not sure I understand what you mean -- are you asking why we need to
> > do memchr_inv(src + size, 0, rest) earlier?
>
> I'm asking why bother with __ and separate access_ok().
>
> > > if ((unsigned long)addr & 1) {
> > > u8 v;
> > > if (get_user(v, (__u8 __user *)addr))
> > > return -EFAULT;
> > > if (v)
> > > return -E2BIG;
> > > addr++;
> > > }
> > > if ((unsigned long)addr & 2) {
> > > u16 v;
> > > if (get_user(v, (__u16 __user *)addr))
> > > return -EFAULT;
> > > if (v)
> > > return -E2BIG;
> > > addr +=2;
> > > }
> > > if ((unsigned long)addr & 4) {
> > > u32 v;
> > > if (get_user(v, (__u32 __user *)addr))
> > > return -EFAULT;
> > > if (v)
> > > return -E2BIG;
> > > }
> > > <read the rest like you currently do>
>
> Actually, this is a dumb way to do it - page size on anything
> is going to be a multiple of 8, so you could just as well
> read 8 bytes from an address aligned down. Then mask the
> bytes you don't want to check out and see if there's anything
> left.
>
> You can have readability boundaries inside a page - it's either
> the entire page (let alone a single word) being readable, or
> it's EFAULT for all parts.
>
> > > would be saner, and things like x86 could trivially add an
> > > asm variant - it's not hard. Incidentally, memchr_inv() is
> > > an overkill in this case...
> >
> > Why is memchr_inv() overkill?
>
> Look at its implementation; you only care if there are
> non-zeroes, you don't give a damn where in the buffer
> the first one would be. All you need is the same logics
> as in "from userland" case
> if (!count)
> return true;
> offset = (unsigned long)from & 7
> p = (u64 *)(from - offset);
> v = *p++;
> if (offset) { // unaligned
> count += offset;
> v &= ~aligned_byte_mask(offset); // see strnlen_user.c
> }
> while (count > 8) {
> if (v)
> return false;
> v = *p++;
> count -= 8;
> }
> if (count != 8)
> v &= aligned_byte_mask(count);
> return v == 0;
>
> All there is to it...

... and __user case would be pretty much this with
if (user_access_begin(from, count)) {
....
user_access_end();
}
wrapped around the damn thing - again, see strnlen_user.c, with
unsafe_get_user(v, p++, efault);
instead of those
v = *p++;

Calling conventions might need some thinking - it might be
* all read, all zeroes
* non-zero found
* read failed
so we probably want to map the "all zeroes" case to 0,
"read failed" to -EFAULT and "non-zero found" to something
else. Might be positive, might be some other -E.... - not
sure if E2BIG (or EFBIG) makes much sense here. Need to
look at the users...