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

Al Viro
2019-09-05T23:49:44Z

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...