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

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On 2019-09-06, Al Viro <viro@xxxxxxxxxxxxxxxxxx> 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().

Ah right, it was a dumb "optimisation" (since we need to do access_ok()
anyway since we should early -EFAULT in that case). I've dropped the __
usages in my working copy.

> > > 	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...

Alright, will do (for some reason I hadn't made the connection that
memchr_inv() is doing effectively the same word-by-word comparison but
also detecting where the first byte is).

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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