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

On 2019-09-06, Al Viro [off-list ref] wrote:

On Fri, Sep 06, 2019 at 09:00:03AM +1000, Aleksa Sarai wrote:

quoted

quoted

quoted

+			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.

quoted

quoted

	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.

quoted

quoted

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/>