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Date:   Fri, 6 Sep 2019 00:49:44 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Aleksa Sarai <cyphar@cyphar.com>
Cc:     Jeff Layton <jlayton@kernel.org>,
        "J. Bruce Fields" <bfields@fieldses.org>,
        Arnd Bergmann <arnd@arndb.de>,
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Subject: Re: [PATCH v12 01/12] lib: introduce copy_struct_{to,from}_user
 helpers
Message-ID: <20190905234944.GT1131@ZenIV.linux.org.uk>
References: <20190904201933.10736-1-cyphar@cyphar.com>
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 <20190905230003.bek7vqdvruzi4ybx@yavin.dot.cyphar.com>
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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...