On 2019-09-06, 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(). 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; > > > } > > > > > 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