On 2019-09-06, Al Viro viro@zeniv.linux.org.uk 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).