On Fri, Sep 06, 2019 at 12:49:44AM +0100, 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().
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...
... and __user case would be pretty much this with
if (user_access_begin(from, count)) {
....
user_access_end();
}
wrapped around the damn thing - again, see strnlen_user.c, with
unsafe_get_user(v, p++, efault);
instead of those
v = *p++;
Calling conventions might need some thinking - it might be
* all read, all zeroes
* non-zero found
* read failed
so we probably want to map the "all zeroes" case to 0,
"read failed" to -EFAULT and "non-zero found" to something
else. Might be positive, might be some other -E.... - not
sure if E2BIG (or EFBIG) makes much sense here. Need to
look at the users...
