On Fri, Sep 06, 2019 at 05:56:18AM +1000, Aleksa Sarai wrote:
On 2019-09-05, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
On Thu, Sep 05, 2019 at 08:23:03PM +0200, Christian Brauner wrote:
Because every caller of that function right now has that limit set
anyway iirc. So we can either remove it from here and place it back for
the individual callers or leave it in the helper.
Also, I'm really asking, why not? Is it unreasonable to have an upper
bound on the size (for a long time probably) or are you disagreeing with
PAGE_SIZE being used? PAGE_SIZE limit is currently used by sched, perf,
bpf, and clone3 and in a few other places.
For a primitive that can be safely used with any size (OK, any within
the usual 2Gb limit)? Why push the random policy into the place where
it doesn't belong?
Seriously, what's the point? If they want to have a large chunk of
userland memory zeroed or checked for non-zeroes - why would that
be a problem?
Thinking about it some more, there isn't really any r/w amplification --
so there isn't much to gain by passing giant structs. Though, if we are
going to permit 2GB buffers, isn't that also an argument to use
memchr_inv()? :P
I'm not sure I understand the last bit. If you look at what copy_from_user()
does on misaligned source/destination, especially on architectures that
really, really do not like unaligned access...
Case in point: alpha (and it's not unusual in that respect). What it boils
down to is
copy bytes until the destination is aligned
if source and destination are both aligned
copy word by word
else
read word by word, storing the mix of two adjacent words
copy the rest byte by byte
The unpleasant case (to and from having different remainders modulo 8) is
basically
if (count >= 8) {
u64 *aligned = (u64 *)(from & ~7);
u64 *dest = (u64 *)to;
int bitshift = (from & 7) * 8;
u64 prev, next;
prev = aligned[0];
do {
next = aligned[1];
prev <<= bitshift;
prev |= next >> (64 - bitshift);
*dest++ = prev;
aligned++;
prev = next;
from += 8;
to += 8;
count -= 8;
} while (count >= 8);
}
Now, mix that with "... and do memchr_inv() on the copy to find if we'd
copied any non-zeroes, nevermind where" and it starts looking really
ridiculous.
We should just read the fscking source, aligned down to word boundary
and check each word being read. The first and the last ones - masked.
All there is to it. On almost all architectures that'll work well
enough; s390 might want something more elaborate (there even word-by-word
copies are costly, but I'd suggest talking to them for details).
Something like bool all_zeroes_user(const void __user *p, size_t count)
would probably be a sane API...
