RE: [PATCH 02/11] mm: call import_iovec() instead of rw_copy_check_uvector() in process_vm_rw()

David Laight
2020-09-21T15:21:48Z

From: Al Viro
> Sent: 21 September 2020 16:02
>
> On Mon, Sep 21, 2020 at 04:34:25PM +0200, Christoph Hellwig wrote:
> > From: David Laight <David.Laight@ACULAB.COM>
> >
> > This is the only direct call of rw_copy_check_uvector(). Removing it
> > will allow rw_copy_check_uvector() to be inlined into import_iovec(),
> > while only paying a minor price by setting up an otherwise unused
> > iov_iter in the process_vm_readv/process_vm_writev syscalls that aren't
> > in a super hot path.
>
> > @@ -443,7 +443,7 @@ void iov_iter_init(struct iov_iter *i, unsigned int direction,
> > const struct iovec *iov, unsigned long nr_segs,
> > size_t count)
> > {
> > - WARN_ON(direction & ~(READ | WRITE));
> > + WARN_ON(direction & ~(READ | WRITE | CHECK_IOVEC_ONLY));
> > direction &= READ | WRITE;
>
> Ugh...
>
> > - rc = rw_copy_check_uvector(CHECK_IOVEC_ONLY, rvec, riovcnt, UIO_FASTIOV,
> > - iovstack_r, &iov_r);
> > + rc = import_iovec(CHECK_IOVEC_ONLY, rvec, riovcnt, UIO_FASTIOV, &iov_r,
> > + &iter_r);
> > if (rc <= 0)
> > goto free_iovecs;
> >
> > - rc = process_vm_rw_core(pid, &iter, iov_r, riovcnt, flags, vm_write);
> > + rc = process_vm_rw_core(pid, &iter_l, iter_r.iov, iter_r.nr_segs,
> > + flags, vm_write);
>
> ... and ugh^2, since now you are not only setting a meaningless iov_iter,
> you are creating a new place that pokes directly into struct iov_iter
> guts.
>
> Sure, moving rw_copy_check_uvector() over to lib/iov_iter.c makes sense.
> But I would rather split the access_ok()-related checks out of that thing
> and bury CHECK_IOVEC_ONLY.
>
> Step 1: move the damn thing to lib/iov_iter.c (same as you do, but without
> making it static)
>
> Step 2: split it in two:
>
> ssize_t rw_copy_check_uvector(const struct iovec __user * uvector,
> unsigned long nr_segs, unsigned long fast_segs,
> struct iovec *fast_pointer,
> struct iovec **ret_pointer)
> {
> unsigned long seg;
...
> ret = 0;
> for (seg = 0; seg < nr_segs; seg++) {
> void __user *buf = iov[seg].iov_base;
> ssize_t len = (ssize_t)iov[seg].iov_len;
>
> /* see if we we're about to use an invalid len or if
> * it's about to overflow ssize_t */
> if (len < 0)
> return -EINVAL;
> if (len > MAX_RW_COUNT - ret) {
> len = MAX_RW_COUNT - ret;
> iov[seg].iov_len = len;
> }
> ret += len;
> }
> return ret;
> }
>
> /*
> * This is merely an early sanity check; we do _not_ rely upon
> * it when we get to the actual memory accesses.
> */
> static bool check_iovecs(const struct iovec *iov, int nr_segs)
> {
> for (seg = 0; seg < nr_segs; seg++) {
> void __user *buf = iov[seg].iov_base;
> ssize_t len = (ssize_t)iov[seg].iov_len;
>
> if (unlikely(!access_ok(buf, len)))
> return false;
> }
> return true;
> }

You really don't want to be looping through the array twice.
In fact you don't really want to be doing all those tests at all.
This code makes a significant fraction of the not-insignificant
difference between the 'costs' of send() and sendmsg().

I think the 'length' check can be optimised to do something like:
for (...) {
ssize_t len = (ssize_t)iov[seg].iov_len;
ret += len;
len_hi += (unsigned long)len >> 20;
}
if (len_hi) {
/* Something potentially odd in the lengths.
* Might just be a very long fragment.
* Check the individial values. */
Add the exiting loop here.
}

David

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