Re: RFD: x32 ABI system call numbers

[Arnd Bergmann <arnd@arndb.de>]

On Monday 05 September 2011 12:59:50 H. Peter Anvin wrote:
> On 09/05/2011 12:54 PM, H.J. Lu wrote:
> > 
> > Since  readv/writev/preadv/pwritev have const struct iovec *iov, I
> > have to copy the whole array.  compat_sys seems more efficient.
> > 
> 
> compat_sys for these do exactly what we want, right?

Quoting from compat_rw_copy_check_uvector():

        if (nr_segs > fast_segs) {
                ret = -ENOMEM;
                iov = kmalloc(nr_segs*sizeof(struct iovec), GFP_KERNEL);
                if (iov == NULL)
                        goto out;
        }
        *ret_pointer = iov;

        /*
         * Single unix specification:
         * We should -EINVAL if an element length is not >= 0 and fitting an
         * ssize_t.
         *
         * In Linux, the total length is limited to MAX_RW_COUNT, there is
         * no overflow possibility.
         */
        tot_len = 0;
        ret = -EINVAL;
        for (seg = 0; seg < nr_segs; seg++) {
                compat_uptr_t buf;
                compat_ssize_t len;

                if (__get_user(len, &uvector->iov_len) ||
                   __get_user(buf, &uvector->iov_base)) {
                        ret = -EFAULT;
                        goto out;
                }
                if (len < 0)    /* size_t not fitting in compat_ssize_t .. */
                        goto out;
                if (!access_ok(vrfy_dir(type), compat_ptr(buf), len)) {
                        ret = -EFAULT;
                        goto out;
                }
                if (len > MAX_RW_COUNT - tot_len)
                        len = MAX_RW_COUNT - tot_len;
                tot_len += len;
                iov->iov_base = compat_ptr(buf);
                iov->iov_len = (compat_size_t) len;
                uvector++;
                iov++;
        }


compared to native rw_copy_check_uvector():

        if (copy_from_user(iov, uvector, nr_segs*sizeof(*uvector))) {
                ret = -EFAULT;
                goto out;
        }

        /*
         * According to the Single Unix Specification we should return EINVAL
         * if an element length is < 0 when cast to ssize_t or if the
         * total length would overflow the ssize_t return value of the
         * system call.
         *
         * Linux caps all read/write calls to MAX_RW_COUNT, and avoids the
         * overflow case.
         */
        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) {
                        ret = -EINVAL;
                        goto out;
                }
                if (unlikely(!access_ok(vrfy_dir(type), buf, len))) {
                        ret = -EFAULT;
                        goto out;
                }
                if (len > MAX_RW_COUNT - ret) {
                        len = MAX_RW_COUNT - ret;
                        iov[seg].iov_len = len;
                }
                ret += len;
        }

This is better than I thought for the compat version. The only overhead
is in reading the array in word chunks as opposed to a single memcpu for
the native case. This should barely be noticeably within the other stuff
done in the same function. So you are both right, the compat case is good.

I was assuming that this would do something worse, like an extra copy
of the data back to userspace.

	Arnd