qemu linux-user doesn't translate host/target data for iovec I/O

[Mike Gelfand]

When using iovec I/O functions (like `readv`), no data translation happens. I'm hitting this issue with libevent upon constructing a bufferevent over an inotify descriptor, and then building for either ppc64 or s390x (both big-endian) on x86_64 (little-endian) and running resulting code with qemu-ppc64 or qemu-s390x on Gentoo using latest QEMU version available (5.0.0-r2).

The code in question is in https://github.com/transmission/transmission/blob/master/libtransmission/watchdir-inotify.c (`tr_watchdir_inotify_new`, `tr_watchdir_inotify_on_event`).

While `read` syscall is handled properly, `readv` (which libevent is using in my case) doesn't have any logic to call `host_to_target_data_inotify` or any other translation function, leaving inotify data unchanged (with values in little-endian), which then leads to unit test failures. Quoting `do_syscall1` implementation bits for the reference:

---8<---begin---
    case TARGET_NR_read:
        if (arg2 == 0 && arg3 == 0) {
            return get_errno(safe_read(arg1, 0, 0));
        } else {
            if (!(p = lock_user(VERIFY_WRITE, arg2, arg3, 0)))
                return -TARGET_EFAULT;
            ret = get_errno(safe_read(arg1, p, arg3));
            if (ret >= 0 &&
                fd_trans_host_to_target_data(arg1)) {
                ret = fd_trans_host_to_target_data(arg1)(p, ret);
            }
            unlock_user(p, arg2, ret);
        }
        return ret;
...
    case TARGET_NR_readv:
        {
            struct iovec *vec = lock_iovec(VERIFY_WRITE, arg2, arg3, 0);
            if (vec != NULL) {
                ret = get_errno(safe_readv(arg1, vec, arg3));
                unlock_iovec(vec, arg2, arg3, 1);
            } else {
                ret = -host_to_target_errno(errno);
            }
        }
        return ret;
---8<---end---

To reiterate, the issue is not only with `readv` but with other iovec functions as well.

[Mike Gelfand (mike.dld)]

The attached patch fixes the issue for me, but is incomplete (and not thoroughly tested) as I've only implemented inotify data translation for readv syscall.

[Thomas Huth (th-huth)]

The QEMU project is currently moving its bug tracking to another system.
For this we need to know which bugs are still valid and which could be
closed already. Thus we are setting the bug state to "Incomplete" now.

If the bug has already been fixed in the latest upstream version of QEMU,
then please close this ticket as "Fix released".

If it is not fixed yet and you think that this bug report here is still
valid, then you have two options:

1) If you already have an account on gitlab.com, please open a new ticket
for this problem in our new tracker here:

    https://gitlab.com/qemu-project/qemu/-/issues

and then close this ticket here on Launchpad (or let it expire auto-
matically after 60 days). Please mention the URL of this bug ticket on
Launchpad in the new ticket on GitLab.

2) If you don't have an account on gitlab.com and don't intend to get
one, but still would like to keep this ticket opened, then please switch
the state back to "New" or "Confirmed" within the next 60 days (other-
wise it will get closed as "Expired"). We will then eventually migrate
the ticket automatically to the new system (but you won't be the reporter
of the bug in the new system and thus you won't get notified on changes
anymore).

Thank you and sorry for the inconvenience.

[Thomas Huth (th-huth)]

This is an automated cleanup. This bug report has been moved to QEMU's
new bug tracker on gitlab.com and thus gets marked as 'expired' now.
Please continue with the discussion here:

 https://gitlab.com/qemu-project/qemu/-/issues/426