[Bug 1893003] Re: qemu linux-user doesn't translate host/target data for iovec I/O

[Mike Gelfand]

** Summary changed:

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

** Tags added: linux-user

** Tags added: ppc s390x

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https://bugs.launchpad.net/bugs/1893003

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

Status in QEMU:
  New

Bug description:
  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.

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