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


From: Thomas Huth
Subject: [Bug 1893003] Re: qemu linux-user doesn't translate host/target data for iovec I/O
Date: Thu, 17 Jun 2021 07:12:15 -0000

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


** Changed in: qemu
       Status: Incomplete => Expired

** Bug watch added: gitlab.com/qemu-project/qemu/-/issues #426
   https://gitlab.com/qemu-project/qemu/-/issues/426

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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:
  Expired

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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