[Bug 1902109] Re: rsync uses lchmod and fails in Ubuntu >= 20.10 if /proc isn't mounted

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** Bug watch added: Red Hat Bugzilla #1894485
   https://bugzilla.redhat.com/show_bug.cgi?id=1894485

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https://bugs.launchpad.net/bugs/1902109

Title:
  rsync uses lchmod and fails in Ubuntu >= 20.10 if /proc isn't mounted

Status in GLibC:
  Confirmed
Status in glibc package in Ubuntu:
  New
Status in rsync package in Ubuntu:
  Triaged

Bug description:
  Rsync in Ubuntu 20.10 fails when /proc isn't mounted, while it worked before.
  This happens because AC_CHECK_FUNC(lchmod) returns "yes" in 20.10, while it returned "no" before.

  Steps to reproduce:

  # Emulate /proc not being mounted
  $ mount --bind / /mnt
  $ chroot /mnt rsync -a /bin/ls .
  rsync: [receiver] failed to set permissions on "/.ls.CDExhu": Operation not supported (95)
  rsync error: some files/attrs were not transferred (see previous errors) (code 3) at main.c(1330) [sender=3.2.3]

  I reported this issue upstream in
  https://github.com/WayneD/rsync/issues/109 but the rsync developer
  says it's a problem in libc, and it might well be.

  Simple C code to reproduce the problem without rsync:

  printf("lchmod returned: %d\n", lchmod("/tmp/ls", 0755));

  If /tmp/ls is e.g. mode=0123, and needs to be changed, lchmod fails
  when /proc isn't mounted, yet it succeeds if it is mounted.

  Python had a similar issue, and they ended up avoiding
  AC_CHECK_FUNC(lchmod) under Linux:

  https://bugs.python.org/issue34652
  https://github.com/python/cpython/commit/69e96910153219b0b15a18323b917bd74336d229#diff-49473dca262eeab3b4a43002adb08b4db31020d190caaad1594b47f1d5daa810R3140

  ```c
  if test "$MACHDEP" != linux; then
    AC_CHECK_FUNC(lchmod)
  fi
  ```

  So I'm not sure which package is causing the bug here. Should autoconf
  return false? Should libc implement lchown without the bug? Or should
  rsync skip lchmod under Linux, like python did?

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