[CentOS] Writing to a symlink on a read-only file system that land on a read-write file system

Mon Apr 22 12:42:54 UTC 2013
James Pearson <james-p at moving-picture.com>

We've come across a problem with 6.4 kernels that we didn't have with 
6.2 kernels - which involves writing to a symlink that is on a read-only 
file system - but the symlink lands on a read-write file system

The following shows the issue:

  mkdir -p /mnt/tmp
  mount -t tmpfs -o size=1% none /mnt/tmp
  rm -f /tmp/file
  ln -s /tmp/file /mnt/tmp/file
  mount -o remount,ro /mnt/tmp
  echo "some text" > /mnt/tmp/file

On a machine with a 6.2 kernel, the above works fine - the target of the 
symlink (/tmp/file) is created etc. with no error

But on a machine with a 6.4 kernel, the above fails with:

  /mnt/tmp/file: Read-only file system.

Strace'ing a process that fails gives:

open("/mnt/tmp/file", O_WRONLY|O_CREAT|O_TRUNC, 0666) = -1 EROFS 
(Read-only file system)

I don't have a machine with a 6.3 kernel, so I'm not sure when the 
change in behaviour happened, but does anyone know as to why this change 
was made in the kernel?

I've had a look through the kernel changelog - and the following entry 
mentions EROFS and read-only file systems:

- [fs] vfs: prefer EEXIST to EROFS when creating on an RO filesystem 
(Eric Sandeen) [878091]

I can't access that BZ (878091) entry - so don't know if the above is 
anything to do with what I'm seeing ...


James Pearson