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

[m.roth at 5-cent.us]

James Pearson wrote:
> 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
<snip>
That's weird, all right... but I would *never* have tried that, because I
assume that ro mean READ ONLY. IMO, if you could write *anything* to a
read-only filesystem, that was a serious bug, both in design and in
security (gee, what a *great* way to get malware where it shouldn't be!).

       mark