[uml-devel] hostfs oddities

[Ingo van Lil <inguin@gmx.de>]

Hi there,

I noticed two little misfeatures in the hostfs file system. The first 
one is most certainly a bug, the second one might be functioning as 
intended.

1. If a file is held read-only by one process it cannot by creat()'ed by 
another one:

   [root@localhost ~]# mount -t hostfs none /mnt
   [root@localhost ~]# cd /mnt/tmp
   [root@localhost tmp]# touch foo
   [root@localhost tmp]# tail -f foo &
   [1] 895
   [root@localhost tmp]# > foo
   -bash: foo: Invalid argument
   [root@localhost tmp]# kill 895
   [1]+  Terminated              tail -f foo
   [root@localhost tmp]# > foo

I tracked that problem down to the set_attr() function in hostfs_user.c: 
It tries to ftruncate() the read-only file descriptor. The file should 
be re-opened writable before changing its length.

2. As long as a file is held open changes made outside the UML kernel 
(i.e. on the host side) may not be visible:

   [root@localhost tmp]# tail -f foo &
   [1] 897
   # Host system: echo "Hello World" > /tmp/foo
   [root@localhost tmp]# cat foo
   [root@localhost tmp]# kill 897
   [1]+  Terminated              tail -f foo
   [root@localhost tmp]# cat foo
   Hello World

I guess this is some kind of caching effect, because strace shows that 
there is only a single read to the actual physical file. Maybe hostfs 
should check whether the outside file has changed and invalidate the 
cache if necessary.

Cheers,
Ingo

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