NFS Permission denied instead of EROFS

[Jan Engelhardt <jengelh@linux01.gwdg.de>]

Hello lists,


I am extending this topic to LKML to post a question that goes beyond the 
scope of unionfs. First, a description:

unionfs depends on filesystems being read-only returning EROFS to properly 
do copyup.
Unfortunately, NFS mounts that are exported with the "ro" flag, and which 
have been imported/mounted with the "rw" flag, either implicit (i.e. 
automatically added by /bin/mount) or explicit using "-o rw", do return 
EACCES instead of EROFS for opening already-existing files with the write 
bit. Or, in other words:


# grep export /etc/exports 
/export *(ro,async,no_root_squash)
# mount localhost:/export /mnt -t nfs -oro
# logout
$ cat /proc/mounts | grep nfs
localhost:/export /mnt nfs ro,v3,rsize=32768,wsize=32768,hard,tcp,lock,addr=localhost 0 0
$ ls -l /mnt/file
-rw-r--r--  1 root root 0 Oct 27 22:42 /mnt/file
$ touch /mnt/file
touch: cannot touch `file': Read-only file system
$ strace -e trace=file touch /mnt/file
open("/mnt/file", O_WRONLY|O_NONBLOCK|O_CREAT|O_NOCTTY|O_LARGEFILE, 0666) = 
-1 EACCES (Permission denied)
utimes("/mnt/file", NULL) = -1 EACCES (Permission denied)
  ...
touch: cannot touch `/mnt/file': Permission denied


If the file does not exist however, everything is ok:
$ touch /mnt/file
touch: cannot touch /mnt/file': Read-only file system
$ strace -e trace=file touch /mnt/file
open("/mnt/file", O_WRONLY|O_NONBLOCK|O_CREAT|O_NOCTTY|O_LARGEFILE, 0666) =
-1 EROFS (Read-only file system)
utimes("/mnt/file", NULL) = -1 ENOENT (No such file or directory)


Which brings me right to question... should mountd or knfsd be adjusted to
refuse a rw mount request if an nfs export is only available as ro? Like it is
already the case with normal block devices:

# strace -e trace=mount mount /dev/hdb /F -o rw -t iso9660
mount("/dev/hd/b", "/F", "iso9660", MS_POSIXACL|MS_ACTIVE|MS_NOUSER|0xec0000,
0x8067cd0) = -1 EROFS (Read-only file system)
mount: block device /dev/hd/b is write-protected, mounting read-only
mount("/dev/hd/b", "/F", "iso9660",
MS_RDONLY|MS_POSIXACL|MS_ACTIVE|MS_NOUSER|0xec0000, 0x80680c8) = 0



Jan Engelhardt
-- 
| Alphagate Systems, http://alphagate.hopto.org/
| jengelh's site, http://jengelh.hopto.org/
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