The Samba-Bugzilla – Bug 11606
vfs_stream_xattr incorrect assumption of null byte in attribute value
Last modified: 2018-06-29 21:28:47 UTC
At least these two places in ADS over xattr vfs make assume that extended attribute value must be null-terminated:
"source3/modules/vfs_streams_xattr.c" line 491
* Darn, xattrs need at least 1 byte
char null = '\0';
"source3/modules/vfs_streams_xattr.c" line 1024
length = ea.value.length-1;
DEBUG(10, ("streams_xattr_pread: get_ea_value returned %d bytes\n",
However linux extended attribute man pages are clear that zero-length values are in fact allowed:
setxattr() sets the value of the extended attribute identified by
name and associated with the given path in the filesystem. The size
argument specifies the size (in bytes) of value; a zero-length value
My use case is that I want to set extended attributes natively on the samba server and have those readable as ADS for windows clients.
Currently if I set 8-byte value, clients see 7-byte value (sans last byte) and I'll probably have to work around samba's vfs module and include a dummy byte at the end.
Linux EA's are not the only EA's Samba uses.
We also run on all the *BSD's and also Solaris (and older HPUX and AIX).
It's very likely this restriction might come from another platform than Linux (shock, horror :-). They do exist :-).
Adding an extra null byte on Linux isn't the end of the world, and I'm having a hard time thinking of this as a Samba bug.
This affects xattr reads and is a real problem there. The Samba read path forces the final byte to be NULL even if it isn't! It doesn't extend the response with an extra NULL pad. It just overwrites the final byte with NULL.
Produce the following using macOS Sierra as the client and Samba 4.6 on FreeBSD/ZFS server:
On the FreeBSD server on a ZFS backed share:
> touch asdf
> setextattr user foo bar asdf
> getextattr -x user foo asdf
asdf 62 61 72
the "foo" attribute is 3 bytes, not NULL padded.
On the macOS client:
> xattr -p foo asdf
> xattr -x -p foo asdf
FreeBSD does have an "-n" option to setextattr, which NULL pads and produces the expected result on the client. This is not the default though and it seems reasonable for Samba to handle both cases.
You could make a case for not extending attributes when writing from the client, too, especially when the attribute is binary and not a string.
So, the assumption of NULL padding on read is a bug and, perhaps NULL padding should be an option on write instead of forced.
Linux, FreeBSD and Solaris have 0-byte size attributes, so vfs_streams_xattr should default to not append a 0 byte. Alas, we can't change the default, we can only add this as an optional behaviour.
Created attachment 14265 [details]
WIP patch that addresses the problem
(In reply to Dima Tisnek from comment #0)
Here is the patch I've sent to the Samba ML for the evaluation. If you happen to run the FreeBSD as a Samba server it is already in the ports.