Indication of extended attributes availability.

[Václav Haisman]

Robert Watson wrote, On 22.3.2008 19:05:
> 
> On Fri, 21 Mar 2008, Václav Haisman wrote:
> 
>> I would like to have some sort of indication of extended attributes 
>> availability for given FS. It seems that things like this (MAC, ACLs 
>> etc.) are indicated using mount flags and those are available through 
>> statfs() call. The following is tentative patch that would expose 
>> extended attributes availability as mount flag. It is completely 
>> untested. I would just like to know if it is a viable approach to the 
>> problem or should I scratch it and try something else?
> 
> I think the preferred programmatic approach is actually via 
> fpathconf(2).  I don't know if any other OS's have assigned a _PC 
> constant for extended attributes, but if they have we should probably 
> use the same one.  However, I guess there's a meta-question: is your 
> goal to allow programs to be able to tell if extended attributes are 
> available, or for administrators to be able to tell?
> 
My original intent was to just extend /bin/cp with switch that would allow 
copying of extended attributes together with the contents of files. When I 
looked at its source I noticed that it uses fpathconf() for querying for ACLs 
capability. Because I have not found extended attributes in fpathconf(2) I 
have looked at statfs(2) but there is nothing there either. So I thought the 
information would have to be conveyed to either of the syscalls somehow. The 
mnt_flag field of struct mount seems like a logical place to put the 
information into. From there it seems it could be used by either fpathconf() 
or statfs() or both.

So, to answer the question, the goal is to allow programs to detect extended 
attributes availability.

As to what other OSes do, Solaris mentions _PC_XATTR_ENABLED and 
_PC_XATTR_EXISTS in fpathconf(2).

--
VH

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