Re: [vox-tech] good reference source for fstab permissions

[Ken Bloom <kabloom@uMAPScdavis.edu>]

On 2003.07.03 09:18, Jonathan Stickel wrote:
> Donald Childs wrote:
> > Thanks for all the assistance . I'm able to write to the vfat 
> > partition
> > now, using:
> >
> > /dev/hdf4    /mnt/fat    vfat user,rw,uid=[user],gid=[workgroup] 0 0
> >
> > but will go back and add the showexec attribute.
> This is different from what I use for my fat32 partition(s).  I have 
> in my fstab:
>
> /dev/hda5  /Documents  vfat  defaults,quiet,umask=0000  0 0
>
> I'd like to learn a little more about these mount options, if someone 
> cares to enlighten me:
>
> My method mounts on bootup, allows all users read/write permission, 
> and quiets failed attempts to change file ownership/permissions.  
> Reading a little of the man page, I get the impression that 
> "umask=0000" and "quiet" essentially overrides the "suid" implied by 
> using the "default" option.  Is this about right?
It appears that this umask is in agreement with the suid option. 
Setting umask=7000 would seem to be in disagreement the suid option but 
would seem to be in consonance with the nosuid option.

> It seems to me Donald's fstab entry only gives read/write permission 
> to [user] and [workgroup].  Also, it is not clear to me whether the 
> partition would mount on startup or with "mount -a" ("auto" option is 
> not specified).  But I can see the ownership/permission problems of 
> fat is solved by assigning the entire partition to just one user.  
> How am I here?
I believe it follows the default umask of 022, so only the owner can 
write, but everyone else can read and execute. By saying user, anybody 
can mount and unmount the partition (I'd bet that this isn't Donald's 
intended effect). It appears that it is mounted automatically (like my 
vfat partition).

My vfat partition:
/dev/hda6  /home/bloom/mydocs   vfat    umask=073,uid=1000,gid=1000,
noexec,showexec  0 0

> I am also curious about the "showexec" option.  If I used it, would 
> it make "quiet" unnecessary?
No. Showexec just sets the default permissions differently. If you list 
your vfat drive now, then you will notice that all directories and 
*all* files are rwxrwxrwx. You would probably prefer to have your files 
rw-rw-rw- (I found the execute bits especially annoying when working 
with xftree, and when burning CD-ROMs) This is what showexec does.

Kernel 2.5.43 and later (according to mount(8)) replace this with fmask 
and dmask which set umasks for files and directories, respectively. I 
haven't used 2.5.x kernels, so I don't know for sure whether showexec 
will be removed, or merely deprecated. It took me a while to discover 
showexec because I had been hoping to find something more like fmask 
and dmask to begin with.

Quiet prevents chmods and chowns from returning errors. It can be used 
with showexec the only difference is the exact set of permissions that 
cannot be changed.

Also note that the write bits can actually be changed permenantly (at 
least without the quiet option) if all three are the same they affect 
the native FAT read-only bit that can also be set from and has an 
effect in Windows (if they're not all the same, see your kernel source 
to see who wins). And I found that (again, without quiet) other file 
permissions (like execute) can be changed temporarly and Linux will 
remember the permissions until the operations involving file dates 
would fail though (I don't know whether they do with quiet).

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