time stamp on msdosfs could not be set by general user

[Mark Day]

On Saturday, April 12, 2003, at 10:06  AM, Yoshihiko Sarumaru wrote:

> mistral% cp -p somefile /dos/
> cp: utimes: /dos/somefile: Operation not permitted
> cp: chmod: /dos/somefile: Operation not permitted
>
> I can understand errors about chmod, but I can not understand errors
> about utimes and modified time could not be set at all.

This is a consequence of the user and group IDs not being settable 
per-file on DOS volumes.  In effect, the user and group IDs are being 
changed behind your back.

> Below patch ignores unmatching of user and file owner

Which means that the user who did the "cp" is not the same as the user 
associated with the volume (the one who owns everything on that volume 
-- which is settable via a mount option).

But since you were able to create the file in the first place, the user 
doing the cp must have had write access (as part of the group, or 
world).

Workarounds would be to do the cp as root, or mount the volume as owned 
by the same user as the one doing the cp.

> Any objection ?

Hard to say.  It violates the documented behavior of utimes -- that 
only the owner should be able to modify the times.  But if the volume 
properly stored user and group IDs, you would have been the owner of 
the file, and the utimes would have worked in this case.

Your change would allow utimes to work even for a file you didn't just 
create, as long as you had write access.  That's potentially a security 
problem, but msdosfs doesn't really have security to begin with.

For comparison, Darwin and Mac OS X generally avoid the problem by the 
way they manage the user ID.  By default, everything on a msdosfs 
volume is owned by a special user ID that gets mapped dynamically to 
whoever is logged in at the console.  For the one-user-at-a-time case, 
this works well.  But a different user would get the same behavior you 
see.

-Mark