Redundant syscalls?

Redundant syscalls?

[Lorenzo Allegrucci]

I was wondering why do we need fsetxattr(2), fgetxattr(2) etc when we 
already have setxattr(2), getxattr(2) etc working on file names
instead of file descriptors.
truncate(2)/ftruncate(2) is another more traditional example.

Thanks

Re: Redundant syscalls?

[Doug McNaught]

Lorenzo Allegrucci <l.allegrucci@tiscalinet.it> writes:

> I was wondering why do we need fsetxattr(2), fgetxattr(2) etc when we 
> already have setxattr(2), getxattr(2) etc working on file names
> instead of file descriptors.
> truncate(2)/ftruncate(2) is another more traditional example.

Because you can't reliably derive a file name from an open file
descriptor, so it's useful to have a way to act on the file directly
through the descriptor. 

-Doug
-- 
Let us cross over the river, and rest under the shade of the trees.
   --T. J. Jackson, 1863

Re: Redundant syscalls?

[Jesse Pollard]

---------  Received message begins Here  ---------

> 
> 
> I was wondering why do we need fsetxattr(2), fgetxattr(2) etc when we 
> already have setxattr(2), getxattr(2) etc working on file names
> instead of file descriptors.
> truncate(2)/ftruncate(2) is another more traditional example.

Atomic actions.

Consider the case of a file that doesn't exist yet. first you
open it, then perform the fsetxattr. If you use the name, then it becomes
possible to rename the file and substitute a different one before the
setxattr. Now, the open file will be missing the attribute(s).
-------------------------------------------------------------------------
Jesse I Pollard, II
Email: pollard@navo.hpc.mil

Any opinions expressed are solely my own.

Re: Redundant syscalls?

[Andi Kleen]

Lorenzo Allegrucci <l.allegrucci@tiscalinet.it> writes:

> I was wondering why do we need fsetxattr(2), fgetxattr(2) etc when we 
> already have setxattr(2), getxattr(2) etc working on file names
> instead of file descriptors.
> truncate(2)/ftruncate(2) is another more traditional example.

The f* variant can be race free. For example you want to stat something
first to make sure it is what you expect it to be and not a symlink
to your /etc/passwd. When you use first stat() and then do random
operation on filename with name there is a small window where someone
could replace the name with something else. This could be security relevant.
fd = open(name, ...); fstat(fd, ..); check fsomething(fd, ...); close(fd);
guarantees that you're always working on the same object without any race 
windows.

-Andi