Re: Redundant syscalls?

[Andi Kleen <ak@suse.de>]

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