Re: Union mounts and fchown/fchmod/utimensat

["J. R. Okajima" <hooanon05@yahoo.co.jp>]

Valerie Aurora:
> My question: How often do applications actually attempt fchown(),
> fchmod(), or utimensat() on an O_RDONLY file descriptor?  Would it
> break a lot of applications if a union mount file returned EBADF or
> EPERM in this case?

Unfortunately I don't know how often it is.
But I am afraid it will be insufficient even if you implement the error.
Do you remember the sample case I've written before?
For instance,

	fd1 = open(path, O_RDONLY);
	fd2 = open(path, O_RDWR);
	fchmod(fd2);
	fstat(fd1, &st);

In this case, fchmod will not return the error because fd2 is opened for
write and the file would be copied-up. And the next fstat for fd1 will
return a bogus info since fd1 still refers the file on the lower layer.
Regardless fchmod() is issued by the same process or different one,
fstat() lies.
This is the same scenario I wrote before essentially (Jul 2009).

Also we should take care of the file-locking.
The copyup by UnionMount will break fcntl(F_SETLK) easily in the same
scenario. But I am not sure how many applications depend upon file lock
and UnionMount copyup affect.