My feeling is that it should be in glibc: as Mike mentioned, we don't normally
change the behavior of existing system calls unless they are obviously
broken to start with. If we want to keep fchmodat getting the implicit
"." directory, and at the same time keep fchmod returning an error, the fchmod
wrapper around fchmodat is the only place that can enforce this.
My point was that it's quite arguable that the *at syscall interfaces were
broken to begin with. I've never seen anything suggesting their intent was
other than to permit relative pathnames, and the empty string has never
been a valid relative pathname. To fit the POSIX requirements as I read
them, the *at functions must refuse to resolve the empty string. So if the
kernel does not change and my interpretation of POSIX stands, then libc
must wrap all the *at syscalls with a function that checks for the empty
string and fails with ENOENT as a special case.
I don't have any strong opinion about this subject, but it makes the most
sense to me for the kernel's behavior to change. I know of no reason to
think that the current treatment of the empty string was ever intended at
the creation of the *at interfaces.
Thanks,
Roland