exec: Move initialization of bprm->filename into alloc_bprm

exec: Move initialization of bprm->filename into alloc_bprm

Currently it is necessary for the usermode helper code and the code
that launches init to use set_fs so that pages coming from the kernel
look like they are coming from userspace.

To allow that usage of set_fs to be removed cleanly the argument
copying from userspace needs to happen earlier.  Move the computation
of bprm->filename and possible allocation of a name in the case
of execveat into alloc_bprm to make that possible.

The exectuable name, the arguments, and the environment are
copied into the new usermode stack which is stored in bprm
until exec passes the point of no return.

As the executable name is copied first onto the usermode stack
it needs to be known.  As there are no dependencies to computing
the executable name, compute it early in alloc_bprm.

As an implementation detail if the filename needs to be generated
because it embeds a file descriptor store that filename in a new field
bprm->fdpath, and free it in free_bprm.  Previously this was done in
an independent variable pathbuf.  I have renamed pathbuf fdpath
because fdpath is more suggestive of what kind of path is in the
variable.  I moved fdpath into struct linux_binprm because it is
tightly tied to the other variables in struct linux_binprm, and as
such is needed to allow the call alloc_binprm to move.

Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lkml.kernel.org/r/87k0z66x8f.fsf@x220.int.ebiederm.org
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>