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.  Factor bprm_execve
out of do_execve_common to separate out the copying of arguments
to the newe stack, and the rest of exec.

In separating bprm_execve from do_execve_common the copying
of the arguments onto the new stack happens earlier.

As the copying of the arguments does not depend any security hooks,
files, the file table, current->in_execve, current->fs->in_exec,
bprm->unsafe, or creds this is safe.

Likewise the security hook security_creds_for_exec does not depend upon
preventing the argument copying from happening.

In addition to making it possible to implement kernel_execve that
performs the copying differently, this separation of bprm_execve from
do_execve_common makes for a nice separation of responsibilities making
the exec code easier to navigate.

Signed-off-by: "Eric W. Biederman" <redacted>
---
 fs/exec.c | 108 +++++++++++++++++++++++++++++-------------------------
 1 file changed, 58 insertions(+), 50 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index afb168bf5e23..50508892fa71 100644
--- a/fs/exec.c
+++ b/fs/exec.c

@@ -1856,44 +1856,16 @@ static int exec_binprm(struct linux_binprm *bprm)
 /*
  * sys_execve() executes a new program.
  */
-static int do_execveat_common(int fd, struct filename *filename,
-			      struct user_arg_ptr argv,
-			      struct user_arg_ptr envp,
-			      int flags)
+static int bprm_execve(struct linux_binprm *bprm,
+		       int fd, struct filename *filename, int flags)