exec: do unshare_files after de_thread
POSIX mandates that open fds and their associated file locks should be
preserved across an execve. This works, unless the process is
multithreaded at the time that execve is called.
In that case, we'll end up unsharing the files_struct but the locks will
still have their fl_owner set to the address of the old one. Eventually,
when the other threads die and the last reference to the old
files_struct is put, any POSIX locks get torn down since it looks like
a close occurred on them.
The result is that all of your open files will be intact with none of
the locks you held before execve. The simple answer to this is "use OFD
locks", but this is a nasty surprise and it violates the spec.
Fix this by doing unshare_files later during exec, after we've already
killed off the other threads in the process. This helps ensure that we
only unshare the files_struct during exec when it is truly shared with
other processes.
Note that because the unshare_files call is now done just after
de_thread, we need a mechanism to pass the displaced files_struct back
up to __do_execve_file. This is done via a new displaced_files field
inside the linux_binprm.
Cc: Eric W. Biederman <ebiederm@xmission.com>
Reported-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
2 files changed
