sys_unshare: remove the dead CLONE_THREAD/SIGHAND/VM code

Cleanup: kill the dead code which does nothing but complicates the code
and confuses the reader.

sys_unshare(CLONE_THREAD/SIGHAND/VM) is not really implemented, and I
doubt very much it will ever work.  At least, nobody even tried since the
original 99d1419d96d7df9cfa56 ("unshare system call -v5: system call
handler function") was applied more than 4 years ago.

And the code is not consistent.  unshare_thread() always fails
unconditionally, while unshare_sighand() and unshare_vm() pretend to work
if there is nothing to unshare.

Remove unshare_thread(), unshare_sighand(), unshare_vm() helpers and
related variables and add a simple CLONE_THREAD | CLONE_SIGHAND| CLONE_VM
check into check_unshare_flags().

Also, move the "CLONE_NEWNS needs CLONE_FS" check from
check_unshare_flags() to sys_unshare().  This looks more consistent and
matches the similar do_sysvsem check in sys_unshare().

Note: with or without this patch "atomic_read(mm->mm_users) > 1" can give
a false positive due to get_task_mm().

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Roland McGrath <roland@redhat.com>
Cc: Janak Desai <janak@us.ibm.com>
Cc: Daniel Lezcano <daniel.lezcano@free.fr>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 25 insertions, 98 deletions

diff --git a/kernel/fork.c b/kernel/fork.c
index a8f64f8ec7e1..f2b494d7c557 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1519,38 +1519,24 @@ void __init proc_caches_init(void)
 }
 
 /*
- * Check constraints on flags passed to the unshare system call and
- * force unsharing of additional process context as appropriate.
+ * Check constraints on flags passed to the unshare system call.
  */
-static void check_unshare_flags(unsigned long *flags_ptr)
+static int check_unshare_flags(unsigned long unshare_flags)
 {
+        if (unshare_flags & ~(CLONE_THREAD|CLONE_FS|CLONE_NEWNS|CLONE_SIGHAND|
+                                CLONE_VM|CLONE_FILES|CLONE_SYSVSEM|
+                                CLONE_NEWUTS|CLONE_NEWIPC|CLONE_NEWNET))
+                return -EINVAL;
         /*
-         * If unsharing a thread from a thread group, must also
-         * unshare vm.
-         */
-        if (*flags_ptr & CLONE_THREAD)
-                *flags_ptr |= CLONE_VM;
-
-        /*
-         * If unsharing vm, must also unshare signal handlers.
-         */
-        if (*flags_ptr & CLONE_VM)
-                *flags_ptr |= CLONE_SIGHAND;
-
-        /*
-         * If unsharing namespace, must also unshare filesystem information.
+         * Not implemented, but pretend it works if there is nothing to
+         * unshare. Note that unsharing CLONE_THREAD or CLONE_SIGHAND
+         * needs to unshare vm.
          */
-        if (*flags_ptr & CLONE_NEWNS)
-                *flags_ptr |= CLONE_FS;
-}
-
-/*
- * Unsharing of tasks created with CLONE_THREAD is not supported yet
- */
-static int unshare_thread(unsigned long unshare_flags)
-{
-        if (unshare_flags & CLONE_THREAD)
-                return -EINVAL;
+        if (unshare_flags & (CLONE_THREAD | CLONE_SIGHAND | CLONE_VM)) {
+                /* FIXME: get_task_mm() increments ->mm_users */
+                if (atomic_read(&current->mm->mm_users) > 1)
+                        return -EINVAL;
+        }
 
         return 0;
 }
@@ -1577,34 +1563,6 @@ static int unshare_fs(unsigned long unshare_flags, struct fs_struct **new_fsp)
 }
 
 /*
- * Unsharing of sighand is not supported yet
- */
-static int unshare_sighand(unsigned long unshare_flags, struct sighand_struct **new_sighp)
-{
-        struct sighand_struct *sigh = current->sighand;
-
-        if ((unshare_flags & CLONE_SIGHAND) && atomic_read(&sigh->count) > 1)
-                return -EINVAL;
-        else
-                return 0;
-}
-
-/*
- * Unshare vm if it is being shared
- */
-static int unshare_vm(unsigned long unshare_flags, struct mm_struct **new_mmp)
-{
-        struct mm_struct *mm = current->mm;
-
-        if ((unshare_flags & CLONE_VM) &&
-            (mm && atomic_read(&mm->mm_users) > 1)) {
-                return -EINVAL;
-        }
-
-        return 0;
-}
-
-/*
  * Unshare file descriptor table if it is being shared
  */
 static int unshare_fd(unsigned long unshare_flags, struct files_struct **new_fdp)
@@ -1632,45 +1590,37 @@ static int unshare_fd(unsigned long unshare_flags, struct files_struct **new_fdp
  */
 SYSCALL_DEFINE1(unshare, unsigned long, unshare_flags)
 {
-        int err = 0;
         struct fs_struct *fs, *new_fs = NULL;
-        struct sighand_struct *new_sigh = NULL;
-        struct mm_struct *mm, *new_mm = NULL, *active_mm = NULL;
         struct files_struct *fd, *new_fd = NULL;
         struct nsproxy *new_nsproxy = NULL;
         int do_sysvsem = 0;
+        int err;
 
-        check_unshare_flags(&unshare_flags);
-
-        /* Return -EINVAL for all unsupported flags */
-        err = -EINVAL;
-        if (unshare_flags & ~(CLONE_THREAD|CLONE_FS|CLONE_NEWNS|CLONE_SIGHAND|
-                                CLONE_VM|CLONE_FILES|CLONE_SYSVSEM|
-                                CLONE_NEWUTS|CLONE_NEWIPC|CLONE_NEWNET))
+        err = check_unshare_flags(unshare_flags);
+        if (err)
                 goto bad_unshare_out;
 
         /*
+         * If unsharing namespace, must also unshare filesystem information.
+         */
+        if (unshare_flags & CLONE_NEWNS)
+                unshare_flags |= CLONE_FS;
+        /*
          * CLONE_NEWIPC must also detach from the undolist: after switching
          * to a new ipc namespace, the semaphore arrays from the old
          * namespace are unreachable.
          */
         if (unshare_flags & (CLONE_NEWIPC|CLONE_SYSVSEM))
                 do_sysvsem = 1;
-        if ((err = unshare_thread(unshare_flags)))
-                goto bad_unshare_out;
         if ((err = unshare_fs(unshare_flags, &new_fs)))
-                goto bad_unshare_cleanup_thread;
-        if ((err = unshare_sighand(unshare_flags, &new_sigh)))
-                goto bad_unshare_cleanup_fs;
-        if ((err = unshare_vm(unshare_flags, &new_mm)))
-                goto bad_unshare_cleanup_sigh;
+                goto bad_unshare_out;
         if ((err = unshare_fd(unshare_flags, &new_fd)))
-                goto bad_unshare_cleanup_vm;
+                goto bad_unshare_cleanup_fs;
         if ((err = unshare_nsproxy_namespaces(unshare_flags, &new_nsproxy,
                         new_fs)))
                 goto bad_unshare_cleanup_fd;
 
-        if (new_fs ||  new_mm || new_fd || do_sysvsem || new_nsproxy) {
+        if (new_fs || new_fd || do_sysvsem || new_nsproxy) {
                 if (do_sysvsem) {
                         /*
                          * CLONE_SYSVSEM is equivalent to sys_exit().
@@ -1696,19 +1646,6 @@ SYSCALL_DEFINE1(unshare, unsigned long, unshare_flags)
                         spin_unlock(&fs->lock);
                 }
 
-                if (new_mm) {
-                        mm = current->mm;
-                        active_mm = current->active_mm;
-                        current->mm = new_mm;
-                        current->active_mm = new_mm;
-                        if (current->signal->oom_score_adj == OOM_SCORE_ADJ_MIN) {
-                                atomic_dec(&mm->oom_disable_count);
-                                atomic_inc(&new_mm->oom_disable_count);
-                        }
-                        activate_mm(active_mm, new_mm);
-                        new_mm = mm;
-                }
-
                 if (new_fd) {
                         fd = current->files;
                         current->files = new_fd;
@@ -1725,20 +1662,10 @@ bad_unshare_cleanup_fd:
         if (new_fd)
                 put_files_struct(new_fd);
 
-bad_unshare_cleanup_vm:
-        if (new_mm)
-                mmput(new_mm);
-
-bad_unshare_cleanup_sigh:
-        if (new_sigh)
-                if (atomic_dec_and_test(&new_sigh->count))
-                        kmem_cache_free(sighand_cachep, new_sigh);
-
 bad_unshare_cleanup_fs:
         if (new_fs)
                 free_fs_struct(new_fs);
 
-bad_unshare_cleanup_thread:
 bad_unshare_out:
         return err;
 }