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sys_unshare: remove the dead CLONE_THREAD/SIGHAND/VM code
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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 99d1419 ("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>
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Oleg Nesterov authored and Linus Torvalds committed Mar 23, 2011
1 parent 4d51985 commit 9bfb23f
Showing 1 changed file with 25 additions and 98 deletions.
123 changes: 25 additions & 98 deletions kernel/fork.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -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;
}
Expand All @@ -1576,34 +1562,6 @@ static int unshare_fs(unsigned long unshare_flags, struct fs_struct **new_fsp)
return 0;
}

/*
* 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
*/
Expand Down Expand Up @@ -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().
Expand All @@ -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;
Expand All @@ -1725,20 +1662,10 @@ SYSCALL_DEFINE1(unshare, unsigned long, unshare_flags)
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;
}
Expand Down

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