* [PATCH -mm] sys_unshare: simplify the not-really-implemented CLONE_THREAD/SIGHAND/VM code
@ 2010-03-23 17:08 Oleg Nesterov
2010-03-23 21:02 ` Eric W. Biederman
2010-04-09 20:03 ` Roland McGrath
0 siblings, 2 replies; 5+ messages in thread
From: Oleg Nesterov @ 2010-03-23 17:08 UTC (permalink / raw)
To: Andrew Morton
Cc: Badari Pulavarty, Christoph Hellwig, Eric W. Biederman,
Janak Desai, Roland McGrath, Stanislaw Gruszka,
Sukadev Bhattiprolu, linux-kernel
(on top of check_unshare_flags-kill-the-bogus-clone_sighand-sig-count-check.patch)
Cleanup.
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
"unshare system call -v5: system call handler function" commit
99d1419d96d7df9cfa56bc977810be831bd5ef64 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>
---
kernel/fork.c | 119 ++++++++++++----------------------------------------------
1 file changed, 25 insertions(+), 94 deletions(-)
--- 34-rc1/kernel/fork.c~UNSHARE_KILL_THE_BOGUS_CODE 2010-03-21 19:08:06.000000000 +0100
+++ 34-rc1/kernel/fork.c 2010-03-23 17:54:27.000000000 +0100
@@ -1500,38 +1500,24 @@ 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(¤t->mm->mm_users) > 1)
+ return -EINVAL;
+ }
return 0;
}
@@ -1558,34 +1544,6 @@ static int unshare_fs(unsigned long unsh
}
/*
- * 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)
@@ -1613,45 +1571,37 @@ static int unshare_fd(unsigned long unsh
*/
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().
@@ -1677,15 +1627,6 @@ SYSCALL_DEFINE1(unshare, unsigned long,
write_unlock(&fs->lock);
}
- if (new_mm) {
- mm = current->mm;
- active_mm = current->active_mm;
- current->mm = new_mm;
- current->active_mm = new_mm;
- activate_mm(active_mm, new_mm);
- new_mm = mm;
- }
-
if (new_fd) {
fd = current->files;
current->files = new_fd;
@@ -1702,20 +1643,10 @@ 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;
}
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH -mm] sys_unshare: simplify the not-really-implemented CLONE_THREAD/SIGHAND/VM code
2010-03-23 17:08 [PATCH -mm] sys_unshare: simplify the not-really-implemented CLONE_THREAD/SIGHAND/VM code Oleg Nesterov
@ 2010-03-23 21:02 ` Eric W. Biederman
2010-03-23 23:05 ` Oleg Nesterov
2010-04-09 20:03 ` Roland McGrath
1 sibling, 1 reply; 5+ messages in thread
From: Eric W. Biederman @ 2010-03-23 21:02 UTC (permalink / raw)
To: Oleg Nesterov
Cc: Andrew Morton, Badari Pulavarty, Christoph Hellwig, Janak Desai,
Roland McGrath, Stanislaw Gruszka, Sukadev Bhattiprolu,
linux-kernel
Oleg Nesterov <oleg@redhat.com> writes:
> (on top of check_unshare_flags-kill-the-bogus-clone_sighand-sig-count-check.patch)
>
> Cleanup.
>
> 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
> "unshare system call -v5: system call handler function" commit
> 99d1419d96d7df9cfa56bc977810be831bd5ef64 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.
This is setting off alarm bells in my head.
I haven't traced this all through but I like your logic a lot less, and
I think it is buggy. Why don't we need to look at sigh->count ?
The current logic is very fine grained but it does a lot of simple logical
checks and it ties those checks together if a very maintainable way.
You require that we know upfront all of the dependencies, which is things
change subtlety can be a maintenance challenge.
> 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().
I think the number of times get_task_mm is called on not current this isn't
an interesting race.
Eric
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH -mm] sys_unshare: simplify the not-really-implemented CLONE_THREAD/SIGHAND/VM code
2010-03-23 21:02 ` Eric W. Biederman
@ 2010-03-23 23:05 ` Oleg Nesterov
2010-03-31 23:53 ` Oleg Nesterov
0 siblings, 1 reply; 5+ messages in thread
From: Oleg Nesterov @ 2010-03-23 23:05 UTC (permalink / raw)
To: Eric W. Biederman
Cc: Andrew Morton, Badari Pulavarty, Christoph Hellwig, Janak Desai,
Roland McGrath, Stanislaw Gruszka, Sukadev Bhattiprolu,
linux-kernel
On 03/23, Eric W. Biederman wrote:
>
> Oleg Nesterov <oleg@redhat.com> writes:
>
> > (on top of check_unshare_flags-kill-the-bogus-clone_sighand-sig-count-check.patch)
> >
> > Cleanup.
> >
> > 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
> > "unshare system call -v5: system call handler function" commit
> > 99d1419d96d7df9cfa56bc977810be831bd5ef64 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.
>
> This is setting off alarm bells in my head.
>
> I haven't traced this all through but I like your logic a lot less, and
> I think it is buggy. Why don't we need to look at sigh->count ?
CLONE_SIGHAND needs CLONE_VM in copy_process(). It is not possible that
sighand->count > 1 while mm->mm_users <= 1.
> The current logic is very fine grained but it does a lot of simple logical
> checks and it ties those checks together if a very maintainable way.
I'd say the current simple logic is simple but wrong ;)
Before the recent changes check_unshare_flags() did
if (*flags_ptr & CLONE_THREAD)
*flags_ptr |= CLONE_VM;
...
if ((*flags_ptr & CLONE_SIGHAND) &&
(atomic_read(¤t->signal->count) > 1))
*flags_ptr |= CLONE_THREAD;
Now, if we add CLONE_THREAD, why we do not add CLONE_VM here? This is
not right.
And why unshare_thread() always fails even in single-threaded case?
But,
> You require that we know upfront all of the dependencies, which is things
> change subtlety can be a maintenance challenge.
Fortunately this all is not implemented anyway.
My point was: lets simplify this code, mainly to reduce the output from, say,
"grep CLONE_SIGHAND". In my opinion, it is a bit strange that the code which
doesn't really work adds the unnecessary dependencies to CLONE_THREAD/etc
subtleness.
> > Note: with or without this patch "atomic_read(mm->mm_users) > 1" can give
> > a false positive due to get_task_mm().
>
> I think the number of times get_task_mm is called on not current this isn't
> an interesting race.
Sure. I just meant that this check is wrong, but it was copied from the
current code. We could use current_is_single_threaded() though.
That said, I do not really care about this cleanup. I did it just because
I sent another patch which touches check_unshare_flags(), and I was really
surprised that ~70 lines in kernel/fork.c do nothing but confuse the reader.
Please nack this patch and lets forget it ;)
Oleg.
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH -mm] sys_unshare: simplify the not-really-implemented CLONE_THREAD/SIGHAND/VM code
2010-03-23 23:05 ` Oleg Nesterov
@ 2010-03-31 23:53 ` Oleg Nesterov
0 siblings, 0 replies; 5+ messages in thread
From: Oleg Nesterov @ 2010-03-31 23:53 UTC (permalink / raw)
To: Eric W. Biederman
Cc: Andrew Morton, Badari Pulavarty, Christoph Hellwig, Janak Desai,
Roland McGrath, Stanislaw Gruszka, Sukadev Bhattiprolu,
linux-kernel
On 03/24, Oleg Nesterov wrote:
>
> That said, I do not really care about this cleanup. I did it just because
> I sent another patch which touches check_unshare_flags(), and I was really
> surprised that ~70 lines in kernel/fork.c do nothing but confuse the reader.
I changed my mind. I do care ;)
Seriously, Eric, it is just stupid this code does nothing but complicates
fork.c, and unless you prove this patch is wrong you can't convince me
this patch is bad idea.
> Please nack this patch and lets forget it ;)
Yes. You have all rights to nack it and I won't insist even if I disagree.
But please do this explicitly, otherwise I'll resend it.
Oleg.
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH -mm] sys_unshare: simplify the not-really-implemented CLONE_THREAD/SIGHAND/VM code
2010-03-23 17:08 [PATCH -mm] sys_unshare: simplify the not-really-implemented CLONE_THREAD/SIGHAND/VM code Oleg Nesterov
2010-03-23 21:02 ` Eric W. Biederman
@ 2010-04-09 20:03 ` Roland McGrath
1 sibling, 0 replies; 5+ messages in thread
From: Roland McGrath @ 2010-04-09 20:03 UTC (permalink / raw)
To: Oleg Nesterov
Cc: Andrew Morton, Badari Pulavarty, Christoph Hellwig,
Eric W. Biederman, Janak Desai, Stanislaw Gruszka,
Sukadev Bhattiprolu, linux-kernel
Acked-by: Roland McGrath <roland@redhat.com>
Thanks,
Roland
^ permalink raw reply [flat|nested] 5+ messages in thread
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2010-03-23 17:08 [PATCH -mm] sys_unshare: simplify the not-really-implemented CLONE_THREAD/SIGHAND/VM code Oleg Nesterov
2010-03-23 21:02 ` Eric W. Biederman
2010-03-23 23:05 ` Oleg Nesterov
2010-03-31 23:53 ` Oleg Nesterov
2010-04-09 20:03 ` Roland McGrath
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