Jan Kiszka wrote:
> Philippe Gerum wrote:
>> On Mon, 2009-11-02 at 17:41 +0100, Jan Kiszka wrote:
>>> Philippe Gerum wrote:
>>>> On Sat, 2009-10-24 at 19:22 +0200, Philippe Gerum wrote:
>>>>> On Tue, 2009-10-20 at 13:37 +0200, Jan Kiszka wrote:
>>>>>> Allowing xnheap_delete_mapped to return an error and then attempting to
>>>>>> recover from it does not work out very well: Corner cases are racy,
>>>>>> intransparent to the user, and proper error handling imposes a lot of
>>>>>> complexity on the caller - if it actually bothers to check the return
>>>>>> value...
>>>>>>
>>>>>> Fortunately, there is no reason for this function to fail: If the heap
>>>>>> is still mapped, just install the provide cleanup handler and switch to
>>>>>> deferred removal. If the unmapping fails, we either raced with some
>>>>>> other caller of unmap or user space provided a bogus address, or
>>>>>> something else is wrong. In any case, leaving the cleanup callback
>>>>>> behind is the best we can do anyway.
>>>>>>
>>>>>> Removing the return value immediately allows to simplify the callers,
>>>>>> namemly rt_queue_delete and rt_heap_delete.
>>>>>>
>>>>>> Note: This is still not 100% waterproof. If we issue
>>>>>> xnheap_destroy_mapped from module cleanup passing a release handler
>>>>>> that belongs to the module text, deferred release will cause a crash.
>>>>>> But this corner case is no new regression, so let's keep the head in the
>>>>>> sand.
>>>>> I agree with this one, eventually. This does make things clearer, and
>>>>> removes some opportunities for the upper interfaces to shot themselves
>>>>> in the foot. Merged, thanks.
>>>> Well, actually, it does make things clearer, but it is broken. Enabling
>>>> list debugging makes the nucleus pull the break after a double unlink in
>>>> vmclose().
>>>>
>>>> Basically, the issue is that calling rt_queue/heap_delete() explicitly
>>>> from userland will break, due to the vmclose() handler being indirectly
>>>> called by do_munmap() for the last mapping. The nasty thing is that
>>>> without debugs on, kheapq is just silently trashed.
>>>>
>>>> Fix is on its way, along with nommu support for shared heaps as well.
>>> OK, I see. Just on minor add-on to your fix:
>>>
>>> diff --git a/ksrc/nucleus/heap.c b/ksrc/nucleus/heap.c
>>> index ec14f73..1ae6af6 100644
>>> --- a/ksrc/nucleus/heap.c
>>> +++ b/ksrc/nucleus/heap.c
>>> @@ -1241,6 +1241,7 @@ void xnheap_destroy_mapped(xnheap_t *heap,
>>> down_write(¤t->mm->mmap_sem);
>>> heap->archdep.release = NULL;
>>> do_munmap(current->mm, (unsigned long)mapaddr, len);
>>> + heap->archdep.release = release;
>>> up_write(¤t->mm->mmap_sem);
>>> }
>>>
>>> @@ -1252,7 +1253,6 @@ void xnheap_destroy_mapped(xnheap_t *heap,
>>> if (heap->archdep.numaps > 0) {
>>> /* The release handler is supposed to clean up the rest. */
>>> XENO_ASSERT(NUCLEUS, release != NULL, /* nop */);
>>> - heap->archdep.release = release;
>>> return;
>>> }
>>>
>>>
>>> This is safer than leaving a potential race window open between dropping
>>> mmap_sem and fixing up archdep.release again.
>>>
>> Actually, we have to hold the kheap lock, in case weird code starts
>> mapping randomly from userland without getting a valid descriptor
>> through a skin call.
>
> Yep, that as well.
>
Note that 6b1a185b46 doesn't obsolete my patch (pull it from my tree if
you like).
Jan
--
Siemens AG, Corporate Technology, CT SE 2
Corporate Competence Center Embedded Linux
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