From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <4AEF1E91.6030407@domain.hid> Date: Mon, 02 Nov 2009 19:01:53 +0100 From: Jan Kiszka MIME-Version: 1.0 References: <20091020113724.9069.23594.stgit@domain.hid> <20091020113725.9069.28060.stgit@domain.hid> <1256404942.2862.498.camel@domain.hid> <1257177854.2065.616.camel@domain.hid> <4AEF0BCC.2020208@domain.hid> <1257180716.2065.618.camel@domain.hid> <4AEF0F7F.2060603@domain.hid> In-Reply-To: <4AEF0F7F.2060603@domain.hid> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Subject: Re: [Xenomai-core] [PATCH v3 5/9] nucleus: Avoid returning errors from xnheap_destroy_mapped List-Id: Xenomai life and development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Philippe Gerum Cc: "xenomai@xenomai.org" 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