From mboxrd@z Thu Jan 1 00:00:00 1970 From: Philippe Gerum In-Reply-To: <4AEF0BCC.2020208@domain.hid> 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> Content-Type: text/plain; charset="UTF-8" Date: Mon, 02 Nov 2009 17:51:56 +0100 Message-ID: <1257180716.2065.618.camel@domain.hid> Mime-Version: 1.0 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: Jan Kiszka Cc: "xenomai@xenomai.org" 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. > Jan > -- Philippe.