From mboxrd@z Thu Jan 1 00:00:00 1970 From: Shaobo He Subject: Re: [PATCH] cxgb4: fix undefined behavior in mem.c Date: Thu, 28 Feb 2019 16:18:40 -0700 Message-ID: References: <1551393519-96595-1-git-send-email-shaobo@cs.utah.edu> <1551394596.31902.209.camel@acm.org> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <1551394596.31902.209.camel@acm.org> Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org To: Bart Van Assche , linux-rdma@vger.kernel.org Cc: Steve Wise , Doug Ledford , Jason Gunthorpe , open list List-Id: linux-rdma@vger.kernel.org I can't afford a pdf version of the C standard. So I looked at the draft version used in the link I put in the commit message. It says (in 6.2.4:2), ``` The lifetime of an object is the portion of program execution during which storage is guaranteed to be reserved for it. An object exists, has a constant address, and retains its last-stored value throughout its lifetime. If an object is referred to outside of its lifetime, the behavior is undefined. The value of a pointer becomes indeterminate when the object it points to (or just past) reaches the end of its lifetime. ``` I couldn't find the definition of lifetime over a dynamically allocated object in the draft of C standard. I refer to this link (https://en.cppreference.com/w/c/language/lifetime) which suggests that the lifetime of an allocated object ends after the deallocation function is called upon it. I think maybe the more problematic issue is that the value of a freed pointer is intermediate. Shaobo On 2/28/19 3:56 PM, Bart Van Assche wrote: > On Thu, 2019-02-28 at 15:38 -0700, Shaobo He wrote: >> In function `c4iw_dealloc_mw`, variable mhp's value is printed after >> freed, which triggers undefined behavior according to this post: >> https://trust-in-soft.com/dangling-pointer-indeterminate/. >> >> This commit fixes it by swapping the order of `kfree` and `pr_debug`. >> >> Signed-off-by: Shaobo He >> --- >> drivers/infiniband/hw/cxgb4/mem.c | 2 +- >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >> diff --git a/drivers/infiniband/hw/cxgb4/mem.c b/drivers/infiniband/hw/cxgb4/mem.c >> index 7b76e6f..bb8e0bc 100644 >> --- a/drivers/infiniband/hw/cxgb4/mem.c >> +++ b/drivers/infiniband/hw/cxgb4/mem.c >> @@ -684,8 +684,8 @@ int c4iw_dealloc_mw(struct ib_mw *mw) >> mhp->wr_waitp); >> kfree_skb(mhp->dereg_skb); >> c4iw_put_wr_wait(mhp->wr_waitp); >> - kfree(mhp); >> pr_debug("ib_mw %p mmid 0x%x ptr %p\n", mw, mmid, mhp); >> + kfree(mhp); >> return 0; >> } > > Please quote the relevant paragraphs from the C standard. All I have found > about free() in ISO/IEC 9899:2017 is the following: > > Description > The free function causes the space pointed to by ptr to be deallocated, that > is, made available for further allocation. If ptr is a null pointer, no > action occurs. Otherwise, if the argument does not match a pointer earlier > returned by a memory management function, or if the space has been > deallocated by a call to free or realloc, the behavior is undefined. > > That is not sufficient to claim that the above code triggers undefined > behavior. > > Bart. >