From mboxrd@z Thu Jan 1 00:00:00 1970 From: Cyrill Gorcunov Subject: Re: [patch v4 resend 2/2] kcmp: Add KCMP_EPOLL_TFD mode to compare epoll target files Date: Sat, 13 May 2017 01:53:40 +0300 Message-ID: <20170512225340.GD1881@uranus.lan> References: <20170424154423.511592110@gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org To: Jann Horn Cc: linux-fsdevel@vger.kernel.org, kernel list , Linux API , Al Viro , akpm@linuxfoundation.org, avagin@virtuozzo.com, xemul@virtuozzo.com, Michael Kerrisk-manpages , avagin@openvz.org, jbaron@akamai.com, Andy Lutomirski List-Id: linux-api@vger.kernel.org On Sat, May 13, 2017 at 12:41:30AM +0200, Jann Horn wrote: > [resending as plaintext] > > I realize that the existing kcmp code has the same issue, but: > > Why are you not taking a reference to filp or filp_tgt? This can end up > performing a comparison between a pointer to a freed struct file and a > pointer to a struct file that was allocated afterwards, right? So it can > return a false "is equal" result when the two files aren't actually the same > if one of the target tasks is running? This looks like it unnecessarily > exposes information about whether an allocation reuses the memory of > a previously freed allocation. It work with unlocked data on purpose for speed sake. Moreover even if we grap a reference it is valid _only_ during comparision operation, next we drop ref and it can be easily freed by os. Thus it's up to a caller to keep references to files/task and other resources used. Cyrill