From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mathieu Desnoyers Subject: Re: [RFC PATCH for 4.21 01/16] rseq/selftests: Add reference counter to coexist with glibc Date: Thu, 11 Oct 2018 11:13:28 -0400 (EDT) Message-ID: <1917048565.2402.1539270808972.JavaMail.zimbra@efficios.com> References: <20181010191936.7495-1-mathieu.desnoyers@efficios.com> <20181010191936.7495-2-mathieu.desnoyers@efficios.com> <38596780-30f7-0763-0c17-7517dbf0bf59@arm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <38596780-30f7-0763-0c17-7517dbf0bf59@arm.com> Sender: linux-kernel-owner@vger.kernel.org To: Szabolcs Nagy Cc: Peter Zijlstra , "Paul E. McKenney" , Boqun Feng , nd , linux-kernel , linux-api , Thomas Gleixner , Andy Lutomirski , Dave Watson , Paul Turner , Andrew Morton , Russell King , Ingo Molnar , "H. Peter Anvin" , Andi Kleen , Chris Lameter , Ben Maurer , rostedt , Josh Triplett , Linus Torvalds , Catalin Marinas List-Id: linux-api@vger.kernel.org ----- On Oct 11, 2018, at 6:37 AM, Szabolcs Nagy Szabolcs.Nagy@arm.com wrote: > On 10/10/18 20:19, Mathieu Desnoyers wrote: >> In order to integrate rseq into user-space applications, add a reference >> counter field after the struct rseq TLS ABI so many rseq users can be >> linked into the same application (e.g. librseq and glibc). The >> reference count ensures that rseq syscall registration/unregistration >> happens only for the most early/late user for each thread, thus ensuring >> that rseq is registered across the lifetime of all rseq users for a >> given thread. > ... >> +__attribute__((visibility("hidden"))) __thread >> +volatile struct libc_rseq __lib_rseq_abi = { > ... >> +extern __attribute__((weak, alias("__lib_rseq_abi"))) __thread >> +volatile struct rseq __rseq_abi; > ... >> @@ -70,7 +86,7 @@ int rseq_register_current_thread(void) >> sigset_t oldset; >> >> signal_off_save(&oldset); >> - if (refcount++) >> + if (__lib_rseq_abi.refcount++) >> goto end; >> rc = sys_rseq(&__rseq_abi, sizeof(struct rseq), 0, RSEQ_SIG); > > why do you use a local refcounter instead of the __rseq_abi one? There is no refcount in struct rseq (the ABI between kernel and user-space). The registration refcount was part of an earlier version of the rseq system call, but we decided against keeping it in the kernel. So I'm adding one _after_ struct rseq, purely to allow interaction between various user-space components (program/libraries). > > what prevents calling rseq_register_current_thread more than 4G times? Nothing. It would indeed be cleaner to error out if we detect that refcount is at INT_MAX. Is that what you have in mind ? > > why cant the kernel see that the same address is registered again and succeed? It can, and it does. However, refcounting at user-level is needed to ensure the registration "lifetime" for rseq covers its entire use. If we have two libraries using rseq, we end up with the following scenario: Thread 1 libA registers rseq libB registers rseq libB unregisters rseq libA uses rseq -> bug! it's been unregistered by libB. libA unregisters rseq -> unexpected, it's already been unregistered. same applies if libA unregisters rseq before libB (and libB try to use rseq after libA has unregistered). The refcount in user-space fixes this. Thoughts ? Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com