Messages in this thread Patch in this message | | | Date | Thu, 11 Oct 2018 11:13:28 -0400 (EDT) | From | Mathieu Desnoyers <> | Subject | Re: [RFC PATCH for 4.21 01/16] rseq/selftests: Add reference counter to coexist with glibc |
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----- 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
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