Re: [RFC PATCH for 4.21 01/16] rseq/selftests: Add reference counter to coexist with glibc

From: Szabolcs Nagy <Szabolcs.Nagy@arm.com>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Peter Zijlstra <peterz@infradead.org>,
	"Paul E . McKenney" <paulmck@linux.vnet.ibm.com>,
	Boqun Feng <boqun.feng@gmail.com>
Cc: nd <nd@arm.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-api@vger.kernel.org" <linux-api@vger.kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Andy Lutomirski <luto@amacapital.net>,
	Dave Watson <davejwatson@fb.com>, Paul Turner <pjt@google.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Russell King <linux@arm.linux.org.uk>,
	Ingo Molnar <mingo@redhat.com>, "H . Peter Anvin" <hpa@zytor.com>,
	Andi Kleen <andi@firstfloor.org>, Chris Lameter <cl@linux.com>,
	Ben Maurer <bmaurer@fb.com>, Steven Rostedt <rostedt@goodmis.org>,
	Josh Triplett <josh@joshtriplett.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Catalin Marinas <Catalin.Marinas@arm.com>,
	Will Deacon <Will.Deacon@arm.com>,
	Michael Kerrisk <mtk.manpages@gmail.com>,
	Joel Fernandes <joelaf@google.com>, Shuah Khan <shuah@kernel.org>,
	Carlos O'Donell <carlos@redhat.com>,
	Florian Weimer <fweimer@redhat.com>,
	Joseph Myers <joseph@codesourcery.com>
Subject: Re: [RFC PATCH for 4.21 01/16] rseq/selftests: Add reference counter to coexist with glibc
Date: Thu, 11 Oct 2018 10:37:25 +0000	[thread overview]
Message-ID: <38596780-30f7-0763-0c17-7517dbf0bf59@arm.com> (raw)
In-Reply-To: <20181010191936.7495-2-mathieu.desnoyers@efficios.com>

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?

what prevents calling rseq_register_current_thread more than 4G times?

why cant the kernel see that the same address is registered again and succeed?