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Re: [RFC PATCH glibc 1/4] glibc: Perform rseq(2) registration at C startup and thread creation (v5)
- From: Mathieu Desnoyers <mathieu dot desnoyers at efficios dot com>
- To: carlos <carlos at redhat dot com>
- Cc: Florian Weimer <fweimer at redhat dot com>, Joseph Myers <joseph at codesourcery dot com>, Szabolcs Nagy <szabolcs dot nagy at arm dot com>, libc-alpha <libc-alpha at sourceware dot org>, Thomas Gleixner <tglx at linutronix dot de>, Ben Maurer <bmaurer at fb dot com>, Peter Zijlstra <peterz at infradead dot org>, "Paul E. McKenney" <paulmck at linux dot vnet dot ibm dot com>, Boqun Feng <boqun dot feng at gmail dot com>, Will Deacon <will dot deacon at arm dot com>, Dave Watson <davejwatson at fb dot com>, Paul Turner <pjt at google dot com>, Rich Felker <dalias at libc dot org>, linux-kernel <linux-kernel at vger dot kernel dot org>, linux-api <linux-api at vger dot kernel dot org>
- Date: Mon, 21 Jan 2019 16:23:55 -0500 (EST)
- Subject: Re: [RFC PATCH glibc 1/4] glibc: Perform rseq(2) registration at C startup and thread creation (v5)
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- References: <20190115015148.32155-1-mathieu.desnoyers@efficios.com> <1887968822.1146.1547833305059.JavaMail.zimbra@efficios.com>
----- On Jan 18, 2019, at 12:41 PM, Mathieu Desnoyers mathieu.desnoyers@efficios.com wrote:
> ----- On Jan 14, 2019, at 8:51 PM, Mathieu Desnoyers
> mathieu.desnoyers@efficios.com wrote:
>
> [...]
>
>> diff --git a/sysdeps/unix/sysv/linux/rseq-sym.c
>> b/sysdeps/unix/sysv/linux/rseq-sym.c
>> new file mode 100644
>> index 0000000000..6856d0388a
> [...]
>> +/* volatile because fields can be read/updated by the kernel. */
>> +__thread volatile struct rseq __rseq_abi = {
>> + .cpu_id = RSEQ_CPU_ID_UNINITIALIZED,
>> +};
>> +
>> +/* volatile because refcount can be read/updated by signal handlers. */
>> +__thread volatile uint32_t __rseq_refcount;
>
> Back to the weak vs non-weak question about those two symbols. I understand
> that tagging them as weak symbols has little effect on the dynamic loader
> when it loads libc.so. However, I'm worried about that happens when
> libc is statically linked into an application, and there happens to
> be more than one instance of those symbols (e.g. libc and another library
> define the same symbols, and both are statically linked into the same
> application). Isn't it a situation where tagging those symbols as "weak"
> becomes useful ?
Testing shows that it seems fine to statically link two archives within an
executable in a scenario where each .a defines the same symbol, without
using "weak", so I won't worry about this further.
Thanks,
Mathieu
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com