Date:   Wed, 17 Apr 2019 15:56:29 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Joseph Myers <joseph@...esourcery.com>
Cc:     carlos <carlos@...hat.com>, Will Deacon <will.deacon@....com>,
        Florian Weimer <fweimer@...hat.com>,
        Szabolcs Nagy <szabolcs.nagy@....com>,
        libc-alpha <libc-alpha@...rceware.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ben Maurer <bmaurer@...com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Dave Watson <davejwatson@...com>, Paul Turner <pjt@...gle.com>,
        Rich Felker <dalias@...c.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-api <linux-api@...r.kernel.org>
Subject: Re: [PATCH 1/5] glibc: Perform rseq(2) registration at C startup
 and thread creation (v8)

----- On Apr 17, 2019, at 12:17 PM, Joseph Myers joseph@...esourcery.com wrote:

> On Wed, 17 Apr 2019, Mathieu Desnoyers wrote:
> 
>> > +/* RSEQ_SIG is a signature required before each abort handler code.
>> > +
>> > +   It is a 32-bit value that maps to actual architecture code compiled
>> > +   into applications and libraries. It needs to be defined for each
>> > +   architecture. When choosing this value, it needs to be taken into
>> > +   account that generating invalid instructions may have ill effects on
>> > +   tools like objdump, and may also have impact on the CPU speculative
>> > +   execution efficiency in some cases.  */
>> > +
>> > +#define RSEQ_SIG 0xd428bc00	/* BRK #0x45E0.  */
>> 
>> After further investigation, we should probably do the following
>> to handle compiling with -mbig-endian on aarch64, which generates
>> binaries with mixed code vs data endianness (little endian code,
>> big endian data):
> 
> First, the comment on RSEQ_SIG should specify whether it is to be
> interpreted in the code or the data endianness.

Right. The signature passed as argument to the rseq registration
system call needs to be in data endianness (currently exposed kernel
ABI).

Ideally for userspace, we want to define a signature in code endianness
that happens to nicely match specific code patterns.

> 
>> For ARM32, the situation is a bit more complex. Only armv6+
>> generates mixed-endianness code vs data with -mbig-endian.
>> Prior to armv6, the code and data endianness matches. Therefore,
>> I plan to #ifdef the reversed endianness handling with:
>> 
>> #if __ARM_ARCH >= 6 && __ARM_BIG_ENDIAN
>> 
>> on arm32.
> 
> That doesn't work well because BE code (.o files) can be built for v5te
> (for example) and used on a range of different architecture variants with
> both BE32 and BE8 - the choice between BE32 and BE8 is a link-time choice,
> not a compile-time choice.  So if the value for Arm is a compile-time
> constant, it should also work for both BE32 and BE8.

Good to know! Then we need to be even more careful.

> 
> In turn, that suggests to me that RSEQ_SIG should be defined to be a value
> that is always in the code endianness (and whatever corresponding kernel
> code handles RSEQ_SIG values should act accordingly on architectures where
> the two endiannesses can differ).  If the kernel ABI is already fixed in a
> way that prevents such a definition of RSEQ_SIG semantics as using code
> endianness, a value should be chosen for Arm that works for both
> endiannesses.

It might be tricky to pick up a trap instruction that is a palindrome
endianness-wise.

> 
> (Also, installed glibc headers are supposed to work with older compilers,
> and support for __ARM_ARCH was only added in GCC 4.8.  Before that you
> need to test lots of separate macros for different architecture variants
> to determine a version number.)

Good point!

Here is an alternative to the palindrome approach. I'm taking arm32
as an example:

* We define RSEQ_SIG_CODE in code endianness, meant to be used with
  .inst in rseq assembly:

#define RSEQ_SIG_CODE 0xe7f5def3

* We define RSEQ_SIG_DATA in data endianness:

#define RSEQ_SIG_DATA \
        ({ \
                int sig; \
                asm volatile (  "b 2f\n\t" \
                                ".arm\n\t" \
                                "1: .inst 0xe7f5def3\n\t" \
                                "2:\n\t" \
                                "ldr %[sig], 1b\n\t" \
                                : [sig] "=r" (sig)); \
                sig; \
        })

Technically, only glibc and early-adopter libraries wishing to
register rseq need to use RSEQ_SIG_DATA. The RSEQ_SIG_CODE needs
to be used from inline assembly to create the signatures before
each abort handler.

Thoughts ?

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com

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