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Date:   Tue, 23 Apr 2019 08:36:04 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To:     Ramana Radhakrishnan <ramana.gcc@googlemail.com>
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Subject: Re: [PATCH 1/5] glibc: Perform rseq(2) registration at C startup
 and thread creation (v8)
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Thread-Topic: glibc: Perform rseq(2) registration at C startup and thread creation (v8)
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----- On Apr 23, 2019, at 7:59 AM, Ramana Radhakrishnan ramana.gcc@googlemail.com wrote:

> On Tue, Apr 23, 2019 at 12:16 PM Szabolcs Nagy <Szabolcs.Nagy@arm.com> wrote:
>>
>> On 18/04/2019 19:17, Mathieu Desnoyers wrote:
>> > ----- On Apr 18, 2019, at 1:37 PM, Szabolcs Nagy Szabolcs.Nagy@arm.com wrote:
>> >> you have to add a documentation comment somewhere
>> >> explaining if RSEQ_SIG is the value that's passed to
>> >> the kernel and then aarch64 asm code has to use
>> >>
>> >> .inst endianfixup(RSEQ_SIG) // or
>> >> .word RSEQ_SIG
>> >
>> > Using ".word" won't allow objdump to show the instruction it
>> > maps to. It will consider it as data. So .inst is preferred here.
>>
>> is there some specific reason you prefer .inst?
> 
> I believe the reasoning here is that in the disassembly you want to
> see an instruction pattern for an architecture rather than a magic bit
> pattern that appears to be an "inline" literal pool entry.  I would
> support the .inst variant so that the disassembler shows the
> instruction for what it is when debugging. If control reaches the
> marker instruction, something's gone wrong and thus from a user
> friendliness perspective I would prefer to see an instruction that
> clearly indicates that it's undefined .inst directive so that someone
> disassembling this in an assembly view in GDB sees the right thing
> (TM) instead of having to reach for the manual and disassembling this
> by hand.

That's my line of thinking exactly. I might add that having data in a
literal pool within the instruction stream might be unexpected in some
compilation environments, e.g. when compiling with -mno-text-section-literals .

So even though the signature may likely end up being placed in a literal
pool, it's preferable to ensure it is a valid uncommon trap instruction.

Thanks,

Mathieu

> 
>>
>> disassembling a canary value as data (that is
>> never executed, but loaded and compared by the
>> kernel as data) sounds more semantically correct
>> to me than showing it as an instruction.
>>
> 
> 
> 
> Ramana
> 
> 
>> i guess having it as an instruction can avoid
>> issues if some tools dislike .word in .text,
> > but otherwise .word seems better.

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