lkml.org 
[lkml]   [2019]   [Apr]   [18]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH 1/4] glibc: Perform rseq(2) registration at C startup and thread creation (v7)
----- On Apr 9, 2019, at 5:29 AM, Alan Modra amodra@gmail.com wrote:

> On Tue, Apr 09, 2019 at 02:23:53PM +1000, Michael Ellerman wrote:
>> I'd much rather we use a trap with a specific immediate value. Otherwise
>> someone's going to waste time one day puzzling over why userspace is
>> doing mtmsr.
>
> It's data. We have other data in executable sections. Anyone who
> wonders about odd disassembly just hasn't realized they are
> disassembling data.
>
>> It would also complicate things if we ever wanted to emulate mtmsr.
>
> No, because it won't be executed. If I understand correctly, the only
> reason to choose an illegal, trap or privileged insn is to halt
> execution earlier rather than later when a program goes off in the
> weeds.
>
>> If we want something that is a trap rather than a nop then use 0x0fe50553.
>>
>> That's "compare the value in r5 with 0x553 and then trap unconditionally".
>>
>> It shows up in objdump as:
>>
>> 10000000: 53 05 e5 0f twui r5,1363
>>
>>
>> The immediate can be anything, I chose that value to mimic the x86 value
>> Mathieu mentioned.
>>
>> There's no reason that instruction would ever be generated because the
>> immediate value serves no purpose. So it satisfies the "very unlikely
>> to appear" criteria AFAICS.
>
> Yes, looks fine to me, except that in VLE mode (do we care?)
> ".long 0x0fe50553" disassembles as
> 0: 0f e5 se_cmphl r5,r30
> 2: 05 53 se_mullw r3,r5
> No illegal/trap/privileged insn there.
>
> ".long 0x0fe5000b" might be better to cover VLE.

Can you share with us the objdump output of ".long 0x0fe5000b" in
VLE mode ? VLE mode support does not appear to be available in typical
toolchains. Also, is VLE mode only for powerpc 32 be, or also for
powerpc 64 be/le ?

Thanks,

Mathieu



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

\
 
 \ /
  Last update: 2019-04-18 17:32    [W:0.102 / U:2.896 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site