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SubjectRe: [PATCH 1/4] glibc: Perform rseq(2) registration at C startup and thread creation (v7)
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.

--
Alan Modra
Australia Development Lab, IBM

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