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Date:   Wed, 10 Apr 2019 17:52:43 +0200
From:   Martin Schwidefsky <schwidefsky@de.ibm.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc:     heiko carstens <heiko.carstens@de.ibm.com>,
        gor <gor@linux.ibm.com>, libc-alpha <libc-alpha@sourceware.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        carlos <carlos@redhat.com>
Subject: Re: rseq/s390: choosing code signature
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On Wed, 10 Apr 2019 11:50:39 -0400 (EDT)
Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:

> ----- On Apr 10, 2019, at 6:32 AM, schwidefsky schwidefsky@de.ibm.com wrote:
> 
> > On Tue, 9 Apr 2019 15:32:22 -0400 (EDT)
> > Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:
> >   
> >> Hi,
> >> 
> >> We are about to include the code signature required prior to restartable
> >> sequences abort handlers into glibc, which will make this ABI choice final.
> >> We need architecture maintainer input on that signature value.
> >> 
> >> That code signature is placed before each abort handler, so the kernel can
> >> validate that it is indeed jumping to an abort handler (and not some
> >> arbitrary attacker-chosen code). The signature is never executed.
> >> 
> >> The current discussion thread on the glibc mailing list leads us towards
> >> using a trap with uncommon immediate operand, which simplifies integration
> >> with disassemblers, emulators, makes it easier to debug if the control
> >> flow gets redirected there by mistake, and is nicer for some architecture's
> >> speculative execution.
> >> 
> >> We can have different signatures for each sub-architecture, as long as they
> >> don't have to co-exist within the same process. We can special-case with
> >> #ifdef for each sub-architecture and endianness if need be. If the architecture
> >> has instruction set extensions that can co-exist with the architecture
> >> instruction set within the same process, we need to take into account to which
> >> instruction the chosen signature value would map (and possibly decide if we
> >> need to extend rseq to support many signatures).
> >> 
> >> Here is an example of rseq signature definition template:
> >> 
> >> /*
> >>  * TODO: document trap instruction objdump output on each sub-architecture
> >>  * instruction sets, as well as instruction set extensions.
> >>  */
> >> #define RSEQ_SIG 0x########
> >> 
> >> Ideally we'd need a patch on top of the Linux kernel
> >> tools/testing/selftests/rseq/rseq-s390.h file that updates
> >> the signature value, so I can then pick it up for the glibc
> >> patchset.  
> > 
> > The trap4 instruction is a suitable one. The patch would look like this  
> 
> Great! I'm picking it up into my rseq tree if that's OK with you.

Just added the patch to s390/linux:features for the next merge window as well.

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.