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From:   Tulio Magno Quites Machado Filho <tuliom@ascii.art.br>
To:     Carlos O'Donell <codonell@redhat.com>,
        Florian Weimer <fweimer@redhat.com>,
        Michael Meissner <meissner@linux.ibm.com>,
        Alan Modra <amodra@gmail.com>,
        Peter Bergner <bergner@vnet.ibm.com>,
        Michael Ellerman <mpe@ellerman.id.au>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Paul Burton <paul.burton@mips.com>,
        Will Deacon <will.deacon@arm.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Heiko Carstens <heiko.carstens@de.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Martin Schwidefsky <schwidefsky@de.ibm.com>,
        Russell King <linux@armlinux.org.uk>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>, carlos <carlos@redhat.com>,
        Joseph Myers <joseph@codesourcery.com>,
        Szabolcs Nagy <szabolcs.nagy@arm.com>,
        libc-alpha <libc-alpha@sourceware.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ben Maurer <bmaurer@fb.com>,
        Peter Zijlstra <peterz@infradead.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Dave Watson <davejwatson@fb.com>, Paul Turner <pjt@google.com>,
        Rich Felker <dalias@libc.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        linux-api <linux-api@vger.kernel.org>
Subject: Re: [PATCH 1/4] glibc: Perform rseq(2) registration at C startup and thread creation (v7)
In-Reply-To: <43f97ddb-c8df-27ea-9517-63252ebd3183@redhat.com>
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Date:   Mon, 08 Apr 2019 16:20:37 -0300
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Carlos O'Donell <codonell@redhat.com> writes:

> On 4/5/19 5:16 AM, Florian Weimer wrote:
>> * Carlos O'Donell:
>>> It is valuable that it be a trap, particularly for constant pools because
>>> it means that a jump into the constant pool will trap.
>> 
>> Sorry, I don't understand why this matters in this context.  Would you
>> please elaborate?
>
> Sorry, I wasn't very clear.
>
> My point is only that any accidental jumps, either with off-by-one (like you
> fixed in gcc/glibc's signal unwinding most recently), result in a process fault
> rather than executing RSEQ_SIG as a valid instruction *and then* continuing
> onwards to the handler.
>
> A process fault is achieved either by a trap, or an invalid instruction, or
> a privileged insn (like suggested for MIPS in this thread).

In that case, mtmsr (Move to Machine State Register) seems a good candidate.

mtmsr is available both on 32 and 64 bits since their first implementations.

It's a privileged instruction and should never appear in userspace
code (causes SIGILL).

Any comments?

-- 
Tulio Magno