linux-kernel - rseq: How to test for compat task at signal delivery

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Date:   Tue, 26 Jun 2018 13:38:07 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Boqun Feng <boqun.feng@...il.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: rseq: How to test for compat task at signal delivery

Hi Andy,

I would like to make the behavior rseq on compat tasks more robust
by ensuring that kernel/rseq.c:rseq_get_rseq_cs() clears the high
bits of rseq_cs->abort_ip, rseq_cs->start_ip and
rseq_cs->post_commit_offset when a 32-bit binary is run on a 64-bit
kernel.

The intent here is that if user-space has garbage rather than zeroes
in its struct rseq_cs fields padding, the behavior will be the same
whether the binary is run on 32-bit or 64 kernels.

I know that internally, the kernel is making a transition from
is_compat_task() to in_compat_syscall().

I'm fine with using in_compat_syscall() when rseq_get_rseq_cs() is
invoked from a system call, but is it OK to call it when it is
invoked from signal delivery ? AFAIU, signals can be delivered
upon return from interrupt as well.

If not, what strategy do you recommend for arch-agnostic code ?

Thanks,

Mathieu

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