netdev - RE: [PATCH 6/9] x86/bpf: Fix JIT frame pointer usage

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Message-ID: <57f6e69da6b3461a9c39d71aa1b58662@AcuMS.aculab.com>
Date:   Fri, 14 Jun 2019 10:50:23 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Alexei Starovoitov' <alexei.starovoitov@...il.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>
CC:     "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        "Daniel Borkmann" <daniel@...earbox.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "Peter Zijlstra" <peterz@...radead.org>,
        Song Liu <songliubraving@...com>,
        "Kairui Song" <kasong@...hat.com>
Subject: RE: [PATCH 6/9] x86/bpf: Fix JIT frame pointer usage

On Thu, Jun 13, 2019 at 08:21:03AM -0500, Josh Poimboeuf wrote:
> The BPF JIT code clobbers RBP.  This breaks frame pointer convention and
> thus prevents the FP unwinder from unwinding through JIT generated code.
>
> RBP is currently used as the BPF stack frame pointer register.  The
> actual register used is opaque to the user, as long as it's a
> callee-saved register.  Change it to use R12 instead.

Could you maintain the system %rbp chain through the BPF stack?
It might even be possible to put something relevant in the %rip
location.

	David

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