From: Daniel Borkmann <daniel@iogearbox.net>
Subject: Re: [PATCH 31/33] bpf: Add __bpf_prog_run() to stacktool whitelist
Date: Thu, 21 Jan 2016 23:57:41 +0100
Message-ID: <56A16265.9080409@iogearbox.net>
References: <cover.1453405861.git.jpoimboe@redhat.com> <8ee01c195432e4c1ebaa79c20e91525c7af22384.1453405861.git.jpoimboe@redhat.com>
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Cc: linux-kernel@vger.kernel.org, live-patching@vger.kernel.org,
	Michal Marek <mmarek@suse.cz>,
	Peter Zijlstra <peterz@infradead.org>,
	Andy Lutomirski <luto@kernel.org>,
	Borislav Petkov <bp@alien8.de>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Andi Kleen <andi@firstfloor.org>,
	Pedro Alves <palves@redhat.com>,
	Namhyung Kim <namhyung@gmail.com>,
	Bernd Petrovitsch <bernd@petrovitsch.priv.at>,
	Chris J Arges <chris.j.arges@canonical.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Jiri Slaby <jslaby@suse.cz>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	Alexei Starovoitov <ast@kernel.org>, netdev@vger.kernel.org
To: Josh Poimboeuf <jpoimboe@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>,
	"H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org
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In-Reply-To: <8ee01c195432e4c1ebaa79c20e91525c7af22384.1453405861.git.jpoimboe@redhat.com>
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List-Id: netdev.vger.kernel.org

On 01/21/2016 11:49 PM, Josh Poimboeuf wrote:
> stacktool reports the following false positive warnings:
>
>    stacktool: kernel/bpf/core.o: __bpf_prog_run()+0x5c: sibling call from callable instruction with changed frame pointer
>    stacktool: kernel/bpf/core.o: __bpf_prog_run()+0x60: function has unreachable instruction
>    stacktool: kernel/bpf/core.o: __bpf_prog_run()+0x64: function has unreachable instruction
>    [...]
>
> It's confused by the following dynamic jump instruction in
> __bpf_prog_run()::
>
>    jmp     *(%r12,%rax,8)
>
> which corresponds to the following line in the C code:
>
>    goto *jumptable[insn->code];
>
> There's no way for stacktool to deterministically find all possible
> branch targets for a dynamic jump, so it can't verify this code.
>
> In this case the jumps all stay within the function, and there's nothing
> unusual going on related to the stack, so we can whitelist the function.
>
> Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
> Cc: Alexei Starovoitov <ast@kernel.org>
> Cc: netdev@vger.kernel.org

Fine by me:

Acked-by: Daniel Borkmann <daniel@iogearbox.net>