netdev - RE: [PATCH net] x86: bpf_jit: fix compilation of large bpf programs

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <063D6719AE5E284EB5DD2968C1650D6D1CB3EA6A@AcuExch.aculab.com>
Date:	Tue, 26 May 2015 13:40:15 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Alexei Starovoitov' <ast@...mgrid.com>,
	"David S. Miller" <davem@...emloft.net>
CC:	Daniel Borkmann <daniel@...earbox.net>,
	Eric Dumazet <edumazet@...gle.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: RE: [PATCH net] x86: bpf_jit: fix compilation of large bpf programs

From: Alexei Starovoitov
> Sent: 22 May 2015 23:43
> x86 has variable length encoding. x86 JIT compiler is trying
> to pick the shortest encoding for given bpf instruction.
> While doing so the jump targets are changing, so JIT is doing
> multiple passes over the program. Typical program needs 3 passes.
> Some very short programs converge with 2 passes. Large programs
> may need 4 or 5. But specially crafted bpf programs may hit the
> pass limit and if the program converges on the last iteration
> the JIT compiler will be producing an image full of 'int 3' insns.
> Fix this corner case by doing final iteration over bpf program.

If the JIT compiler is only changing the encoding of the constants
in the x86 instructions (rather than changing the instructions themselves)
then there is likely to me an unmeasurable change in the execution time.
For instance I don't remember there being a difference in execution time
between long and short branches - the only difference is the amount of
cache they use.

	David

--
To unsubscribe from this list: send the line "unsubscribe netdev" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html