netdev - Re: [PATCH net-next 2/5] bpf: move clearing of A/X into classic to eBPF migration prologue

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Message-ID: <CABg9mcuVj7XL=3LyQHxjep_k4eBYe7OcV3W4jrOzXrHaD6iCaA@mail.gmail.com>
Date:	Thu, 17 Dec 2015 16:00:45 -0800
From:	Z Lim <zlim.lnx@...il.com>
To:	Daniel Borkmann <daniel@...earbox.net>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Alexei Starovoitov <ast@...nel.org>,
	Michael Holzheu <holzheu@...ux.vnet.ibm.com>,
	Network Development <netdev@...r.kernel.org>,
	Yang Shi <yang.shi@...aro.org>
Subject: Re: [PATCH net-next 2/5] bpf: move clearing of A/X into classic to
 eBPF migration prologue

On Thu, Dec 17, 2015 at 2:51 PM, Daniel Borkmann <daniel@...earbox.net> wrote:
> Back in the days where eBPF (or back then "internal BPF" ;->) was not
> exposed to user space, and only the classic BPF programs internally
> translated into eBPF programs, we missed the fact that for classic BPF
> A and X needed to be cleared. It was fixed back then via 83d5b7ef99c9
> ("net: filter: initialize A and X registers"), and thus classic BPF
> specifics were added to the eBPF interpreter core to work around it.
>
> This added some confusion for JIT developers later on that take the
> eBPF interpreter code as an example for deriving their JIT. F.e. in
> f75298f5c3fe ("s390/bpf: clear correct BPF accumulator register"), at
> least X could leak stack memory. Furthermore, since this is only needed
> for classic BPF translations and not for eBPF (verifier takes care
> that read access to regs cannot be done uninitialized), more complexity
> is added to JITs as they need to determine whether they deal with
> migrations or native eBPF where they can just omit clearing A/X in
> their prologue and thus reduce image size a bit, see f.e. cde66c2d88da
> ("s390/bpf: Only clear A and X for converted BPF programs"). In other
> cases (x86, arm64), A and X is being cleared in the prologue also for
> eBPF case, which is unnecessary.
>
> Lets move this into the BPF migration in bpf_convert_filter() where it
> actually belongs as long as the number of eBPF JITs are still few. It
> can thus be done generically; allowing us to remove the quirk from
> __bpf_prog_run() and to slightly reduce JIT image size in case of eBPF,
> while reducing code duplication on this matter in current(/future) eBPF
> JITs.
>
> Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
> Acked-by: Alexei Starovoitov <ast@...nel.org>
> Reviewed-by: Michael Holzheu <holzheu@...ux.vnet.ibm.com>
> Tested-by: Michael Holzheu <holzheu@...ux.vnet.ibm.com>
> Cc: Zi Shen Lim <zlim.lnx@...il.com>
> Cc: Yang Shi <yang.shi@...aro.org>
> ---
>  arch/arm64/net/bpf_jit_comp.c |  6 ------

For the arm64 bits:
Acked-by: Zi Shen Lim <zlim.lnx@...il.com>
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