From: Alexei Starovoitov <ast@plumgrid.com>
To: Andy Lutomirski <luto@amacapital.net>
Cc: "David S. Miller" <davem@davemloft.net>,
Ingo Molnar <mingo@kernel.org>,
Daniel Borkmann <daniel@iogearbox.net>,
Michael Holzheu <holzheu@linux.vnet.ibm.com>,
Zi Shen Lim <zlim.lnx@gmail.com>,
Linux API <linux-api@vger.kernel.org>,
Network Development <netdev@vger.kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH net-next 2/4] x86: bpf_jit: implement bpf_tail_call() helper
Date: Wed, 20 May 2015 09:29:28 -0700 [thread overview]
Message-ID: <555CB668.2090901@plumgrid.com> (raw)
In-Reply-To: <CALCETrV8CStuwvDXDPp5zsZw5FsSpYWBDXMYjLh6Qq703a=cgQ@mail.gmail.com>
On 5/20/15 9:05 AM, Andy Lutomirski wrote:
>>>
>>> What causes the stack pointer to be right? Is there some reason that
>>> the stack pointer is the same no matter where you are in the generated
>>> code?
>>
>>
>> that's why I said 'it's _roughly_ expressed in C' this way.
>> Stack pointer doesn't change. It uses the same stack frame.
>>
>
> I think the more relevant point is that (I think) eBPF never changes
> the stack pointer after the prologue (i.e. the stack depth is truly
> constant).
ahh, that's what you were referring to.
Yes, there is no alloca(). stack cannot grow and always fixed.
That's critical for safety verification.
On a JIT side though, x64 has ugly div/mod, so JIT is doing
push/pop rax/rdx to compile 'dst_reg /= src_reg' bpf insn.
But that doesn't change 'same stack depth' rule at the time
of bpf_tail_call.
Note, s390 JIT can generate different prologue/epilogue
for every program, so it will likely be doing stack unwind
and jump. Like I was doing in my tail_call_v2 version of x64 jit:
https://git.kernel.org/cgit/linux/kernel/git/ast/bpf.git/diff/arch/x86/net/bpf_jit_comp.c?h=tail_call_v2&id=bfd60c3135c8f010a6497dfc5e7d3070e26ca4d1
In case of interrupt happens sometime during this jumping process
it's also fine. no-red-zone business is very dear to my heart :)
I always keep it in mind when doing assembler/jit changes.
next prev parent reply other threads:[~2015-05-20 16:29 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-05-19 23:59 [PATCH net-next 0/4] bpf: introduce bpf_tail_call() helper Alexei Starovoitov
2015-05-19 23:59 ` [PATCH net-next 1/4] bpf: allow bpf programs to tail-call other bpf programs Alexei Starovoitov
2015-05-20 0:13 ` Andy Lutomirski
2015-05-20 0:18 ` Alexei Starovoitov
2015-05-21 16:20 ` Andy Lutomirski
2015-05-21 16:40 ` Alexei Starovoitov
2015-05-21 16:43 ` Andy Lutomirski
2015-05-21 16:53 ` Alexei Starovoitov
2015-05-21 16:57 ` Andy Lutomirski
2015-05-21 17:16 ` Alexei Starovoitov
2015-05-21 16:17 ` Daniel Borkmann
2015-05-19 23:59 ` [PATCH net-next 2/4] x86: bpf_jit: implement bpf_tail_call() helper Alexei Starovoitov
2015-05-20 0:11 ` Andy Lutomirski
2015-05-20 0:14 ` Alexei Starovoitov
2015-05-20 16:05 ` Andy Lutomirski
2015-05-20 16:29 ` Alexei Starovoitov [this message]
2015-05-19 23:59 ` [PATCH net-next 3/4] samples/bpf: bpf_tail_call example for tracing Alexei Starovoitov
2015-05-19 23:59 ` [PATCH net-next 4/4] samples/bpf: bpf_tail_call example for networking Alexei Starovoitov
2015-05-21 21:08 ` [PATCH net-next 0/4] bpf: introduce bpf_tail_call() helper David Miller
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