linux-kernel - Re: [PATCH net-next 2/4] x86: bpf_jit: implement bpf_tail

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: <CALCETrVZyB+m8Oaad1JGwrLx1rTUoL0F6Afrvg_hQWiYyTTang@mail.gmail.com>
Date:	Tue, 19 May 2015 17:11:59 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Alexei Starovoitov <ast@...mgrid.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Ingo Molnar <mingo@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Michael Holzheu <holzheu@...ux.vnet.ibm.com>,
	Zi Shen Lim <zlim.lnx@...il.com>,
	Linux API <linux-api@...r.kernel.org>,
	Network Development <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next 2/4] x86: bpf_jit: implement bpf_tail_call() helper

On Tue, May 19, 2015 at 4:59 PM, Alexei Starovoitov <ast@...mgrid.com> wrote:
> bpf_tail_call() arguments:
> ctx - context pointer
> jmp_table - one of BPF_MAP_TYPE_PROG_ARRAY maps used as the jump table
> index - index in the jump table
>
> In this implementation x64 JIT bypasses stack unwind and jumps into the
> callee program after prologue, so the callee program reuses the same stack.
>
> The logic can be roughly expressed in C like:
>
> u32 tail_call_cnt;
>
> void *jumptable[2] = { &&label1, &&label2 };
>
> int bpf_prog1(void *ctx)
> {
> label1:
>     ...
> }
>
> int bpf_prog2(void *ctx)
> {
> label2:
>     ...
> }
>
> int bpf_prog1(void *ctx)
> {
>     ...
>     if (tail_call_cnt++ < MAX_TAIL_CALL_CNT)
>         goto *jumptable[index]; ... and pass my 'ctx' to callee ...
>
>     ... fall through if no entry in jumptable ...
> }
>

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?

--Andy
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/