netdev - Re: [PATCH bpf] bpf, x64: implement retpoline for tail call

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Date:   Wed, 21 Feb 2018 19:53:22 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Daniel Borkmann <daniel@...earbox.net>, ast@...nel.org,
        torvalds@...ux-foundation.org, netdev@...r.kernel.org
Subject: Re: [PATCH bpf] bpf, x64: implement retpoline for tail call

On Wed, 2018-02-21 at 19:43 -0800, Alexei Starovoitov wrote:
> On Wed, Feb 21, 2018 at 07:04:02PM -0800, Eric Dumazet wrote:
> > On Thu, 2018-02-22 at 01:05 +0100, Daniel Borkmann wrote:
> > 
> > ...
> > 
> > > +/* Instead of plain jmp %rax, we emit a retpoline to control
> > > + * speculative execution for the indirect branch.
> > > + */
> > > +static void emit_retpoline_rax_trampoline(u8 **pprog)
> > > +{
> > > +	u8 *prog = *pprog;
> > > +	int cnt = 0;
> > > +
> > > +	EMIT1_off32(0xE8, 7);	 /* callq <set_up_target> */
> > > +	/* capture_spec: */
> > > +	EMIT2(0xF3, 0x90);	 /* pause */
> > > +	EMIT3(0x0F, 0xAE, 0xE8); /* lfence */
> > > +	EMIT2(0xEB, 0xF9);	 /* jmp <capture_spec> */
> > > +	/* set_up_target: */
> > > +	EMIT4(0x48, 0x89, 0x04, 0x24); /* mov %rax,(%rsp) */
> > > +	EMIT1(0xC3);		 /* retq */
> > > +
> > > +	BUILD_BUG_ON(cnt != RETPOLINE_SIZE);
> > > +	*pprog = prog;
> > 
> > You might define the actual code sequence (and length) in 
> > arch/x86/include/asm/nospec-branch.h
> > 
> > If we need to adjust code sequences for RETPOLINE, then we wont
> > forget/miss that arch/x86/net/bpf_jit_comp.c had it hard-coded.
> 
> like adding a comment to asm/nospec-branch.h that says
> "dont forget to adjust bpf_jit_comp.c" ?
> but clang/gcc generate slightly different sequences for
> retpoline anyway, so even if '.macro RETPOLINE_JMP' in
> nospec-branch.h changes it doesn't mean that x64 jit has to change.
> So what kinda comment there would make sense?

I was thinking of something very explicit :

/* byte sequence for following assembly code used by eBPF
   call ...
   ...
   retq
*/
#define RETPOLINE_RAX_DIRECT_FOR_EBPF                         \
       EMIT1_off32(0xE8, 7);    /* callq <set_up_target> */   \
       /* capture_spec: */                                    \
       EMIT2(0xF3, 0x90);       /* pause */                   \
       EMIT3(0x0F, 0xAE, 0xE8); /* lfence */                  \
       EMIT2(0xEB, 0xF9);       /* jmp <capture_spec> */      \
       /* set_up_target: */                                   \
       EMIT4(0x48, 0x89, 0x04, 0x24); /* mov %rax,(%rsp) */   \
       EMIT1(0xC3);             /* retq */                    \

Might be simply byte encoded, (array of 17 bytes)

Well, something like that anyway...