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?
