RE: [RFC PATCH net-next 2/6] x86: bpf_jit_comp: support BPF_S_ANC_SECCOMP_LD_W instruction

Eric Dumazet
2013-04-26T15:27:46Z

On Fri, 2013-04-26 at 16:15 +0100, David Laight wrote:
> > >> +#ifdef CONFIG_SECCOMP_FILTER
> > >> + case BPF_S_ANC_SECCOMP_LD_W:
> > >> + if (K == offsetof(struct seccomp_data, arch)) {
> > >> + int arch = syscall_get_arch(current, NULL);
> > >> +
> > >> + EMIT1_off32(0xb8, arch); /* mov arch,%eax */
> > >> + break;
> > >> + }
> > >> + func = (u8 *)seccomp_bpf_load;
> > >> + t_offset = func - (image + addrs[i]);
> > >> + EMIT1_off32(0xbf, K); /* mov imm32,%edi */
> > >> + EMIT1_off32(0xe8, t_offset); /* call seccomp_bpf_load */
> > >> + break;
> > >> +#endif
> > >
> > > This seems seriously wrong to me.
> >
> > Can you elaborate?
>
> The 'call seccomp_bpf_load' needs a pc-relative offset,
> I assume that is what EMIT1_off32() generates.
>
> The other two instructions want an absolute 32 bit value...

Hmm, this part is fine, we perform the relative adjustments in
t_offset = func - (image + addrs[i]);