linux-kernel - RE: [RFC PATCH net-next 2/6] x86: bpf_jit_comp: support BPF_S_ANC_SECCOMP_LD

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Date:	Fri, 26 Apr 2013 16:38:09 +0100
From:	"David Laight" <David.Laight@...LAB.COM>
To:	"Eric Dumazet" <eric.dumazet@...il.com>
Cc:	"Xi Wang" <xi.wang@...il.com>,
	"Daniel Borkmann" <dborkman@...hat.com>,
	"David S. Miller" <davem@...emloft.net>,
	"Russell King" <linux@....linux.org.uk>,
	"Heiko Carstens" <heiko.carstens@...ibm.com>,
	"Eric Dumazet" <edumazet@...gle.com>,
	"Will Drewry" <wad@...omium.org>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	<netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: RE: [RFC PATCH net-next 2/6] x86: bpf_jit_comp: support BPF_S_ANC_SECCOMP_LD_W instruction

> > > >> +                             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]);

The call needs the displacement from the address of
the instruction following the call.
I can't imagine any way in which above can allow for the 5 byte
'mov imm32,%edi' instruction.

I'd have thought there would be an EMIT1_imm32().
(I've written a lot of x86 asm in my days!)

	David