[PATCH v2] arm64: bpf: Fix branch offset in JIT

[Jean-Philippe Brucker]

On Tue, Sep 15, 2020 at 02:11:03PM +0100, Will Deacon wrote:
> >  		ret = build_insn(insn, ctx, extra_pass);
> >  		if (ret > 0) {
> >  			i++;
> >  			if (ctx->image == NULL)
> > -				ctx->offset[i] = ctx->idx;
> > +				ctx->offset[i] = ctx->offset[i - 1];
> 
> Does it matter that we set the offset for both halves of a 16-byte BPF
> instruction? I think that's a change in behaviour here.

After testing this patch a bit, I think setting only the first slot should
be sufficient, and we can drop these two lines. It does make a minor
difference, because although the BPF verifier normally rejects a program
that jumps into the middle of a 16-byte instruction, it can validate it in
some cases: 

        BPF_LD_IMM64(BPF_REG_0, 2)		// 16-byte immediate load
        BPF_JMP_IMM(BPF_JLE, BPF_REG_0, 1, -2)	// If r0 <= 1, branch to
        BPF_EXIT_INSN()				//  the middle of the insn

The verifier detects that the condition is always false and doesn't follow
the branch, hands the program to the JIT. So if we don't set the second
slot, then we generate an invalid branch offset. But that doesn't really
matter as the branch is never taken.

Thanks,
Jean