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

[Ilias Apalodimas]

On Tue, Sep 15, 2020 at 02:11:03PM +0100, Will Deacon wrote:
> Hi Ilias,
> 
> On Mon, Sep 14, 2020 at 07:03:55PM +0300, Ilias Apalodimas wrote:
> > Running the eBPF test_verifier leads to random errors looking like this:
> > 
> > [ 6525.735488] Unexpected kernel BRK exception at EL1
> > [ 6525.735502] Internal error: ptrace BRK handler: f2000100 [#1] SMP
> 
> Does this happen because we poison the BPF memory with BRK instructions?
> Maybe we should look at using a special immediate so we can detect this,
> rather than end up in the ptrace handler.

As discussed offline this is what aarch64_insn_gen_branch_imm() will return for
offsets > 128M and yes replacing the handler with a more suitable message would 
be good.

> 
> > diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
> > index f8912e45be7a..0974effff58c 100644
> > --- a/arch/arm64/net/bpf_jit_comp.c
> > +++ b/arch/arm64/net/bpf_jit_comp.c
> > @@ -143,9 +143,13 @@ static inline void emit_addr_mov_i64(const int reg, const u64 val,
> >  	}
> >  }
> >  
> > -static inline int bpf2a64_offset(int bpf_to, int bpf_from,
> > +static inline int bpf2a64_offset(int bpf_insn, int off,
> >  				 const struct jit_ctx *ctx)
> >  {
> > +	/* arm64 offset is relative to the branch instruction */
> > +	int bpf_from = bpf_insn + 1;
> > +	/* BPF JMP offset is relative to the next instruction */
> > +	int bpf_to = bpf_insn + off + 1;
> >  	int to = ctx->offset[bpf_to];
> >  	/* -1 to account for the Branch instruction */
> >  	int from = ctx->offset[bpf_from] - 1;
> 
> I think this is a bit confusing with all the variables. How about just
> doing:
> 
> 	/* BPF JMP offset is relative to the next BPF instruction */
> 	bpf_insn++;
> 
> 	/*
> 	 * Whereas arm64 branch instructions encode the offset from the
> 	 * branch itself, so we must subtract 1 from the instruction offset.
> 	 */
> 	return ctx->offset[bpf_insn + off] - ctx->offset[bpf_insn] - 1;
> 

Sure

> > @@ -642,7 +646,7 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx,
> >  
> >  	/* JUMP off */
> >  	case BPF_JMP | BPF_JA:
> > -		jmp_offset = bpf2a64_offset(i + off, i, ctx);
> > +		jmp_offset = bpf2a64_offset(i, off, ctx);
> >  		check_imm26(jmp_offset);
> >  		emit(A64_B(jmp_offset), ctx);
> >  		break;
> > @@ -669,7 +673,7 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx,
> >  	case BPF_JMP32 | BPF_JSLE | BPF_X:
> >  		emit(A64_CMP(is64, dst, src), ctx);
> >  emit_cond_jmp:
> > -		jmp_offset = bpf2a64_offset(i + off, i, ctx);
> > +		jmp_offset = bpf2a64_offset(i, off, ctx);
> >  		check_imm19(jmp_offset);
> >  		switch (BPF_OP(code)) {
> >  		case BPF_JEQ:
> > @@ -912,18 +916,26 @@ static int build_body(struct jit_ctx *ctx, bool extra_pass)
> >  		const struct bpf_insn *insn = &prog->insnsi[i];
> >  		int ret;
> >  
> > +		/*
> > +		 * offset[0] offset of the end of prologue, start of the
> > +		 * first insn.
> > +		 * offset[x] - offset of the end of x insn.
> 
> So does offset[1] point at the last arm64 instruction for the first BPF
> instruction, or does it point to the first arm64 instruction for the second
> BPF instruction?
> 

Right this isn't exactly a good comment. 
I'll change it to something like:

offset[0] - offset of the end of prologue, start of the 1st insn.
offset[1] - offset of the end of 1st insn.

> > +		 */
> > +		if (ctx->image == NULL)
> > +			ctx->offset[i] = ctx->idx;
> > +
> >  		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.

Yes it is, but from reading around that's what I understood.
for 16-byte eBPF instructions both should point to the start of 
the corresponding jited arm64 instruction.
If I am horribly wrong about this, please shout.

> 
> >  			continue;
> >  		}
> > -		if (ctx->image == NULL)
> > -			ctx->offset[i] = ctx->idx;
> >  		if (ret)
> >  			return ret;
> >  	}
> > +	if (ctx->image == NULL)
> > +		ctx->offset[i] = ctx->idx;
> 
> I think it would be cleared to set ctx->offset[0] before the for loop (with
> a comment about what it is) and then change the for loop to iterate from 1
> all the way to prog->len.
> 

Sure

> Will

Thanks
/Ilias