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

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