Re: [PATCH bpf] riscv, bpf: Fix inconsistent JIT image generation

[Palmer Dabbelt]

On Mon, 10 Jul 2023 00:41:31 PDT (-0700), bjorn@xxxxxxxxxx wrote:
> From: Björn Töpel <bjorn@xxxxxxxxxxxx>
>
> In order to generate the prologue and epilogue, the BPF JIT needs to
> know which registers that are clobbered. Therefore, the during
> pre-final passes, the prologue is generated after the body of the
> program body-prologue-epilogue. Then, in the final pass, a proper
> prologue-body-epilogue JITted image is generated.
>
> This scheme has worked most of the time. However, for some large
> programs with many jumps, e.g. the test_kmod.sh BPF selftest with
> hardening enabled (blinding constants), this has shown to be
> incorrect. For the final pass, when the proper prologue-body-epilogue
> is generated, the image has not converged. This will lead to that the
> final image will have incorrect jump offsets. The following is an
> excerpt from an incorrect image:
>
>   | ...
>   |     3b8:       00c50663                beq     a0,a2,3c4 <.text+0x3c4>
>   |     3bc:       0020e317                auipc   t1,0x20e
>   |     3c0:       49630067                jalr    zero,1174(t1) # 20e852 <.text+0x20e852>
>   | ...
>   |  20e84c:       8796                    c.mv    a5,t0
>   |  20e84e:       6422                    c.ldsp  s0,8(sp)    # Epilogue start
>   |  20e850:       6141                    c.addi16sp      sp,16
>   |  20e852:       853e                    c.mv    a0,a5       # Incorrect jump target
>   |  20e854:       8082                    c.jr    ra
>
> The image has shrunk, and the epilogue offset is incorrect in the
> final pass.
>
> Correct the problem by always generating proper prologue-body-epilogue
> outputs, which means that the first pass will only generate the body
> to track what registers that are touched.
>
> Fixes: 2353ecc6f91f ("bpf, riscv: add BPF JIT for RV64G")
> Signed-off-by: Björn Töpel <bjorn@xxxxxxxxxxxx>
> ---
>  arch/riscv/net/bpf_jit.h      |  6 +++---
>  arch/riscv/net/bpf_jit_core.c | 19 +++++++++++++------
>  2 files changed, 16 insertions(+), 9 deletions(-)
>
> diff --git a/arch/riscv/net/bpf_jit.h b/arch/riscv/net/bpf_jit.h
> index bf9802a63061..2717f5490428 100644
> --- a/arch/riscv/net/bpf_jit.h
> +++ b/arch/riscv/net/bpf_jit.h
> @@ -69,7 +69,7 @@ struct rv_jit_context {
>  	struct bpf_prog *prog;
>  	u16 *insns;		/* RV insns */
>  	int ninsns;
> -	int body_len;
> +	int prologue_len;
>  	int epilogue_offset;
>  	int *offset;		/* BPF to RV */
>  	int nexentries;
> @@ -216,8 +216,8 @@ static inline int rv_offset(int insn, int off, struct rv_jit_context *ctx)
>  	int from, to;
>
>  	off++; /* BPF branch is from PC+1, RV is from PC */
> -	from = (insn > 0) ? ctx->offset[insn - 1] : 0;
> -	to = (insn + off > 0) ? ctx->offset[insn + off - 1] : 0;
> +	from = (insn > 0) ? ctx->offset[insn - 1] : ctx->prologue_len;
> +	to = (insn + off > 0) ? ctx->offset[insn + off - 1] : ctx->prologue_len;
>  	return ninsns_rvoff(to - from);
>  }
>
> diff --git a/arch/riscv/net/bpf_jit_core.c b/arch/riscv/net/bpf_jit_core.c
> index 737baf8715da..7a26a3e1c73c 100644
> --- a/arch/riscv/net/bpf_jit_core.c
> +++ b/arch/riscv/net/bpf_jit_core.c
> @@ -44,7 +44,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
>  	unsigned int prog_size = 0, extable_size = 0;
>  	bool tmp_blinded = false, extra_pass = false;
>  	struct bpf_prog *tmp, *orig_prog = prog;
> -	int pass = 0, prev_ninsns = 0, prologue_len, i;
> +	int pass = 0, prev_ninsns = 0, i;
>  	struct rv_jit_data *jit_data;
>  	struct rv_jit_context *ctx;
>
> @@ -83,6 +83,12 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
>  		prog = orig_prog;
>  		goto out_offset;
>  	}
> +
> +	if (build_body(ctx, extra_pass, NULL)) {
> +		prog = orig_prog;
> +		goto out_offset;
> +	}
> +
>  	for (i = 0; i < prog->len; i++) {
>  		prev_ninsns += 32;
>  		ctx->offset[i] = prev_ninsns;
> @@ -91,12 +97,15 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
>  	for (i = 0; i < NR_JIT_ITERATIONS; i++) {
>  		pass++;
>  		ctx->ninsns = 0;
> +
> +		bpf_jit_build_prologue(ctx);
> +		ctx->prologue_len = ctx->ninsns;
> +
>  		if (build_body(ctx, extra_pass, ctx->offset)) {
>  			prog = orig_prog;
>  			goto out_offset;
>  		}
> -		ctx->body_len = ctx->ninsns;
> -		bpf_jit_build_prologue(ctx);
> +
>  		ctx->epilogue_offset = ctx->ninsns;
>  		bpf_jit_build_epilogue(ctx);
>
> @@ -162,10 +171,8 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
>
>  	if (!prog->is_func || extra_pass) {
>  		bpf_jit_binary_lock_ro(jit_data->header);
> -		prologue_len = ctx->epilogue_offset - ctx->body_len;
>  		for (i = 0; i < prog->len; i++)
> -			ctx->offset[i] = ninsns_rvoff(prologue_len +
> -						      ctx->offset[i]);
> +			ctx->offset[i] = ninsns_rvoff(ctx->offset[i]);
>  		bpf_prog_fill_jited_linfo(prog, ctx->offset);
>  out_offset:
>  		kfree(ctx->offset);
>
> base-commit: 496720b7cfb6574a8f6f4d434f23e3d1e6cfaeb9

Acked-by: Palmer Dabbelt <palmer@xxxxxxxxxxxx>
Reviewed-by: Palmer Dabbelt <palmer@xxxxxxxxxxxx>

I'm assuming this is aimed at the BPF tree, but LMK if you guys want me 
to pick it up -- I've already got something for this week, so it's easy 
on my end.  I'm dropping it from my queue and patchwork for now, though.

Thanks for the fix!