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

[Daniel Borkmann]

On 7/11/23 7:50 PM, Palmer Dabbelt wrote:
> On Mon, 10 Jul 2023 00:41:31 PDT (-0700), bjorn at kernel.org wrote:
>> From: Björn Töpel <bjorn at rivosinc.com>
>>
>> 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 at rivosinc.com>
>> ---
>>   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 at rivosinc.com>
> Reviewed-by: Palmer Dabbelt <palmer at rivosinc.com>
> 
> 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.

Sounds good, we applied it to bpf already.

Thanks,
Daniel