[PATCH] riscv, bpf: Sign extend struct ops return values properly

[Pu Lehui]

On 2025/8/27 20:03, Hengqi Chen wrote:
> The ns_bpf_qdisc selftest triggers a kernel panic:
> 
>      Unable to handle kernel paging request at virtual address ffffffffa38dbf58
>      Current test_progs pgtable: 4K pagesize, 57-bit VAs, pgdp=0x00000001109cc000
>      [ffffffffa38dbf58] pgd=000000011fffd801, p4d=000000011fffd401, pud=000000011fffd001, pmd=0000000000000000
>      Oops [#1]
>      Modules linked in: bpf_testmod(OE) xt_conntrack nls_iso8859_1 dm_mod drm drm_panel_orientation_quirks configfs backlight btrfs blake2b_generic xor lzo_compress zlib_deflate raid6_pq efivarfs [last unloaded: bpf_testmod(OE)]
>      CPU: 1 UID: 0 PID: 23584 Comm: test_progs Tainted: G        W  OE       6.17.0-rc1-g2465bb83e0b4 #1 NONE
>      Tainted: [W]=WARN, [O]=OOT_MODULE, [E]=UNSIGNED_MODULE
>      Hardware name: Unknown Unknown Product/Unknown Product, BIOS 2024.01+dfsg-1ubuntu5.1 01/01/2024
>      epc : __qdisc_run+0x82/0x6f0
>       ra : __qdisc_run+0x6e/0x6f0
>      epc : ffffffff80bd5c7a ra : ffffffff80bd5c66 sp : ff2000000eecb550
>       gp : ffffffff82472098 tp : ff60000096895940 t0 : ffffffff8001f180
>       t1 : ffffffff801e1664 t2 : 0000000000000000 s0 : ff2000000eecb5d0
>       s1 : ff60000093a6a600 a0 : ffffffffa38dbee8 a1 : 0000000000000001
>       a2 : ff2000000eecb510 a3 : 0000000000000001 a4 : 0000000000000000
>       a5 : 0000000000000010 a6 : 0000000000000000 a7 : 0000000000735049
>       s2 : ffffffffa38dbee8 s3 : 0000000000000040 s4 : ff6000008bcda000
>       s5 : 0000000000000008 s6 : ff60000093a6a680 s7 : ff60000093a6a6f0
>       s8 : ff60000093a6a6ac s9 : ff60000093140000 s10: 0000000000000000
>       s11: ff2000000eecb9d0 t3 : 0000000000000000 t4 : 0000000000ff0000
>       t5 : 0000000000000000 t6 : ff60000093a6a8b6
>      status: 0000000200000120 badaddr: ffffffffa38dbf58 cause: 000000000000000d
>      [<ffffffff80bd5c7a>] __qdisc_run+0x82/0x6f0
>      [<ffffffff80b6fe58>] __dev_queue_xmit+0x4c0/0x1128
>      [<ffffffff80b80ae0>] neigh_resolve_output+0xd0/0x170
>      [<ffffffff80d2daf6>] ip6_finish_output2+0x226/0x6c8
>      [<ffffffff80d31254>] ip6_finish_output+0x10c/0x2a0
>      [<ffffffff80d31446>] ip6_output+0x5e/0x178
>      [<ffffffff80d2e232>] ip6_xmit+0x29a/0x608
>      [<ffffffff80d6f4c6>] inet6_csk_xmit+0xe6/0x140
>      [<ffffffff80c985e4>] __tcp_transmit_skb+0x45c/0xaa8
>      [<ffffffff80c995fe>] tcp_connect+0x9ce/0xd10
>      [<ffffffff80d66524>] tcp_v6_connect+0x4ac/0x5e8
>      [<ffffffff80cc19b8>] __inet_stream_connect+0xd8/0x318
>      [<ffffffff80cc1c36>] inet_stream_connect+0x3e/0x68
>      [<ffffffff80b42b20>] __sys_connect_file+0x50/0x88
>      [<ffffffff80b42bee>] __sys_connect+0x96/0xc8
>      [<ffffffff80b42c40>] __riscv_sys_connect+0x20/0x30
>      [<ffffffff80e5bcae>] do_trap_ecall_u+0x256/0x378
>      [<ffffffff80e69af2>] handle_exception+0x14a/0x156
>      Code: 892a 0363 1205 489c 8bc1 c7e5 2d03 084a 2703 080a (2783) 0709
>      ---[ end trace 0000000000000000 ]---
> 
> The bpf_fifo_dequeue prog returns a skb which is a pointer.
> The pointer is treated as a 32bit value and sign extend to
> 64bit in epilogue. This behavior is right for most bpf prog
> types but wrong for struct ops which requires RISC-V ABI.

Hi Hengqi,

Nice catch!

Actually, I think commit 7112cd26e606c7ba51f9cc5c1905f06039f6f379 looks 
a little bit wired and related to this issue. I guess I need some time 
to recall this commit.

Thanks.

> 
> So let's sign extend struct ops return values according to
> the return value spec in function model.
> 
> Fixes: 25ad10658dc1 ("riscv, bpf: Adapt bpf trampoline to optimized riscv ftrace framework")
> Signed-off-by: Hengqi Chen <hengqi.chen at gmail.com>
> ---
>   arch/riscv/net/bpf_jit_comp64.c | 33 +++++++++++++++++++++++++++++++++
>   1 file changed, 33 insertions(+)
> 
> diff --git a/arch/riscv/net/bpf_jit_comp64.c b/arch/riscv/net/bpf_jit_comp64.c
> index 549c3063c7f1..11ca56320a3f 100644
> --- a/arch/riscv/net/bpf_jit_comp64.c
> +++ b/arch/riscv/net/bpf_jit_comp64.c
> @@ -954,6 +954,33 @@ static int invoke_bpf_prog(struct bpf_tramp_link *l, int args_off, int retval_of
>   	return ret;
>   }
>   
> +/*
> + * Sign-extend the register if necessary
> + */
> +static int sign_extend(struct rv_jit_context *ctx, int r, u8 size)
> +{
> +	switch (size) {
> +	case 1:
> +		emit_slli(r, r, 56, ctx);
> +		emit_srai(r, r, 56, ctx);
> +		break;
> +	case 2:
> +		emit_slli(r, r, 48, ctx);
> +		emit_srai(r, r, 48, ctx);
> +		break;
> +	case 4:
> +		emit_addiw(r, r, 0, ctx);
> +		break;
> +	case 8:
> +		break;
> +	default:
> +		pr_err("bpf-jit: invalid size %d for sign_extend\n", size);
> +		return -EINVAL;
> +	}
> +
> +	return 0;
> +}
> +
>   static int __arch_prepare_bpf_trampoline(struct bpf_tramp_image *im,
>   					 const struct btf_func_model *m,
>   					 struct bpf_tramp_links *tlinks,
> @@ -1177,6 +1204,12 @@ static int __arch_prepare_bpf_trampoline(struct bpf_tramp_image *im,
>   	if (save_ret) {
>   		emit_ld(RV_REG_A0, -retval_off, RV_REG_FP, ctx);
>   		emit_ld(regmap[BPF_REG_0], -(retval_off - 8), RV_REG_FP, ctx);
> +		if (is_struct_ops) {
> +			emit_mv(RV_REG_A0, regmap[BPF_REG_0], ctx);
> +			ret = sign_extend(ctx, RV_REG_A0, m->ret_size);
> +			if (ret)
> +				goto out;
> +		}
>   	}
>   
>   	emit_ld(RV_REG_S1, -sreg_off, RV_REG_FP, ctx);