[PATCH] riscv, bpf: Sign extend struct ops return values properly
Pu Lehui
pulehui at huawei.com
Mon Sep 1 01:06:39 PDT 2025
On 2025/8/28 9:53, Pu Lehui wrote:
>
> 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.
Hi Hengqi,
Sorry for late due to busy work. After some backtracking, I dismissed my
doubts about commit 7112cd26e606.
>
> 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;
>> +}
We don't need to sign-ext when return value is 1 or 2 bytes. As for 4
bytes, we have already do that in __build_epilogue. So we only need to
take care of 8 bytes return value. And the real fix would be:
diff --git a/arch/riscv/net/bpf_jit_comp64.c
b/arch/riscv/net/bpf_jit_comp64.c
index 2f7188e0340a..08cc641f8b7c 100644
--- a/arch/riscv/net/bpf_jit_comp64.c
+++ b/arch/riscv/net/bpf_jit_comp64.c
@@ -1177,6 +1177,9 @@ 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);
+ /* Do not truncate return value when it's 8 bytes */
+ if (is_struct_ops && m->ret_size == 8)
+ emit_mv(RV_REG_A0, regmap[BPF_REG_0], ctx);
}
emit_ld(RV_REG_S1, -sreg_off, RV_REG_FP, ctx);
>> +
>> 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);
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