riscv, bpf: Sign extend struct ops return values properly

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 [...] [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.

So let's sign extend struct ops return values according to the function
model and RISC-V ABI([0]).

  [0]: https://riscv.org/wp-content/uploads/2024/12/riscv-calling.pdf

Fixes: 25ad106 ("riscv, bpf: Adapt bpf trampoline to optimized riscv ftrace framework")
Signed-off-by: Hengqi Chen <hengqi.chen@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: Pu Lehui <pulehui@huawei.com>
Reviewed-by: Pu Lehui <pulehui@huawei.com>
Link: https://lore.kernel.org/bpf/20250908012448.1695-1-hengqi.chen@gmail.com

Author: chenhengqi

arch/riscv/net/bpf_jit_comp64.c

@@ -711,6 +711,39 @@ static int emit_atomic_rmw(u8 rd, u8 rs, const struct bpf_insn *insn,
 	return 0;
 }
 
+/*
+ * Sign-extend the register if necessary
+ */
+static int sign_extend(u8 rd, u8 rs, u8 sz, bool sign, struct rv_jit_context *ctx)
+{
+	if (!sign && (sz == 1 || sz == 2)) {
+		if (rd != rs)
+			emit_mv(rd, rs, ctx);
+		return 0;
+	}
+
+	switch (sz) {
+	case 1:
+		emit_sextb(rd, rs, ctx);
+		break;
+	case 2:
+		emit_sexth(rd, rs, ctx);
+		break;
+	case 4:
+		emit_sextw(rd, rs, ctx);
+		break;
+	case 8:
+		if (rd != rs)
+			emit_mv(rd, rs, ctx);
+		break;
+	default:
+		pr_err("bpf-jit: invalid size %d for sign_extend\n", sz);
+		return -EINVAL;
+	}
+
+	return 0;
+}
+
 #define BPF_FIXUP_OFFSET_MASK   GENMASK(26, 0)
 #define BPF_FIXUP_REG_MASK      GENMASK(31, 27)
 #define REG_DONT_CLEAR_MARKER	0	/* RV_REG_ZERO unused in pt_regmap */
@@ -1175,8 +1208,15 @@ static int __arch_prepare_bpf_trampoline(struct bpf_tramp_image *im,
 		restore_args(min_t(int, nr_arg_slots, RV_MAX_REG_ARGS), args_off, ctx);
 
 	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) {
+			ret = sign_extend(RV_REG_A0, regmap[BPF_REG_0], m->ret_size,
+					  m->ret_flags & BTF_FMODEL_SIGNED_ARG, ctx);
+			if (ret)
+				goto out;
+		} else {
+			emit_ld(RV_REG_A0, -retval_off, RV_REG_FP, ctx);
+		}
 	}
 
 	emit_ld(RV_REG_S1, -sreg_off, RV_REG_FP, ctx);