riscv, bpf: Fix missing sign-ext for signed 1-byte and 2-byte kfunc args

On RV64, the ABI requires sign-extension for signed 1-byte and 2-byte kfunc
args. However, the RV64 JIT currently does not perform sign-extension for
such kfunc args.

Before commit 7ce090afbf ("bpf: Infer zext_dst based on static register
liveness analysis"), state pruning could potentially omit zero-extension
of 32-bit subregisters, which inadvertently masked the above issue by making
the args appear as if they had been properly sign-extended. After that
commit, the problem is exposed, causing the kfunc_call/kfunc_call_test4
selftest to fail.

Fix this by extending the existing sign-extension logic to handle signed
1-byte and 2-byte kfunc args as well.

Fixes: 443574b033 ("riscv, bpf: Fix kfunc parameters incompatibility between bpf and riscv abi")
Signed-off-by: Pu Lehui <pulehui@huawei.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20260814064726.3607615-1-pulehui@huaweicloud.com

arch/riscv/net/bpf_jit_comp64.c

@@ -1823,9 +1823,10 @@ int bpf_jit_emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
 
 			for (idx = 0; idx < fm->nr_args; idx++) {
 				u8 reg = bpf_to_rv_reg(BPF_REG_1 + idx, ctx);
+				bool sign = fm->arg_flags[idx] & BTF_FMODEL_SIGNED_ARG;
 
-				if (fm->arg_size[idx] == sizeof(int))
-					emit_sextw(reg, reg, ctx);
+				if (sign_extend(reg, reg, fm->arg_size[idx], sign, ctx))
+					return -EINVAL;
 			}
 		}