[PATCH 1/5] LoongArch: BPF: Gate unsupported arena instructions via bpf_jit_supports_insn()

[George Guo <dongtai.guo@linux.dev>]

From: George Guo <guodongtai@kylinos.cn>

The JIT does not implement atomics on arena pointers (BPF_PROBE_ATOMIC)
nor sign-extending loads from the arena (BPF_PROBE_MEM32SX). Without a
bpf_jit_supports_insn() callback the verifier assumes both are available,
so such programs are accepted only to fail later in the JIT with a
confusing -EINVAL 'unknown opcode'.

Implement bpf_jit_supports_insn() to reject these instructions in the
arena case. The verifier then rejects the program early with a clear
message ('BPF_ATOMIC stores into R<n> ... is not allowed' / 'sign
extending loads from arena are not supported yet'). Regular arena
accesses (BPF_PROBE_MEM32 loads/stores of all sizes) remain supported.

Signed-off-by: George Guo <guodongtai@kylinos.cn>
---
 arch/loongarch/net/bpf_jit.c | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/arch/loongarch/net/bpf_jit.c b/arch/loongarch/net/bpf_jit.c
index 24913dc7f4e8..3f9ffdde2491 100644
--- a/arch/loongarch/net/bpf_jit.c
+++ b/arch/loongarch/net/bpf_jit.c
@@ -2357,6 +2357,26 @@ bool bpf_jit_supports_arena(void)
 	return true;
 }
 
+bool bpf_jit_supports_insn(struct bpf_insn *insn, bool in_arena)
+{
+	if (!in_arena)
+		return true;
+
+	switch (insn->code) {
+	case BPF_STX | BPF_ATOMIC | BPF_W:
+	case BPF_STX | BPF_ATOMIC | BPF_DW:
+		/* Atomics on arena pointers are not implemented yet. */
+		return false;
+	case BPF_LDX | BPF_MEMSX | BPF_B:
+	case BPF_LDX | BPF_MEMSX | BPF_H:
+	case BPF_LDX | BPF_MEMSX | BPF_W:
+		/* Sign-extending loads from arena are not implemented yet. */
+		return false;
+	}
+
+	return true;
+}
+
 bool bpf_jit_supports_fsession(void)
 {
 	return true;
-- 
2.25.1