bpf, x86: Add support for signed arena loads

Currently, signed load instructions into arena memory are unsupported.
The compiler is free to generate these, and on GCC-14 we see a
corresponding error when it happens. The hurdle in supporting them is
deciding which unused opcode to use to mark them for the JIT's own
consumption. After much thinking, it appears 0xc0 / BPF_NOSPEC can be
combined with load instructions to identify signed arena loads. Use
this to recognize and JIT them appropriately, and remove the verifier
side limitation on the program if the JIT supports them.

Co-developed-by: Puranjay Mohan <[email protected]>
Signed-off-by: Kumar Kartikeya Dwivedi <[email protected]>
Signed-off-by: Puranjay Mohan <[email protected]>

Author: kkdwvd

arch/arm64/net/bpf_jit_comp.c

@@ -3073,6 +3073,11 @@ bool bpf_jit_supports_insn(struct bpf_insn *insn, bool in_arena)
 		if (!bpf_atomic_is_load_store(insn) &&
 		    !cpus_have_cap(ARM64_HAS_LSE_ATOMICS))
 			return false;
+		break;
+	case BPF_LDX | BPF_MEMSX | BPF_B:
+	case BPF_LDX | BPF_MEMSX | BPF_H:
+	case BPF_LDX | BPF_MEMSX | BPF_W:
+		return false;
 	}
 	return true;
 }