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

@@ -3064,6 +3064,11 @@ 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;
 }

arch/s390/net/bpf_jit_comp.c

@@ -2967,6 +2967,11 @@ bool bpf_jit_supports_insn(struct bpf_insn *insn, bool in_arena)
 	case BPF_STX | BPF_ATOMIC | BPF_DW:
 		if (bpf_atomic_is_load_store(insn))
 			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;
 }

arch/x86/net/bpf_jit_comp.c

@@ -1152,11 +1152,38 @@ static void emit_ldx_index(u8 **pprog, u32 size, u32 dst_reg, u32 src_reg, u32 i
 	*pprog = prog;
 }
 
+static void emit_ldsx_index(u8 **pprog, u32 size, u32 dst_reg, u32 src_reg, u32 index_reg, int off)
+{
+	u8 *prog = *pprog;
+
+	switch (size) {
+	case BPF_B:
+		/* movsx rax, byte ptr [rax + r12 + off] */
+		EMIT3(add_3mod(0x48, src_reg, dst_reg, index_reg), 0x0F, 0xBE);
+		break;
+	case BPF_H:
+		/* movsx rax, word ptr [rax + r12 + off] */
+		EMIT3(add_3mod(0x48, src_reg, dst_reg, index_reg), 0x0F, 0xBF);
+		break;
+	case BPF_W:
+		/* movsx rax, dword ptr [rax + r12 + off] */
+		EMIT2(add_3mod(0x48, src_reg, dst_reg, index_reg), 0x63);
+		break;
+	}
+	emit_insn_suffix_SIB(&prog, src_reg, dst_reg, index_reg, off);
+	*pprog = prog;
+}
+
 static void emit_ldx_r12(u8 **pprog, u32 size, u32 dst_reg, u32 src_reg, int off)
 {
 	emit_ldx_index(pprog, size, dst_reg, src_reg, X86_REG_R12, off);
 }
 
+static void emit_ldsx_r12(u8 **prog, u32 size, u32 dst_reg, u32 src_reg, int off)
+{
+	emit_ldsx_index(prog, size, dst_reg, src_reg, X86_REG_R12, off);
+}
+
 /* STX: *(u8*)(dst_reg + off) = src_reg */
 static void emit_stx(u8 **pprog, u32 size, u32 dst_reg, u32 src_reg, int off)
 {
@@ -2109,13 +2136,19 @@ st:			if (is_imm8(insn->off))
 		case BPF_LDX | BPF_PROBE_MEM32 | BPF_H:
 		case BPF_LDX | BPF_PROBE_MEM32 | BPF_W:
 		case BPF_LDX | BPF_PROBE_MEM32 | BPF_DW:
+		case BPF_LDX | BPF_PROBE_MEM32SX | BPF_B:
+		case BPF_LDX | BPF_PROBE_MEM32SX | BPF_H:
+		case BPF_LDX | BPF_PROBE_MEM32SX | BPF_W:
 		case BPF_STX | BPF_PROBE_MEM32 | BPF_B:
 		case BPF_STX | BPF_PROBE_MEM32 | BPF_H:
 		case BPF_STX | BPF_PROBE_MEM32 | BPF_W:
 		case BPF_STX | BPF_PROBE_MEM32 | BPF_DW:
 			start_of_ldx = prog;
 			if (BPF_CLASS(insn->code) == BPF_LDX)
-				emit_ldx_r12(&prog, BPF_SIZE(insn->code), dst_reg, src_reg, insn->off);
+				if (BPF_MODE(insn->code) == BPF_PROBE_MEM32SX)
+					emit_ldsx_r12(&prog, BPF_SIZE(insn->code), dst_reg, src_reg, insn->off);
+				else
+					emit_ldx_r12(&prog, BPF_SIZE(insn->code), dst_reg, src_reg, insn->off);
 			else
 				emit_stx_r12(&prog, BPF_SIZE(insn->code), dst_reg, src_reg, insn->off);
 populate_extable:

include/linux/filter.h

@@ -78,6 +78,9 @@ struct ctl_table_header;
 /* unused opcode to mark special atomic instruction */
 #define BPF_PROBE_ATOMIC 0xe0
 
+/* unused opcode to mark special ldsx instruction. Same as BPF_NOSPEC */
+#define BPF_PROBE_MEM32SX 0xc0
+
 /* unused opcode to mark call to interpreter with arguments */
 #define BPF_CALL_ARGS	0xe0
 

kernel/bpf/verifier.c

@@ -21422,10 +21422,14 @@ static int convert_ctx_accesses(struct bpf_verifier_env *env)
 			continue;
 		case PTR_TO_ARENA:
 			if (BPF_MODE(insn->code) == BPF_MEMSX) {
-				verbose(env, "sign extending loads from arena are not supported yet\n");
-				return -EOPNOTSUPP;
+				if (!bpf_jit_supports_insn(insn, true)) {
+					verbose(env, "sign extending loads from arena are not supported yet\n");
+					return -EOPNOTSUPP;
+				}
+				insn->code = BPF_CLASS(insn->code) | BPF_PROBE_MEM32SX | BPF_SIZE(insn->code);
+			} else {
+				insn->code = BPF_CLASS(insn->code) | BPF_PROBE_MEM32 | BPF_SIZE(insn->code);
 			}
-			insn->code = BPF_CLASS(insn->code) | BPF_PROBE_MEM32 | BPF_SIZE(insn->code);
 			env->prog->aux->num_exentries++;
 			continue;
 		default:
@@ -21631,6 +21635,7 @@ static int jit_subprogs(struct bpf_verifier_env *env)
 			if (BPF_CLASS(insn->code) == BPF_LDX &&
 			    (BPF_MODE(insn->code) == BPF_PROBE_MEM ||
 			     BPF_MODE(insn->code) == BPF_PROBE_MEM32 ||
+			     BPF_MODE(insn->code) == BPF_PROBE_MEM32SX ||
 			     BPF_MODE(insn->code) == BPF_PROBE_MEMSX))
 				num_exentries++;
 			if ((BPF_CLASS(insn->code) == BPF_STX ||