[RFC bpf-next 2/2] bpf, mips: Add support for BPF_MOVSX in the JITs

[Nicholas Dudar <main.kalliope@gmail.com>]

The MIPS32 and MIPS64 JITs lower register BPF_MOVSX instructions as
ordinary moves because their register-move paths do not interpret
insn->off. Negative low-width values therefore retain incorrect upper
bits.

Teach the register-move helpers to sign-extend the BPF-defined MOVSX
widths: 8 and 16 for ALU32, and 8, 16, and 32 for ALU64. Propagate the
sign into the MIPS32 high word for ALU64, while retaining the existing
verifier-managed zero extension for ALU32.

Keep the verifier-inserted zero-extension move on its dedicated path
and interpret the raw offset within the MOV helpers.

Assisted-by: Codex:gpt-5.6-sol
Signed-off-by: Nicholas Dudar <main.kalliope@gmail.com>
---
 arch/mips/net/bpf_jit_comp32.c | 50 ++++++++++++++++++++++++++++------
 arch/mips/net/bpf_jit_comp64.c | 46 +++++++++++++++++++++++++------
 2 files changed, 79 insertions(+), 17 deletions(-)

diff --git a/arch/mips/net/bpf_jit_comp32.c b/arch/mips/net/bpf_jit_comp32.c
index bfe73b023983e..66ade9d77638d 100644
--- a/arch/mips/net/bpf_jit_comp32.c
+++ b/arch/mips/net/bpf_jit_comp32.c
@@ -190,20 +190,52 @@ static void emit_zext_ver(struct jit_context *ctx, const u8 dst[])
 	}
 }
 
-/* Register move operation (32-bit) */
+/* Register move operation (32-bit), optionally with sign extension */
 static void emit_mov_r32(struct jit_context *ctx, const u8 dst[],
-			 const u8 src[])
+			 const u8 src[], s16 off)
 {
-	emit_mov_r(ctx, lo(dst), lo(src));
+	switch (off) {
+	case 8:
+		emit(ctx, sll, lo(dst), lo(src), 24);
+		emit(ctx, sra, lo(dst), lo(dst), 24);
+		clobber_reg(ctx, lo(dst));
+		break;
+	case 16:
+		emit(ctx, sll, lo(dst), lo(src), 16);
+		emit(ctx, sra, lo(dst), lo(dst), 16);
+		clobber_reg(ctx, lo(dst));
+		break;
+	default:
+		emit_mov_r(ctx, lo(dst), lo(src));
+		break;
+	}
 	emit_zext_ver(ctx, dst);
 }
 
-/* Register move operation (64-bit) */
+/* Register move operation (64-bit), optionally with sign extension */
 static void emit_mov_r64(struct jit_context *ctx, const u8 dst[],
-			 const u8 src[])
+			 const u8 src[], s16 off)
 {
-	emit_mov_r(ctx, lo(dst), lo(src));
-	emit_mov_r(ctx, hi(dst), hi(src));
+	switch (off) {
+	case 8:
+		emit(ctx, sll, lo(dst), lo(src), 24);
+		emit(ctx, sra, lo(dst), lo(dst), 24);
+		break;
+	case 16:
+		emit(ctx, sll, lo(dst), lo(src), 16);
+		emit(ctx, sra, lo(dst), lo(dst), 16);
+		break;
+	case 32:
+		emit(ctx, move, lo(dst), lo(src));
+		break;
+	default:
+		emit_mov_r(ctx, lo(dst), lo(src));
+		emit_mov_r(ctx, hi(dst), hi(src));
+		return;
+	}
+	clobber_reg(ctx, lo(dst));
+	emit(ctx, sra, hi(dst), lo(dst), 31);
+	clobber_reg(ctx, hi(dst));
 }
 
 /* Load delay slot, if ISA mandates it */
@@ -1501,7 +1533,7 @@ int build_insn(const struct bpf_insn *insn, struct jit_context *ctx)
 			/* Special mov32 for zext */
 			emit_mov_i(ctx, hi(dst), 0);
 		} else {
-			emit_mov_r32(ctx, dst, src);
+			emit_mov_r32(ctx, dst, src, off);
 		}
 		break;
 	/* dst = -dst */
@@ -1570,7 +1602,7 @@ int build_insn(const struct bpf_insn *insn, struct jit_context *ctx)
 		break;
 	/* dst = src (64-bit) */
 	case BPF_ALU64 | BPF_MOV | BPF_X:
-		emit_mov_r64(ctx, dst, src);
+		emit_mov_r64(ctx, dst, src, off);
 		break;
 	/* dst = -dst (64-bit) */
 	case BPF_ALU64 | BPF_NEG:
diff --git a/arch/mips/net/bpf_jit_comp64.c b/arch/mips/net/bpf_jit_comp64.c
index 45fee6f6b87e9..31f73f50caf5d 100644
--- a/arch/mips/net/bpf_jit_comp64.c
+++ b/arch/mips/net/bpf_jit_comp64.c
@@ -120,17 +120,47 @@ static void emit_zext_ver(struct jit_context *ctx, u8 dst)
 		emit_zext(ctx, dst);
 }
 
-/* Register move operation (32-bit) */
-static void emit_mov_r32(struct jit_context *ctx, u8 dst, u8 src)
+/* Register move operation (32-bit), optionally with sign extension */
+static void emit_mov_r32(struct jit_context *ctx, u8 dst, u8 src, s16 off)
 {
-	emit_mov_r(ctx, dst, src);
+	switch (off) {
+	case 8:
+		emit(ctx, dsll32, dst, src, 24);
+		emit(ctx, dsra32, dst, dst, 24);
+		clobber_reg(ctx, dst);
+		break;
+	case 16:
+		emit(ctx, dsll32, dst, src, 16);
+		emit(ctx, dsra32, dst, dst, 16);
+		clobber_reg(ctx, dst);
+		break;
+	default:
+		emit_mov_r(ctx, dst, src);
+		break;
+	}
 	emit_zext_ver(ctx, dst);
 }
 
-/* Register move operation (64-bit) */
-static void emit_mov_r64(struct jit_context *ctx, u8 dst, u8 src)
+/* Register move operation (64-bit), optionally with sign extension */
+static void emit_mov_r64(struct jit_context *ctx, u8 dst, u8 src, s16 off)
 {
-	emit_mov_r(ctx, dst, src);
+	switch (off) {
+	case 8:
+		emit(ctx, dsll32, dst, src, 24);
+		emit(ctx, dsra32, dst, dst, 24);
+		break;
+	case 16:
+		emit(ctx, dsll32, dst, src, 16);
+		emit(ctx, dsra32, dst, dst, 16);
+		break;
+	case 32:
+		emit(ctx, sll, dst, src, 0);
+		break;
+	default:
+		emit_mov_r(ctx, dst, src);
+		return;
+	}
+	clobber_reg(ctx, dst);
 }
 
 /* dst = imm (64-bit) */
@@ -669,7 +699,7 @@ int build_insn(const struct bpf_insn *insn, struct jit_context *ctx)
 			/* Special mov32 for zext */
 			emit_zext(ctx, dst);
 		} else {
-			emit_mov_r32(ctx, dst, src);
+			emit_mov_r32(ctx, dst, src, off);
 		}
 		break;
 	/* dst = -dst */
@@ -754,7 +784,7 @@ int build_insn(const struct bpf_insn *insn, struct jit_context *ctx)
 		break;
 	/* dst = src (64-bit) */
 	case BPF_ALU64 | BPF_MOV | BPF_X:
-		emit_mov_r64(ctx, dst, src);
+		emit_mov_r64(ctx, dst, src, off);
 		break;
 	/* dst = -dst (64-bit) */
 	case BPF_ALU64 | BPF_NEG: