Re: [PATCH bpf-next v2 1/8] bpf: Add UHMUL and SHMUL instructions

["Alexei Starovoitov" <alexei.starovoitov@gmail.com>]

On Tue, Oct 20, 2026 at 10:40 PM tangyazhou@zju.edu.cn <tangyazhou@zju.edu.cn> wrote:

> +static inline bool bpf_insn_is_hmul(const struct bpf_insn *insn)
> +{
> +	return BPF_CLASS(insn->code) == BPF_ALU64 &&
> +	       BPF_OP(insn->code) == BPF_MUL &&
> +	       bpf_is_hmul_variant(insn->off);
> +}

Patch 5 adds this check to 13 JITs and nfp to opt them out.
Lets make JITs opt in instead, like bpf_jit_supports_callx().
The verifier checks it once. x86, arm64 and riscv patches add
bpf_jit_supports_hmul() that returns true. bpf_prog_is_offloaded()
covers nfp.
When JIT is on and it doesn't support them the prog is rejected with
a message in the log instead of silent fallback to the interpreter.
Then patch 5 goes away and only arm64 and riscv acks are needed.

Like:

diff --git a/include/linux/filter.h b/include/linux/filter.h
index e0218e6..6ee8d1a 100644
--- a/include/linux/filter.h
+++ b/include/linux/filter.h
@@ -152,6 +152,13 @@ struct ctl_table_header;
 #define BPF_ALU32_IMM(OP, DST, IMM)				\
 	BPF_ALU32_IMM_OFF(OP, DST, IMM, 0)

+static inline bool bpf_insn_is_hmul(const struct bpf_insn *insn)
+{
+	return BPF_CLASS(insn->code) == BPF_ALU64 &&
+	       BPF_OP(insn->code) == BPF_MUL &&
+	       (insn->off == BPF_UHMUL || insn->off == BPF_SHMUL);
+}
+
 /* Endianess conversion, cpu_to_{l,b}e(), {l,b}e_to_cpu() */

 #define BPF_ENDIAN(TYPE, DST, LEN)				\
@@ -1281,6 +1288,7 @@ bool bpf_jit_supports_kfunc_call(void);
 bool bpf_jit_supports_callx(void);
 bool bpf_jit_supports_kfunc_ret_reg_pair(void);
 bool bpf_jit_supports_unwind(void);
+bool bpf_jit_supports_hmul(void);
 bool bpf_jit_supports_stack_args(void);
 bool bpf_jit_supports_arena_args(void);
 bool bpf_jit_supports_far_kfunc_call(void);
diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index f29b0aa..90c30e5 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -34,6 +34,12 @@
 #define BPF_FROM_LE	BPF_TO_LE
 #define BPF_FROM_BE	BPF_TO_BE

+/* BPF_MUL off values for high-half multiplication; BPF_ALU64 only. */
+enum bpf_mul_variant {
+	BPF_UHMUL = 1,
+	BPF_SHMUL = 2,
+};
+
 /* jmp encodings */
 #define BPF_JNE		0x50	/* jump != */
 #define BPF_JLT		0xa0	/* LT is unsigned, '<' */
diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index 3030beb..e17704a 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -3405,6 +3405,12 @@ bool __weak bpf_jit_supports_unwind(void)
 	return false;
 }

+/* uhmul and shmul */
+bool __weak bpf_jit_supports_hmul(void)
+{
+	return false;
+}
+
 /*
  * How this arch places a by-value kfunc argument, or NULL for one that has
  * not opted in and so only takes an argument of a single eightbyte, which
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 5601d20..9a3c97d 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -21194,6 +21194,17 @@ static int fd_array_get_map_idx(struct bpf_verifier_env *env, u32 idx)
 	return -EPROTO;
 }

+/* The interpreter supports uhmul and shmul. JITs opt in. */
+static int check_hmul_jit(struct bpf_verifier_env *env)
+{
+	if (bpf_prog_is_offloaded(env->prog->aux) ||
+	    (env->prog->jit_requested && !bpf_jit_supports_hmul())) {
+		verbose(env, "JIT doesn't support uhmul and shmul\n");
+		return -EOPNOTSUPP;
+	}
+	return 0;
+}
+
 static int check_alu_fields(struct bpf_verifier_env *env, struct bpf_insn *insn)
 {
 	u8 class = BPF_CLASS(insn->code);
@@ -21249,6 +21260,13 @@ static int check_alu_fields(struct bpf_verifier_env *env, struct bpf_insn *insn)
 	case BPF_MUL:
 	case BPF_DIV:
 	case BPF_MOD:
+		if (bpf_insn_is_hmul(insn)) {
+			if (BPF_SRC(insn->code) == BPF_X ? insn->imm : insn->src_reg) {
+				verbose(env, "BPF_ALU uses reserved fields\n");
+				return -EINVAL;
+			}
+			return check_hmul_jit(env);
+		}
 		if (BPF_SRC(insn->code) == BPF_X) {
 			if (insn->imm != 0 || (insn->off != 0 && insn->off != 1) ||
 			    (insn->off == 1 && opcode != BPF_MOD && opcode != BPF_DIV)) {

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