bpf: Add range tracking for BPF_NEG

Add range tracking for instruction BPF_NEG. Without this logic, a trivial
program like the following will fail

    volatile bool found_value_b;
    SEC("lsm.s/socket_connect")
    int BPF_PROG(test_socket_connect)
    {
        if (!found_value_b)
                return -1;
        return 0;
    }

with verifier log:

"At program exit the register R0 has smin=0 smax=4294967295 should have
been in [-4095, 0]".

This is because range information is lost in BPF_NEG:

0: R1=ctx() R10=fp0
; if (!found_value_b) @ xxxx.c:24
0: (18) r1 = 0xffa00000011e7048       ; R1_w=map_value(...)
2: (71) r0 = *(u8 *)(r1 +0)           ; R0_w=scalar(smin32=0,smax=255)
3: (a4) w0 ^= 1                       ; R0_w=scalar(smin32=0,smax=255)
4: (84) w0 = -w0                      ; R0_w=scalar(range info lost)

Note that, the log above is manually modified to highlight relevant bits.

Fix this by maintaining proper range information with BPF_NEG, so that
the verifier will know:

4: (84) w0 = -w0                      ; R0_w=scalar(smin32=-255,smax=0)

Also updated selftests based on the expected behavior.

Signed-off-by: Song Liu <song@kernel.org>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20250625164025.3310203-2-song@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

include/linux/tnum.h

@@ -40,6 +40,8 @@ struct tnum tnum_arshift(struct tnum a, u8 min_shift, u8 insn_bitness);
 struct tnum tnum_add(struct tnum a, struct tnum b);
 /* Subtract two tnums, return @a - @b */
 struct tnum tnum_sub(struct tnum a, struct tnum b);
+/* Neg of a tnum, return  0 - @a */
+struct tnum tnum_neg(struct tnum a);
 /* Bitwise-AND, return @a & @b */
 struct tnum tnum_and(struct tnum a, struct tnum b);
 /* Bitwise-OR, return @a | @b */

kernel/bpf/tnum.c

@@ -83,6 +83,11 @@ struct tnum tnum_sub(struct tnum a, struct tnum b)
 	return TNUM(dv & ~mu, mu);
 }
 
+struct tnum tnum_neg(struct tnum a)
+{
+	return tnum_sub(TNUM(0, 0), a);
+}
+
 struct tnum tnum_and(struct tnum a, struct tnum b)
 {
 	u64 alpha, beta, v;

kernel/bpf/verifier.c

@@ -15182,6 +15182,7 @@ static bool is_safe_to_compute_dst_reg_range(struct bpf_insn *insn,
 	switch (BPF_OP(insn->code)) {
 	case BPF_ADD:
 	case BPF_SUB:
+	case BPF_NEG:
 	case BPF_AND:
 	case BPF_XOR:
 	case BPF_OR:
@@ -15250,6 +15251,13 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env,
 		scalar_min_max_sub(dst_reg, &src_reg);
 		dst_reg->var_off = tnum_sub(dst_reg->var_off, src_reg.var_off);
 		break;
+	case BPF_NEG:
+		env->fake_reg[0] = *dst_reg;
+		__mark_reg_known(dst_reg, 0);
+		scalar32_min_max_sub(dst_reg, &env->fake_reg[0]);
+		scalar_min_max_sub(dst_reg, &env->fake_reg[0]);
+		dst_reg->var_off = tnum_neg(env->fake_reg[0].var_off);
+		break;
 	case BPF_MUL:
 		dst_reg->var_off = tnum_mul(dst_reg->var_off, src_reg.var_off);
 		scalar32_min_max_mul(dst_reg, &src_reg);
@@ -15473,7 +15481,14 @@ static int check_alu_op(struct bpf_verifier_env *env, struct bpf_insn *insn)
 		}
 
 		/* check dest operand */
-		err = check_reg_arg(env, insn->dst_reg, DST_OP);
+		if (opcode == BPF_NEG) {
+			err = check_reg_arg(env, insn->dst_reg, DST_OP_NO_MARK);
+			err = err ?: adjust_scalar_min_max_vals(env, insn,
+							 &regs[insn->dst_reg],
+							 regs[insn->dst_reg]);
+		} else {
+			err = check_reg_arg(env, insn->dst_reg, DST_OP);
+		}
 		if (err)
 			return err;
 

tools/testing/selftests/bpf/progs/verifier_bounds_deduction.c

@@ -159,13 +159,16 @@ __failure_unpriv
 __naked void deducing_bounds_from_const_10(void)
 {
 	asm volatile ("					\
+	r6 = r1;					\
 	r0 = 0;						\
 	if r0 s<= 0 goto l0_%=;				\
-l0_%=:	/* Marks reg as unknown. */			\
+l0_%=: /* Marks r0 as unknown. */			\
-	r0 = -r0;					\
+	call %[bpf_get_prandom_u32];			\
-	r0 -= r1;					\
+	r0 -= r6;					\
 	exit;						\
-"	::: __clobber_all);
+"	:
+	: __imm(bpf_get_prandom_u32)
+	: __clobber_all);
 }
 
 char _license[] SEC("license") = "GPL";

tools/testing/selftests/bpf/progs/verifier_value_ptr_arith.c

@@ -231,6 +231,10 @@ __retval(1)
 __naked void ptr_unknown_vs_unknown_lt(void)
 {
 	asm volatile ("					\
+	r8 = r1;					\
+	call %[bpf_get_prandom_u32];			\
+	r9 = r0;					\
+	r1 = r8;					\
 	r0 = *(u32*)(r1 + %[__sk_buff_len]);		\
 	r1 = 0;						\
 	*(u64*)(r10 - 8) = r1;				\
@@ -245,11 +249,11 @@ l1_%=:	call %[bpf_map_lookup_elem];			\
 	r4 = *(u8*)(r0 + 0);				\
 	if r4 == 1 goto l3_%=;				\
 	r1 = 6;						\
-	r1 = -r1;					\
+	r1 = r9;					\
 	r1 &= 0x3;					\
 	goto l4_%=;					\
 l3_%=:	r1 = 6;						\
-	r1 = -r1;					\
+	r1 = r9;					\
 	r1 &= 0x7;					\
 l4_%=:	r1 += r0;					\
 	r0 = *(u8*)(r1 + 0);				\
@@ -259,7 +263,8 @@ l2_%=:	r0 = 1;						\
 	: __imm(bpf_map_lookup_elem),
 	  __imm_addr(map_array_48b),
 	  __imm_addr(map_hash_16b),
-	  __imm_const(__sk_buff_len, offsetof(struct __sk_buff, len))
+	  __imm_const(__sk_buff_len, offsetof(struct __sk_buff, len)),
+	  __imm(bpf_get_prandom_u32)
 	: __clobber_all);
 }
 
@@ -271,6 +276,10 @@ __retval(1)
 __naked void ptr_unknown_vs_unknown_gt(void)
 {
 	asm volatile ("					\
+	r8 = r1;					\
+	call %[bpf_get_prandom_u32];			\
+	r9 = r0;					\
+	r1 = r8;					\
 	r0 = *(u32*)(r1 + %[__sk_buff_len]);		\
 	r1 = 0;						\
 	*(u64*)(r10 - 8) = r1;				\
@@ -285,11 +294,11 @@ l1_%=:	call %[bpf_map_lookup_elem];			\
 	r4 = *(u8*)(r0 + 0);				\
 	if r4 == 1 goto l3_%=;				\
 	r1 = 6;						\
-	r1 = -r1;					\
+	r1 = r9;					\
 	r1 &= 0x7;					\
 	goto l4_%=;					\
 l3_%=:	r1 = 6;						\
-	r1 = -r1;					\
+	r1 = r9;					\
 	r1 &= 0x3;					\
 l4_%=:	r1 += r0;					\
 	r0 = *(u8*)(r1 + 0);				\
@@ -299,7 +308,8 @@ l2_%=:	r0 = 1;						\
 	: __imm(bpf_map_lookup_elem),
 	  __imm_addr(map_array_48b),
 	  __imm_addr(map_hash_16b),
-	  __imm_const(__sk_buff_len, offsetof(struct __sk_buff, len))
+	  __imm_const(__sk_buff_len, offsetof(struct __sk_buff, len)),
+	  __imm(bpf_get_prandom_u32)
 	: __clobber_all);
 }