bpf: verifier: Simplify register sign extension with tnum_scast

This patch refactors the verifier's sign-extension logic for narrow
register values to use the new tnum_scast helper.

Previously, coerce_reg_to_size_sx and coerce_subreg_to_size_sx employed
manual logic to determine bounds, sometimes falling back to loose ranges
when sign bits were uncertain.

We simplify said logic by delegating the bounds calculation to tnum_scast
+ the existing bounds synchronization logic:

1. The register's tnum is updated via tnum_scast()
2. The signed bounds (smin/smax) are reset to the maximum theoretical
   range for the target size.
3. The unsigned bounds are reset to the full register width.
4. __update_reg_bounds() is called.

By invoking __update_reg_bounds(), the verifier automatically calculates
the intersection between the theoretical signed range and the bitwise info
in reg->var_off. This ensures bounds are as tight as possible without
requiring custom logic in the coercion functions.

This commit also removes set_sext64_default_val() and
set_sext32_default_val() as they are no longer used.

Signed-off-by: Dimitar Kanaliev <[email protected]>

Author: dkanaliev

kernel/bpf/verifier.c

@@ -6876,147 +6876,57 @@ static void coerce_reg_to_size(struct bpf_reg_state *reg, int size)
 	reg_bounds_sync(reg);
 }
 
-static void set_sext64_default_val(struct bpf_reg_state *reg, int size)
-{
-	if (size == 1) {
-		reg->smin_value = reg->s32_min_value = S8_MIN;
-		reg->smax_value = reg->s32_max_value = S8_MAX;
-	} else if (size == 2) {
-		reg->smin_value = reg->s32_min_value = S16_MIN;
-		reg->smax_value = reg->s32_max_value = S16_MAX;
-	} else {
-		/* size == 4 */
-		reg->smin_value = reg->s32_min_value = S32_MIN;
-		reg->smax_value = reg->s32_max_value = S32_MAX;
-	}
-	reg->umin_value = reg->u32_min_value = 0;
-	reg->umax_value = U64_MAX;
-	reg->u32_max_value = U32_MAX;
-	reg->var_off = tnum_unknown;
-}
-
 static void coerce_reg_to_size_sx(struct bpf_reg_state *reg, int size)
 {
-	s64 init_s64_max, init_s64_min, s64_max, s64_min, u64_cval;
-	u64 top_smax_value, top_smin_value;
-	u64 num_bits = size * 8;
+	s64 smin_value, smax_value;
 
-	if (tnum_is_const(reg->var_off)) {
-		u64_cval = reg->var_off.value;
-		if (size == 1)
-			reg->var_off = tnum_const((s8)u64_cval);
-		else if (size == 2)
-			reg->var_off = tnum_const((s16)u64_cval);
-		else
-			/* size == 4 */
-			reg->var_off = tnum_const((s32)u64_cval);
-
-		u64_cval = reg->var_off.value;
-		reg->smax_value = reg->smin_value = u64_cval;
-		reg->umax_value = reg->umin_value = u64_cval;
-		reg->s32_max_value = reg->s32_min_value = u64_cval;
-		reg->u32_max_value = reg->u32_min_value = u64_cval;
+	if (size >= 8)
 		return;
-	}
 
-	top_smax_value = ((u64)reg->smax_value >> num_bits) << num_bits;
-	top_smin_value = ((u64)reg->smin_value >> num_bits) << num_bits;
+	reg->var_off = tnum_scast(reg->var_off, size);
 
-	if (top_smax_value != top_smin_value)
-		goto out;
+	smin_value = -(1LL << (size * 8 - 1));
+	smax_value = (1LL << (size * 8 - 1)) - 1;
 
-	/* find the s64_min and s64_min after sign extension */
-	if (size == 1) {
-		init_s64_max = (s8)reg->smax_value;
-		init_s64_min = (s8)reg->smin_value;
-	} else if (size == 2) {
-		init_s64_max = (s16)reg->smax_value;
-		init_s64_min = (s16)reg->smin_value;
-	} else {
-		init_s64_max = (s32)reg->smax_value;
-		init_s64_min = (s32)reg->smin_value;
-	}
-
-	s64_max = max(init_s64_max, init_s64_min);
-	s64_min = min(init_s64_max, init_s64_min);
+	reg->smin_value = smin_value;
+	reg->smax_value = smax_value;
 
-	/* both of s64_max/s64_min positive or negative */
-	if ((s64_max >= 0) == (s64_min >= 0)) {
-		reg->s32_min_value = reg->smin_value = s64_min;
-		reg->s32_max_value = reg->smax_value = s64_max;
-		reg->u32_min_value = reg->umin_value = s64_min;
-		reg->u32_max_value = reg->umax_value = s64_max;
-		reg->var_off = tnum_range(s64_min, s64_max);
-		return;
-	}
+	reg->s32_min_value = (s32)smin_value;
+	reg->s32_max_value = (s32)smax_value;
 
-out:
-	set_sext64_default_val(reg, size);
-}
-
-static void set_sext32_default_val(struct bpf_reg_state *reg, int size)
-{
-	if (size == 1) {
-		reg->s32_min_value = S8_MIN;
-		reg->s32_max_value = S8_MAX;
-	} else {
-		/* size == 2 */
-		reg->s32_min_value = S16_MIN;
-		reg->s32_max_value = S16_MAX;
-	}
+	reg->umin_value = 0;
+	reg->umax_value = U64_MAX;
 	reg->u32_min_value = 0;
 	reg->u32_max_value = U32_MAX;
-	reg->var_off = tnum_subreg(tnum_unknown);
+
+	__update_reg_bounds(reg);
 }
 
 static void coerce_subreg_to_size_sx(struct bpf_reg_state *reg, int size)
 {
-	s32 init_s32_max, init_s32_min, s32_max, s32_min, u32_val;
-	u32 top_smax_value, top_smin_value;
-	u32 num_bits = size * 8;
-
-	if (tnum_is_const(reg->var_off)) {
-		u32_val = reg->var_off.value;
-		if (size == 1)
-			reg->var_off = tnum_const((s8)u32_val);
-		else
-			reg->var_off = tnum_const((s16)u32_val);
+	s32 smin_value, smax_value;
 
-		u32_val = reg->var_off.value;
-		reg->s32_min_value = reg->s32_max_value = u32_val;
-		reg->u32_min_value = reg->u32_max_value = u32_val;
+	if (size >= 4)
 		return;
-	}
 
-	top_smax_value = ((u32)reg->s32_max_value >> num_bits) << num_bits;
-	top_smin_value = ((u32)reg->s32_min_value >> num_bits) << num_bits;
+	reg->var_off = tnum_subreg(tnum_scast(reg->var_off, size));
 
-	if (top_smax_value != top_smin_value)
-		goto out;
+	smin_value = -(1 << (size * 8 - 1));
+	smax_value = (1 << (size * 8 - 1)) - 1;
 
-	/* find the s32_min and s32_min after sign extension */
-	if (size == 1) {
-		init_s32_max = (s8)reg->s32_max_value;
-		init_s32_min = (s8)reg->s32_min_value;
-	} else {
-		/* size == 2 */
-		init_s32_max = (s16)reg->s32_max_value;
-		init_s32_min = (s16)reg->s32_min_value;
-	}
-	s32_max = max(init_s32_max, init_s32_min);
-	s32_min = min(init_s32_max, init_s32_min);
-
-	if ((s32_min >= 0) == (s32_max >= 0)) {
-		reg->s32_min_value = s32_min;
-		reg->s32_max_value = s32_max;
-		reg->u32_min_value = (u32)s32_min;
-		reg->u32_max_value = (u32)s32_max;
-		reg->var_off = tnum_subreg(tnum_range(s32_min, s32_max));
-		return;
-	}
+	reg->s32_min_value = smin_value;
+	reg->s32_max_value = smax_value;
 
-out:
-	set_sext32_default_val(reg, size);
+	reg->u32_min_value = 0;
+	reg->u32_max_value = U32_MAX;
+
+	__update_reg32_bounds(reg);
+
+	reg->umin_value = reg->u32_min_value;
+	reg->umax_value = reg->u32_max_value;
+
+	reg->smin_value = reg->umin_value;
+	reg->smax_value = reg->umax_value;
 }
 
 static bool bpf_map_is_rdonly(const struct bpf_map *map)