[PATCH v5 bpf-next 05/23] bpf: derive subreg bounds from full bounds when upper 32 bits are constant

Andrii Nakryiko
2023-10-27T18:16:59Z

Comments in code try to explain the idea behind why this is correct.
Please check the code and comments.

Acked-by: Shung-Hsi Yu <shung-hsi.yu@suse.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
---
kernel/bpf/verifier.c | 45 +++++++++++++++++++++++++++++++++++++++++++
1 file changed, 45 insertions(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 0f66e9092c38..5082ca1ea5dc 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -2324,6 +2324,51 @@ static void __update_reg_bounds(struct bpf_reg_state *reg)
/* Uses signed min/max values to inform unsigned, and vice-versa */
static void __reg32_deduce_bounds(struct bpf_reg_state *reg)
{
+ /* If upper 32 bits of u64/s64 range don't change, we can use lower 32
+ * bits to improve our u32/s32 boundaries.
+ *
+ * E.g., the case where we have upper 32 bits as zero ([10, 20] in
+ * u64) is pretty trivial, it's obvious that in u32 we'll also have
+ * [10, 20] range. But this property holds for any 64-bit range as
+ * long as upper 32 bits in that entire range of values stay the same.
+ *
+ * E.g., u64 range [0x10000000A, 0x10000000F] ([4294967306, 4294967311]
+ * in decimal) has the same upper 32 bits throughout all the values in
+ * that range. As such, lower 32 bits form a valid [0xA, 0xF] ([10, 15])
+ * range.
+ *
+ * Note also, that [0xA, 0xF] is a valid range both in u32 and in s32,
+ * following the rules outlined below about u64/s64 correspondence
+ * (which equally applies to u32 vs s32 correspondence). In general it
+ * depends on actual hexadecimal values of 32-bit range. They can form
+ * only valid u32, or only valid s32 ranges in some cases.
+ *
+ * So we use all these insights to derive bounds for subregisters here.
+ */
+ if ((reg->umin_value >> 32) == (reg->umax_value >> 32)) {
+ /* u64 to u32 casting preserves validity of low 32 bits as
+ * a range, if upper 32 bits are the same
+ */
+ reg->u32_min_value = max_t(u32, reg->u32_min_value, (u32)reg->umin_value);
+ reg->u32_max_value = min_t(u32, reg->u32_max_value, (u32)reg->umax_value);
+
+ if ((s32)reg->umin_value <= (s32)reg->umax_value) {
+ reg->s32_min_value = max_t(s32, reg->s32_min_value, (s32)reg->umin_value);
+ reg->s32_max_value = min_t(s32, reg->s32_max_value, (s32)reg->umax_value);
+ }
+ }
+ if ((reg->smin_value >> 32) == (reg->smax_value >> 32)) {
+ /* low 32 bits should form a proper u32 range */
+ if ((u32)reg->smin_value <= (u32)reg->smax_value) {
+ reg->u32_min_value = max_t(u32, reg->u32_min_value, (u32)reg->smin_value);
+ reg->u32_max_value = min_t(u32, reg->u32_max_value, (u32)reg->smax_value);
+ }
+ /* low 32 bits should form a proper s32 range */
+ if ((s32)reg->smin_value <= (s32)reg->smax_value) {
+ reg->s32_min_value = max_t(s32, reg->s32_min_value, (s32)reg->smin_value);
+ reg->s32_max_value = min_t(s32, reg->s32_max_value, (s32)reg->smax_value);
+ }
+ }
/* if u32 range forms a valid s32 range (due to matching sign bit),
* try to learn from that
*/
--
2.34.1