[PATCH bpf-next 05/13] bpf: remove redundant s{32,64} -> u{32,64} deduction logic

Andrii Nakryiko
2023-11-03T00:08:45Z

Equivalent checks were recently added in more succinct and, arguably,
safer form in:
- f188765f23a5 ("bpf: derive smin32/smax32 from umin32/umax32 bounds");
- 2e74aef782d3 ("bpf: derive smin/smax from umin/max bounds").

The checks we are removing in this patch set do similar checks to detect
if entire u32/u64 range has signed bit set or not set, but does it with
two separate checks.

Further, we forcefully overwrite either smin or smax (and 32-bit equvalents)
without applying normal min/max intersection logic. It's not clear why
that would be correct in all cases and seems to work by accident. This
logic is also "gated" by previous signed -> unsigned derivation, which
returns early.

All this is quite confusing and seems error-prone, while we already have
at least equivalent checks happening earlier. So remove this duplicate
and error-prone logic to simplify things a bit.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
---
kernel/bpf/verifier.c | 36 ------------------------------------
1 file changed, 36 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index af4e2fecbef2..e7b2fe78a07f 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -2411,24 +2411,6 @@ static void __reg32_deduce_bounds(struct bpf_reg_state *reg)
min_t(u32, reg->s32_max_value, reg->u32_max_value);
return;
}
- /* Learn sign from unsigned bounds. Signed bounds cross the sign
- * boundary, so we must be careful.
- */
- if ((s32)reg->u32_max_value >= 0) {
- /* Positive. We can't learn anything from the smin, but smax
- * is positive, hence safe.
- */
- reg->s32_min_value = reg->u32_min_value;
- reg->s32_max_value = reg->u32_max_value =
- min_t(u32, reg->s32_max_value, reg->u32_max_value);
- } else if ((s32)reg->u32_min_value < 0) {
- /* Negative. We can't learn anything from the smax, but smin
- * is negative, hence safe.
- */
- reg->s32_min_value = reg->u32_min_value =
- max_t(u32, reg->s32_min_value, reg->u32_min_value);
- reg->s32_max_value = reg->u32_max_value;
- }
}

static void __reg64_deduce_bounds(struct bpf_reg_state *reg)
@@ -2516,24 +2498,6 @@ static void __reg64_deduce_bounds(struct bpf_reg_state *reg)
reg->umax_value);
return;
}
- /* Learn sign from unsigned bounds. Signed bounds cross the sign
- * boundary, so we must be careful.
- */
- if ((s64)reg->umax_value >= 0) {
- /* Positive. We can't learn anything from the smin, but smax
- * is positive, hence safe.
- */
- reg->smin_value = reg->umin_value;
- reg->smax_value = reg->umax_value = min_t(u64, reg->smax_value,
- reg->umax_value);
- } else if ((s64)reg->umin_value < 0) {
- /* Negative. We can't learn anything from the smax, but smin
- * is negative, hence safe.
- */
- reg->smin_value = reg->umin_value = max_t(u64, reg->smin_value,
- reg->umin_value);
- reg->smax_value = reg->umax_value;
- }
}

static void __reg_deduce_mixed_bounds(struct bpf_reg_state *reg)
--
2.34.1