bpf: remove redundant s{32,64} -> u{32,64} deduction logic

Equivalent checks were recently added in more succinct and, arguably,
safer form in:
  - f188765 ("bpf: derive smin32/smax32 from umin32/umax32 bounds");
  - 2e74aef ("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.

Acked-by: Shung-Hsi Yu <shung-hsi.yu@suse.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20231112010609.848406-6-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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)