[PATCH bpf-next v2 05/13] bpf: keep the range across a sign extension that cannot change it

[Vineet Gupta <vineet.gupta@linux.dev>]

coerce_reg_to_size_sx() decides whether a sign extension is lossless by
comparing the bits above the field in smin and smax:

	top_smax_value = ((u64)reg_smax(reg) >> num_bits) << num_bits;
	top_smin_value = ((u64)reg_smin(reg) >> num_bits) << num_bits;
	if (top_smax_value != top_smin_value)
		goto out;

Equal high bits do imply the truncation is lossless, but the converse does
not hold. Whenever the range straddles zero the high bits necessarily
differ -- smin sign-extends to all ones, smax to all zeroes -- even when
every value in the range fits the field and (sN)v == v throughout. The
second gate, "both of s64_max/s64_min positive or negative", rejects the
same shape again for the same reason.

So a register holding an errno-or-zero value, [-4095, 0], comes out of r0
= (s32)r0 as the full [S32_MIN, S32_MAX] even though the instruction is a
no-op on it. The no_sext test at the call site does not help: it is an
unsigned check, so it only covers non-negative values that fit.

Test the range against the field directly and return early when it fits.
Sign extension is then the identity, so nothing needs updating -- which
also preserves var_off, where the existing path would have replaced known
bits with a coarse tnum_range().

This only tightens: the early return fires exactly where the value is
provably unchanged, and the cases the current tests do accept still take
the same path and produce the same bounds.

Signed-off-by: Vineet Gupta <vineet.gupta@linux.dev>
---
v2: new. coerce_reg_to_size_sx() is fixed rather than special-cased,
which is what was asked on RFC 5/6; it removes the RFC's
call-then-overwrite at the mov site.

 kernel/bpf/verifier.c | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 58e788f53ae5..eb093194e2a3 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -5754,6 +5754,7 @@ static void set_sext64_default_val(struct bpf_reg_state *reg, int size)
 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;
+	s64 field_smin, field_smax;
 	u64 top_smax_value, top_smin_value;
 	u64 num_bits = size * 8;
 
@@ -5773,6 +5774,27 @@ static void coerce_reg_to_size_sx(struct bpf_reg_state *reg, int size)
 		return;
 	}
 
+	if (size == 1) {
+		field_smin = S8_MIN;
+		field_smax = S8_MAX;
+	} else if (size == 2) {
+		field_smin = S16_MIN;
+		field_smax = S16_MAX;
+	} else {
+		/* size == 4 */
+		field_smin = S32_MIN;
+		field_smax = S32_MAX;
+	}
+
+	/*
+	 * The range already fits the field, so (sN)v == v for every value the
+	 * register can hold and the sign extension changes nothing. The tests
+	 * below cannot reach this case once smin is negative: a negative smin
+	 * and a non-negative smax never share their high bits.
+	 */
+	if (reg_smin(reg) >= field_smin && reg_smax(reg) <= field_smax)
+		return;
+
 	top_smax_value = ((u64)reg_smax(reg) >> num_bits) << num_bits;
 	top_smin_value = ((u64)reg_smin(reg) >> num_bits) << num_bits;
 
-- 
2.53.0-Meta