[PATCH bpf-next v3 02/12] selftests/bpf: Cover sign extensions that cannot change the range

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

Two programs, one per field width that coerce_subreg_to_size_sx() handles.
Each builds a 32-bit range that straddles zero but still fits the target
field, so the sign extension is a no-op: [-63, 0] for (s8) and [-4095, 0]
for (s16). The guard that follows can only be resolved statically if the
range survives, and it protects a div by zero, so a widened range is a
verification failure rather than a silently weaker test.

Signed-off-by: Vineet Gupta <vineet.gupta@linux.dev>
---
v3: was v2 6/13.
 - moved with patch 1, and narrowed to the 32-bit mov to match it. The
   ALU64 forms v2 tested now pass with or without the patch.

 .../selftests/bpf/progs/verifier_movsx.c      | 55 +++++++++++++++++++
 1 file changed, 55 insertions(+)

diff --git a/tools/testing/selftests/bpf/progs/verifier_movsx.c b/tools/testing/selftests/bpf/progs/verifier_movsx.c
index 195b27a51224..3fcd03b7a547 100644
--- a/tools/testing/selftests/bpf/progs/verifier_movsx.c
+++ b/tools/testing/selftests/bpf/progs/verifier_movsx.c
@@ -202,6 +202,61 @@ l0_%=:							\
 	: __clobber_all);
 }
 
+/*
+ * A 32-bit range that already fits the field is unchanged by the sign
+ * extension. Both of these straddle zero, so reg_umax() is huge and the high
+ * bits of s32_min and s32_max differ: neither of the gates that predate
+ * testing the range against the field can keep it. The guard that follows can
+ * only be resolved statically if the range survives, and it protects a div by
+ * zero, so a widened range is a verification failure rather than a silently
+ * weaker test.
+ */
+SEC("socket")
+__description("MOV32SX, S8, negative range is preserved")
+__success __success_unpriv __retval(0)
+__naked void mov32sx_s8_negative_range(void)
+{
+	asm volatile ("					\
+	call %[bpf_get_prandom_u32];			\
+	w0 &= 0x3f;					\
+	w0 -= 0x3f;					\
+	/* w0 is [-63, 0], already a valid s8 */	\
+	w0 = (s8)w0;					\
+	if w0 s< -0x3f goto l0_%=;			\
+	w0 = 0;						\
+	exit;						\
+l0_%=:							\
+	/* unreachable unless the range was widened */	\
+	w0 /= 0;					\
+	exit;						\
+"	:
+	: __imm(bpf_get_prandom_u32)
+	: __clobber_all);
+}
+
+SEC("socket")
+__description("MOV32SX, S16, negative range is preserved")
+__success __success_unpriv __retval(0)
+__naked void mov32sx_s16_negative_range(void)
+{
+	asm volatile ("					\
+	call %[bpf_get_prandom_u32];			\
+	w0 &= 0xfff;					\
+	w0 -= 0xfff;					\
+	/* w0 is [-4095, 0], already a valid s16 */	\
+	w0 = (s16)w0;					\
+	if w0 s< -0xfff goto l0_%=;			\
+	w0 = 0;						\
+	exit;						\
+l0_%=:							\
+	/* unreachable unless the range was widened */	\
+	w0 /= 0;					\
+	exit;						\
+"	:
+	: __imm(bpf_get_prandom_u32)
+	: __clobber_all);
+}
+
 SEC("socket")
 __description("MOV64SX, S16, R10 Sign Extension")
 __failure __msg("R1 type=scalar expected=fp, pkt, pkt_meta, map_key, map_value, mem, ringbuf_mem, buf, trusted_ptr_")
-- 
2.53.0-Meta