[PATCH bpf-next v2 13/13] selftests/bpf: cover the low-32 link for narrowing stack spills

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

Two programs:

 - zext_narrowing_spill_keeps_link: a 32-bit spill of a wide register,
   filled back out, still follows a later narrowing of the source
 - zext_narrowing_spill_byte_no_link: a 1-byte spill forms no link, since
   the relation is not expressible

Signed-off-by: Vineet Gupta <vineet.gupta@linux.dev>
---
v2: new, with 12/13.
 .../bpf/progs/verifier_linked_scalars.c       | 54 +++++++++++++++++++
 1 file changed, 54 insertions(+)

diff --git a/tools/testing/selftests/bpf/progs/verifier_linked_scalars.c b/tools/testing/selftests/bpf/progs/verifier_linked_scalars.c
index e8a44e7579c8..ca7471f0293d 100644
--- a/tools/testing/selftests/bpf/progs/verifier_linked_scalars.c
+++ b/tools/testing/selftests/bpf/progs/verifier_linked_scalars.c
@@ -1048,6 +1048,60 @@ __naked void zext_fill_byte_forms_no_link(void)
 	: __clobber_all);
 }
 
+/*
+ * A narrowing spill stores only the source's low 32 bits, so the slot is their
+ * zero-extension. A later narrowing of the source must still reach anything
+ * filled back out of that slot.
+ */
+SEC("socket")
+__success
+__naked void zext_narrowing_spill_keeps_link(void)
+{
+	asm volatile ("						\
+	call %[bpf_get_prandom_u32];				\
+	r6 = r0;						\
+	call %[bpf_get_prandom_u32];				\
+	r0 <<= 32;						\
+	r6 |= r0;		/* r6 = full 64-bit unknown */	\
+	*(u32 *)(r10 - 8) = r6;	/* narrowing spill, forms the link */ \
+	r2 = *(u32 *)(r10 - 8);	/* fill it back */		\
+	if w6 != 0 goto 1f;	/* narrows r6, propagates to r2 */ \
+	if r2 == 0 goto 1f;					\
+	r0 /= 0;						\
+1:								\
+	r0 = 0;							\
+	exit;							\
+"	:
+	: __imm(bpf_get_prandom_u32)
+	: __clobber_all);
+}
+
+/*
+ * A sub-word spill is below the low-32 model, so no link is formed there.
+ */
+SEC("socket")
+__failure __msg("div by zero")
+__naked void zext_narrowing_spill_byte_no_link(void)
+{
+	asm volatile ("						\
+	call %[bpf_get_prandom_u32];				\
+	r6 = r0;						\
+	call %[bpf_get_prandom_u32];				\
+	r0 <<= 32;						\
+	r6 |= r0;						\
+	*(u8 *)(r10 - 8) = r6;	/* 1-byte spill: no link */	\
+	r2 = *(u8 *)(r10 - 8);					\
+	if w6 != 0 goto 1f;					\
+	if r2 == 0 goto 1f;	/* not deduced */		\
+	r0 /= 0;						\
+1:								\
+	r0 = 0;							\
+	exit;							\
+"	:
+	: __imm(bpf_get_prandom_u32)
+	: __clobber_all);
+}
+
 #ifdef CAN_USE_MOVSX
 
 /*
-- 
2.53.0-Meta