[PATCH bpf-next v4 10/18] selftests/bpf: Check that narrow stack stores define no slot

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

The stack liveness analysis records a store as a "def" only for the
4-byte half-slots it covers completely, so a one or two byte store near
the top of the frame must not define anything. Add a test that reads
fp-8, stores one byte at fp-1 and two bytes at fp-4, and reads fp-8
again, expecting no def mark on either store and the second read to
still use fp-8.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 .../selftests/bpf/progs/verifier_live_stack.c | 22 +++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/tools/testing/selftests/bpf/progs/verifier_live_stack.c b/tools/testing/selftests/bpf/progs/verifier_live_stack.c
index 7a1a0670f851..f8758eb62dac 100644
--- a/tools/testing/selftests/bpf/progs/verifier_live_stack.c
+++ b/tools/testing/selftests/bpf/progs/verifier_live_stack.c
@@ -2839,3 +2839,25 @@ static __used __naked void imprecise_dst_spill_join_sub(void)
 	:: __imm(bpf_get_prandom_u32)
 	: __clobber_all);
 }
+
+/*
+ * A store that does not fully cover a 4-byte half-slot defines nothing, so a
+ * narrow store at the top of the frame must not turn any slot into a "def",
+ * least of all every slot of the frame: the earlier data at fp-8 stays live.
+ */
+SEC("socket")
+__log_level(2)
+__msg("0: (79) r0 = *(u64 *)(r10 -8)        ; use: fp0-8")
+__msg("1: (73) *(u8 *)(r10 -1) = r0{{$}}")
+__msg("2: (6b) *(u16 *)(r10 -4) = r0{{$}}")
+__msg("3: (79) r0 = *(u64 *)(r10 -8)        ; use: fp0-8")
+__naked void narrow_store_defines_nothing(void)
+{
+	asm volatile (
+	"r0 = *(u64 *)(r10 - 8);"
+	"*(u8 *)(r10 - 1) = r0;"
+	"*(u16 *)(r10 - 4) = r0;"
+	"r0 = *(u64 *)(r10 - 8);"
+	"exit;"
+	::: __clobber_all);
+}
-- 
2.53.0