selftests/bpf: Test case for refinement improvement using 64b bounds

This new selftest demonstrates the improvement of bounds refinement from
the previous patch. It is inspired from a set of reg_bounds_sync inputs
generated using CBMC [1] by Shung-Hsi:

  reg.smin_value=0x8000000000000002
  reg.smax_value=2
  reg.umin_value=2
  reg.umax_value=19
  reg.s32_min_value=2
  reg.s32_max_value=3
  reg.u32_min_value=2
  reg.u32_max_value=3

reg_bounds_sync returns R=[2; 3] without the previous patch, and R=2
with it. __reg64_deduce_bounds is able to derive that u64=2, but before
the previous patch, those bounds are overwritten in
__reg_deduce_mixed_bounds using the 32bits bounds.

To arrive to these reg_bounds_sync inputs, we bound the 32bits value
first to [2; 3]. We can then upper-bound s64 without impacting u64. At
that point, the refinement to u64=2 doesn't happen because the ranges
still overlap in two points:

  0  umin=2                             umax=0xff..ff00..03   U64_MAX
  |  [xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx]      |
  |----------------------------|------------------------------|
  |xx]                           [xxxxxxxxxxxxxxxxxxxxxxxxxxxx|
  0  smax=2                      smin=0x800..02               -1

With an upper-bound check at value 19, we can reach the above inputs for
reg_bounds_sync. At that point, the refinement to u64=2 happens and
because it isn't overwritten by __reg_deduce_mixed_bounds anymore,
reg_bounds_sync returns with reg=2.

The test validates this result by including an illegal instruction in
the (dead) branch reg != 2.

Link: https://github.com/shunghsiyu/reg_bounds_sync-review/ [1]
Co-developed-by: Shung-Hsi Yu <shung-hsi.yu@suse.com>
Signed-off-by: Shung-Hsi Yu <shung-hsi.yu@suse.com>
Signed-off-by: Paul Chaignon <paul.chaignon@gmail.com>
Tested-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/622dc51c581cd4d652fff362188b2a5f73c1fe99.1773401138.git.paul.chaignon@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 33 insertions, 0 deletions

diff --git a/tools/testing/selftests/bpf/progs/verifier_bounds.c b/tools/testing/selftests/bpf/progs/verifier_bounds.c
index ce09379130aa..3724d5e5bcb3 100644
--- a/tools/testing/selftests/bpf/progs/verifier_bounds.c
+++ b/tools/testing/selftests/bpf/progs/verifier_bounds.c
@@ -2037,4 +2037,37 @@ __naked void signed_unsigned_intersection32_case2(void *ctx)
 	: __clobber_all);
 }
 
+/* After instruction 3, the u64 and s64 ranges look as follows:
+ * 0  umin=2                             umax=0xff..ff00..03   U64_MAX
+ * |  [xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx]      |
+ * |----------------------------|------------------------------|
+ * |xx]                           [xxxxxxxxxxxxxxxxxxxxxxxxxxxx|
+ * 0  smax=2                      smin=0x800..02               -1
+ *
+ * The two ranges can't be refined because they overlap in two places. Once we
+ * add an upper-bound to u64 at instruction 4, the refinement can happen. This
+ * test validates that this refinement does happen and is not overwritten by
+ * the less-precise 32bits ranges.
+ */
+SEC("socket")
+__description("bounds refinement: 64bits ranges not overwritten by 32bits ranges")
+__msg("3: (65) if r0 s> 0x2 {{.*}} R0=scalar(smin=0x8000000000000002,smax=2,umin=smin32=umin32=2,umax=0xffffffff00000003,smax32=umax32=3")
+__msg("4: (25) if r0 > 0x13 {{.*}} R0=2")
+__success __log_level(2)
+__naked void refinement_32bounds_not_overwriting_64bounds(void *ctx)
+{
+	asm volatile("			\
+	call %[bpf_get_prandom_u32];	\
+	if w0 < 2 goto +5;		\
+	if w0 > 3 goto +4;		\
+	if r0 s> 2 goto +3;		\
+	if r0 > 19 goto +2;		\
+	if r0 == 2 goto +1;		\
+	r10 = 0;			\
+	exit;				\
+"	:
+	: __imm(bpf_get_prandom_u32)
+	: __clobber_all);
+}
+
 char _license[] SEC("license") = "GPL";