Re: [PATCH bpf-next 12/15] bpf: Preserve boundaries and track scalars on narrowing fill

On Thu, 04 Jan 2024 at 04:27:00 +0200, Eduard Zingerman wrote:

On Wed, 2023-12-20 at 23:40 +0200, Maxim Mikityanskiy wrote:

[...]

The two tests below were added by the following commit:
ef979017b837 ("bpf: selftest: Add verifier tests for <8-byte scalar spill and refill")

As far as I understand, the original intent was to check the behavior
for stack read/write with non-matching size.
I think these tests are redundant after patch #13. Wdyt?

_6_offset_to_skb_data is for sure not redundant. I don't test a partial
fill from the most significant bits in my patch 13.

u16_offset_to_skb_data is somewhat similar to
fill_32bit_after_spill_64bit, but they aren't exactly the same: the
former spills (u32)20 and fills (u16)20 (the same value), while my test
spills (u64)0xXXXXXXXX00000000 and fills (u32)0 (the most significant
bits are stripped). Maybe u16_offset_to_skb_data is redundant, but more
coverage is better than less coverage, isn't it?

quoted

diff --git a/tools/testing/selftests/bpf/progs/verifier_spill_fill.c b/tools/testing/selftests/bpf/progs/verifier_spill_fill.c
index 809a09732168..de03e72e07a9 100644
--- a/tools/testing/selftests/bpf/progs/verifier_spill_fill.c
+++ b/tools/testing/selftests/bpf/progs/verifier_spill_fill.c

@@ -217,7 +217,7 @@ __naked void uninit_u32_from_the_stack(void)
 
 SEC("tc")
 __description("Spill a u32 const scalar.  Refill as u16.  Offset to skb->data")
-__failure __msg("invalid access to packet")
+__success __retval(0)
 __naked void u16_offset_to_skb_data(void)
 {
 	asm volatile ("					\

@@ -225,19 +225,24 @@ __naked void u16_offset_to_skb_data(void)
 	r3 = *(u32*)(r1 + %[__sk_buff_data_end]);	\
 	w4 = 20;					\
 	*(u32*)(r10 - 8) = r4;				\
-	r4 = *(u16*)(r10 - 8);				\
+	r4 = *(u16*)(r10 - %[offset]);			\
 	r0 = r2;					\
-	/* r0 += r4 R0=pkt R2=pkt R3=pkt_end R4=umax=65535 */\
+	/* r0 += r4 R0=pkt R2=pkt R3=pkt_end R4=20 */\
 	r0 += r4;					\
-	/* if (r0 > r3) R0=pkt,umax=65535 R2=pkt R3=pkt_end R4=umax=65535 */\
+	/* if (r0 > r3) R0=pkt,off=20 R2=pkt R3=pkt_end R4=20 */\
 	if r0 > r3 goto l0_%=;				\
-	/* r0 = *(u32 *)r2 R0=pkt,umax=65535 R2=pkt R3=pkt_end R4=20 */\
+	/* r0 = *(u32 *)r2 R0=pkt,off=20 R2=pkt R3=pkt_end R4=20 */\
 	r0 = *(u32*)(r2 + 0);				\
 l0_%=:	r0 = 0;						\
 	exit;						\
 "	:
 	: __imm_const(__sk_buff_data, offsetof(struct __sk_buff, data)),
-	  __imm_const(__sk_buff_data_end, offsetof(struct __sk_buff, data_end))
+	  __imm_const(__sk_buff_data_end, offsetof(struct __sk_buff, data_end)),
+#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
+	  __imm_const(offset, 8)
+#else
+	  __imm_const(offset, 6)
+#endif
 	: __clobber_all);
 }
 

@@ -270,7 +275,7 @@ l0_%=:	r0 = 0;						\
 }
 
 SEC("tc")
-__description("Spill a u32 const scalar.  Refill as u16 from fp-6.  Offset to skb->data")
+__description("Spill a u32 const scalar.  Refill as u16 from MSB.  Offset to skb->data")
 __failure __msg("invalid access to packet")
 __naked void _6_offset_to_skb_data(void)
 {

@@ -279,7 +284,7 @@ __naked void _6_offset_to_skb_data(void)
 	r3 = *(u32*)(r1 + %[__sk_buff_data_end]);	\
 	w4 = 20;					\
 	*(u32*)(r10 - 8) = r4;				\
-	r4 = *(u16*)(r10 - 6);				\
+	r4 = *(u16*)(r10 - %[offset]);			\
 	r0 = r2;					\
 	/* r0 += r4 R0=pkt R2=pkt R3=pkt_end R4=umax=65535 */\
 	r0 += r4;					\

@@ -291,7 +296,12 @@ l0_%=:	r0 = 0;						\
 	exit;						\
 "	:
 	: __imm_const(__sk_buff_data, offsetof(struct __sk_buff, data)),
-	  __imm_const(__sk_buff_data_end, offsetof(struct __sk_buff, data_end))
+	  __imm_const(__sk_buff_data_end, offsetof(struct __sk_buff, data_end)),
+#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
+	  __imm_const(offset, 6)
+#else
+	  __imm_const(offset, 8)
+#endif
 	: __clobber_all);
 }