Re: [PATCH bpf-next] selftests/bpf: Fix selftest test_global_funcs/global_func1 failure with latest clang

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On 4/25/23 7:47 PM, Yonghong Song wrote:
The selftest test_global_funcs/global_func1 failed with the latest clang17.
The reason is due to upstream ArgumentPromotionPass ([1]),
which may manipulate static function parameters and cause inlining
although the funciton is marked as noinline.

The original code:
   static __attribute__ ((noinline))
   int f0(int var, struct __sk_buff *skb)
   {
         return skb->len;
   }

   __attribute__ ((noinline))
   int f1(struct __sk_buff *skb)
   {
	...
         return f0(0, skb) + skb->len;
   }

   ...

   SEC("tc")
   __failure __msg("combined stack size of 4 calls is 544")
   int global_func1(struct __sk_buff *skb)
   {
         return f0(1, skb) + f1(skb) + f2(2, skb) + f3(3, skb, 4);
   }

After ArgumentPromotionPass, the code is translated to
   static __attribute__ ((noinline))
   int f0(int var, int skb_len)
   {
         return skb_len;
   }

   __attribute__ ((noinline))
   int f1(struct __sk_buff *skb)
   {
	...
         return f0(0, skb->len) + skb->len;
   }

   ...

   SEC("tc")
   __failure __msg("combined stack size of 4 calls is 544")
   int global_func1(struct __sk_buff *skb)
   {
         return f0(1, skb->len) + f1(skb) + f2(2, skb) + f3(3, skb, 4);
   }

And later llvm InstCombine phase recognized that f0()
simplify returns the value of the second argument and removed f0()
completely and the final code looks like:
   __attribute__ ((noinline))
   int f1(struct __sk_buff *skb)
   {
	...
         return skb->len + skb->len;
   }

   ...

   SEC("tc")
   __failure __msg("combined stack size of 4 calls is 544")
   int global_func1(struct __sk_buff *skb)
   {
         return skb->len + f1(skb) + f2(2, skb) + f3(3, skb, 4);
   }

If f0() is not inlined, the verification will fail with stack size
544 for a particular callchain. With f0() inlined, the maximum
stack size is 512 which is in the limit.

Let us add a `asm volatile ("")` in f0() to prevent ArgumentPromotionPass
from hoisting the code to its caller, and this fixed the test failure.

   [1] https://reviews.llvm.org/D148269

Signed-off-by: Yonghong Song <yhs@xxxxxx>
---
  tools/testing/selftests/bpf/progs/test_global_func1.c | 2 ++
  1 file changed, 2 insertions(+)

diff --git a/tools/testing/selftests/bpf/progs/test_global_func1.c b/tools/testing/selftests/bpf/progs/test_global_func1.c
index b85fc8c423ba..17a9f59bf5f3 100644
--- a/tools/testing/selftests/bpf/progs/test_global_func1.c
+++ b/tools/testing/selftests/bpf/progs/test_global_func1.c
@@ -10,6 +10,8 @@
  static __attribute__ ((noinline))
  int f0(int var, struct __sk_buff *skb)
  {
+	asm volatile ("");
+
  	return skb->len;

Is it planned to get this reverted before the final llvm/clang 17 is
officially released (you mentioned the TTI hook in [1])?

Thanks,
Daniel



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