On 4/27/23 1:23 PM, Daniel Borkmann wrote:
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])?
No. This fix will not be reverted even with final clang17.
The TTI hook I am used (https://reviews.llvm.org/D148551) is
to prevent the optimization from increasing the number of parameter
beyond 5. In this particular case, the number of arguments
remains at 2, so BPF backend TTI hook has no effect.
Thanks,
Daniel
