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f1f5553d91
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@fb.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20230425174744.1758515-1-yhs@fb.com
52 lines
984 B
C
52 lines
984 B
C
// SPDX-License-Identifier: GPL-2.0-only
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/* Copyright (c) 2020 Facebook */
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#include <stddef.h>
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#include <linux/bpf.h>
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#include <bpf/bpf_helpers.h>
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#include "bpf_misc.h"
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#define MAX_STACK (512 - 3 * 32 + 8)
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static __attribute__ ((noinline))
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int f0(int var, struct __sk_buff *skb)
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{
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asm volatile ("");
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return skb->len;
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}
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__attribute__ ((noinline))
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int f1(struct __sk_buff *skb)
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{
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volatile char buf[MAX_STACK] = {};
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__sink(buf[MAX_STACK - 1]);
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return f0(0, skb) + skb->len;
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}
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int f3(int, struct __sk_buff *skb, int);
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__attribute__ ((noinline))
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int f2(int val, struct __sk_buff *skb)
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{
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return f1(skb) + f3(val, skb, 1);
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}
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__attribute__ ((noinline))
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int f3(int val, struct __sk_buff *skb, int var)
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{
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volatile char buf[MAX_STACK] = {};
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__sink(buf[MAX_STACK - 1]);
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return skb->ifindex * val * var;
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}
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SEC("tc")
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__failure __msg("combined stack size of 4 calls is 544")
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int global_func1(struct __sk_buff *skb)
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{
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return f0(1, skb) + f1(skb) + f2(2, skb) + f3(3, skb, 4);
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}
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