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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
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.