netdev - Re: [PATCH bpf-next] selftests/bpf: Fix tests after change in struct file

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Message-ID: <8536CB49-0091-4019-839A-B460847995C2@fb.com>
Date: Fri, 28 Mar 2025 17:56:45 +0000
From: Song Liu <songliubraving@...a.com>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>
CC: Song Liu <song@...nel.org>, "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "ast@...nel.org"
	<ast@...nel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "andrii@...nel.org" <andrii@...nel.org>,
        Kernel Team <kernel-team@...a.com>,
        "kuba@...nel.org" <kuba@...nel.org>
Subject: Re: [PATCH bpf-next] selftests/bpf: Fix tests after change in struct
 file



> On Mar 28, 2025, at 10:30 AM, Andrii Nakryiko <andrii.nakryiko@...il.com> wrote:
> 
> On Thu, Mar 27, 2025 at 11:55 AM Song Liu <song@...nel.org> wrote:
>> 
>> Change in struct file [1] moves f_ref to the 3rd cache line. This makes
>> deferencing file pointer as a 8-byte variable invalid, because
>> btf_struct_walk() will walk into f_lock, which is 4-byte long.
>> 
>> Fix the selftests to deference the file pointer as a 4-byte variable.
>> 
>> [1] commit e249056c91a2 ("fs: place f_ref to 3rd cache line in struct
>>                          file to resolve false sharing")
>> Reported-by: Jakub Kicinski <kuba@...nel.org>
>> Signed-off-by: Song Liu <song@...nel.org>
>> ---
>> tools/testing/selftests/bpf/progs/test_module_attach.c    | 2 +-
>> tools/testing/selftests/bpf/progs/test_subprogs_extable.c | 6 +++---
>> 2 files changed, 4 insertions(+), 4 deletions(-)
>> 
>> diff --git a/tools/testing/selftests/bpf/progs/test_module_attach.c b/tools/testing/selftests/bpf/progs/test_module_attach.c
>> index fb07f5773888..7f3c233943b3 100644
>> --- a/tools/testing/selftests/bpf/progs/test_module_attach.c
>> +++ b/tools/testing/selftests/bpf/progs/test_module_attach.c
>> @@ -117,7 +117,7 @@ int BPF_PROG(handle_fexit_ret, int arg, struct file *ret)
>> 
>>        bpf_probe_read_kernel(&buf, 8, ret);
>>        bpf_probe_read_kernel(&buf, 8, (char *)ret + 256);
>> -       *(volatile long long *)ret;
>> +       *(volatile int *)ret;
> 
> we already have `*(volatile int *)&ret->f_mode;` below, do we really
> need this int casting case?.. Maybe instead of guessing the size of
> file's first field, let's just remove `*(volatile long long *)ret;`
> altogether?

I was assuming the original test covers two cases:
  1) deref ret itself;
  2) deref a member of ret (ret->f_mode);

Therefore, instead of doing something like

   *(volatile long long *)&ret->f_ref;  /* first member of file */

I got current version. 

If we don't need the first case, we sure can remove it. 

Thanks,
Song

> 
>>        *(volatile int *)&ret->f_mode;
>>        return 0;