linux-kernel - Re: [PATCH -v2] lib/notifier-error-inject: fix error when writing errno to debugfs file

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Date:   Mon, 19 Sep 2022 13:54:04 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Akinobu Mita <akinobu.mita@...il.com>
Cc:     linux-doc@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        linux-mm@...ck.org, linux-kselftest@...r.kernel.org,
        Jonathan Corbet <corbet@....net>, osalvador@...e.de,
        shuah@...nel.org, Zhao Gongyi <zhaogongyi@...wei.com>,
        Wei Yongjun <weiyongjun1@...wei.com>,
        Yicong Yang <yangyicong@...ilicon.com>
Subject: Re: [PATCH -v2] lib/notifier-error-inject: fix error when writing
 errno to debugfs file

On 19.09.22 13:40, Akinobu Mita wrote:
> 2022年9月19日(月) 18:20 David Hildenbrand <david@...hat.com>:
>>
>> On 17.09.22 09:14, Akinobu Mita wrote:
>>> The simple attribute files do not accept a negative value since the
>>> commit 488dac0c9237 ("libfs: fix error cast of negative value in
>>> simple_attr_write()"), so we can no longer use DEFINE_SIMPLE_ATTRIBUTE() to
>>> define a file operations for errno value.
>>>
>>> Fixes: 488dac0c9237 ("libfs: fix error cast of negative value in simple_attr_write()")
>>> Reported-by: Zhao Gongyi <zhaogongyi@...wei.com>
>>> Signed-off-by: Akinobu Mita <akinobu.mita@...il.com>
>>
>> But shouldn't we fix simple_attr_write() instead?
>>
>> I mean, simple_attr_read() might use attr->fmt to print a signed value,
>> but simple_attr_write() fails on signed values now?
>>
>> I might be wrong, but there is a disconnect. I feel like
>> simple_attr_write() should similarly make decisions based on attr->fmt.
> 
> I agree there is a disconnect, but I have no idea how to fix
> simple_attr_write().
> (strcmp(attr->fmt, "%%lld\n") is ugly)
> If no one seems to come up with a good idea, I'll fix the similar problems
> in fault-injection the same way I did here.
> 

Maybe we simply want another interface for the handful of %lld users. Or 
another way to allow DEFINE_SIMPLE_ATTRIBUTE users to specify it.

Might be good enough to specify instead of/in addition to "%llu" an enum 
that expresses what we want.

$ git grep -C2 DEFINE_SIMPLE  | grep "\%"

tells me that we have

a) %llu
b) %llx
c) %lld
d) 0x%llx
e) 0x%08llx

Maybe we can adjust the debugfs cases to be more similar, to eventually 
get only a, b and c.

-- 
Thanks,

David / dhildenb