linux-kernel - Re: [kbuild-all] [PATCH] OPTIONAL: cpufreq/intel_pstate: fix debugfs_simple

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Date:   Sat, 31 Mar 2018 06:20:32 +0200
From:   Nicolai Stange <nstange@...e.de>
To:     Julia Lawall <julia.lawall@...6.fr>
Cc:     Nicolai Stange <nstange@...e.de>,
        Fabio Estevam <festevam@...il.com>,
        Francisco Jerez <currojerez@...eup.net>,
        linux-pm@...r.kernel.org, Viresh Kumar <viresh.kumar@...aro.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        linux-kernel <linux-kernel@...r.kernel.org>, kbuild-all@...org,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        0day robot <fengguang.wu@...el.com>,
        Len Brown <lenb@...nel.org>
Subject: Re: [kbuild-all] [PATCH] OPTIONAL: cpufreq/intel_pstate: fix debugfs_simple_attr.cocci warnings

Julia Lawall <julia.lawall@...6.fr> writes:

> On Fri, 30 Mar 2018, Nicolai Stange wrote:
>
>> Julia Lawall <julia.lawall@...6.fr> writes:
>>
>> > On Thu, 29 Mar 2018, Fabio Estevam wrote:
>> >
>> >> Hi Julia,
>> >>
>> >> On Thu, Mar 29, 2018 at 4:12 PM, Julia Lawall <julia.lawall@...6.fr> wrote:
>> >> >  Use DEFINE_DEBUGFS_ATTRIBUTE rather than DEFINE_SIMPLE_ATTRIBUTE
>> >> >  for debugfs files.
>> >> >
>> >> > Semantic patch information:
>> >> >  Rationale: DEFINE_SIMPLE_ATTRIBUTE + debugfs_create_file()
>> >> >  imposes some significant overhead as compared to
>> >> >  DEFINE_DEBUGFS_ATTRIBUTE + debugfs_create_file_unsafe().
>> >>
>> >> Just curious: could you please expand on what "imposes some
>> >> significant overhead" means?
>> >
>> > I don't know.  I didn't write this rule.  Nicolai, can you explain?
>>
>> From commit 49d200deaa68 ("debugfs: prevent access to removed files' private
>> data"):
>>
>>     Upon return of debugfs_remove()/debugfs_remove_recursive(), it might
>>     still be attempted to access associated private file data through
>>     previously opened struct file objects. If that data has been freed by
>>     the caller of debugfs_remove*() in the meanwhile, the reading/writing
>>     process would either encounter a fault or, if the memory address in
>>     question has been reassigned again, unrelated data structures could get
>>     overwritten.
>>     [...]
>>     Currently, there are ~1000 call sites of debugfs_create_file() spread
>>     throughout the whole tree and touching all of those struct file_operations
>>     in order to make them file removal aware by means of checking the result of
>>     debugfs_use_file_start() from within their methods is unfeasible.
>>
>>     Instead, wrap the struct file_operations by a lifetime managing proxy at
>>     file open [...]
>>
>> The additional overhead comes in terms of additional memory needed: for
>> debugs files created through debugfs_create_file(), one such struct
>> file_operations proxy is allocated for each struct file instantiation,
>> c.f. full_proxy_open().
>>
>> This was needed to "repair" the ~1000 call sites without touching them.
>>
>> New debugfs users should make their file_operations removal aware
>> themselves by means of DEFINE_DEBUGFS_ATTRIBUTE() and signal that fact to
>> the debugfs core by instantiating them through
>> debugfs_create_file_unsafe().
>>
>> See commit c64688081490 ("debugfs: add support for self-protecting
>> attribute file fops") for further information.
>
> Thanks.  Perhaps it would be good to add a reference to this commit in
> the message generated by the semantic patch.

Thanks for doing this!


>
> Would it be sufficient to just apply the semantic patch everywhere and
> submit the patches?

In principle yes. But I'm note sure whether such a mass application is
worth it: the proxy allocation happens only at file open and the
expectation is that there aren't that many debugfs files kept open at a
time. OTOH, a struct file_operation consumes 256 bytes with
sizeof(long) == 8.

In my opinion, new users should avoid this overhead as it's easily
doable. For existing ones, I don't know.

Thanks,

Nicolai

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