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

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Date:   Fri, 30 Mar 2018 08:14:40 +0200
From:   Nicolai Stange <nstange@...e.de>
To:     Fabio Estevam <festevam@...il.com>
Cc:     Julia Lawall <julia.lawall@...6.fr>,
        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>, Nicolai Stange <nstange@...e.de>
Subject: Re: [kbuild-all] [PATCH] OPTIONAL: cpufreq/intel_pstate: fix debugfs_simple_attr.cocci warnings

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,

Nicolai


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