linux-kernel - RE: [PATCH] sysfs: Remove page boundary align limitation on sysfs_emit and sysfs_emit

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Date:   Thu, 9 Sep 2021 05:52:23 +0000
From:   "Yu, Lang" <Lang.Yu@....com>
To:     Joe Perches <joe@...ches.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] sysfs: Remove page boundary align limitation on
 sysfs_emit and sysfs_emit_at

[Public]



>-----Original Message-----
>From: Joe Perches <joe@...ches.com>
>Sent: Thursday, September 9, 2021 1:44 PM
>To: Yu, Lang <Lang.Yu@....com>; Greg Kroah-Hartman
><gregkh@...uxfoundation.org>; Rafael J . Wysocki <rafael@...nel.org>; linux-
>kernel@...r.kernel.org
>Subject: Re: [PATCH] sysfs: Remove page boundary align limitation on sysfs_emit
>and sysfs_emit_at
>
>On Thu, 2021-09-09 at 05:27 +0000, Yu, Lang wrote:
>> [AMD Official Use Only]
>
>this is a public list and this marker is not appropriate.

Sorry for that.
>
>> > -----Original Message-----
>> > From: Joe Perches <joe@...ches.com>
>> > On Wed, 2021-09-08 at 20:07 +0800, Lang Yu wrote:
>> > > The key purpose of sysfs_emit and sysfs_emit_at is to ensure that
>> > > no overrun is done. Make them more equivalent with scnprintf.
>> >
>> > I can't think of a single reason to do this.
>> > sysfs_emit and sysfs_emit_at are specific to sysfs.
>> >
>> > Use of these functions outside of sysfs is not desired or supported.
>> >
>> Thanks for your reply. But I'm still curious why you put such a limitation.
>> As "Documentation/filesystems/sysfs.rst" described, we can just  use
>> scnprintf(buf, PAGE_SIZE, "%s\n", dev->name) in show functions without
>> such a limitation.
>
>There's nothing particularly wrong with the use of scnprintf as above.
>
>The only real reason that sysfs_emit exists is to be able to reduce the kernel
>treewide quantity of uses of the sprintf family of functions that need to be
>analyzed for possible buffer overruns.
>
>The issue there is that buf is already known to be both a PAGE_SIZE buffer and
>PAGE_SIZE aligned for sysfs show functions so there's no real reason to use
>scnprintf.
>
>sysfs_emit is a shorter/smaller function and using it could avoid some sprintf
>defects.
>
>> Some guys just try to replace scnprintf with sysfs_emit() or sysfs_emit_at() per
>above documents.
>
>So don't do that.
>
>> But sprintf and sysfs_emit/sysfs_emit_at are not totally equivalent(e.g., page
>boundary align).
>>
>> In my opinion, we add a new api and try to replace an old api. Does we
>> need to make it more compatible with old api?
>
>IMO: no.
>
But why you said " - show() should only use sysfs_emit() or sysfs_emit_at() when formatting
the value to be returned to user space. " in Documentation/filesystems/sysfs.rst ?

Obviously, sysfs_emit() and sysfs_emit_at()  can't cover all the cases in show functions. 

Regards,
Lang