RE: [PATCH] sysfs: Remove page boundary align limitation on sysfs_emit and sysfs_emit_at

[Yu, Lang]

[Public]



>-----Original Message-----
>From: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
>Sent: Thursday, September 9, 2021 4:00 PM
>To: Yu, Lang <Lang.Yu@xxxxxxx>
>Cc: Joe Perches <joe@xxxxxxxxxxx>; Rafael J . Wysocki <rafael@xxxxxxxxxx>;
>linux-kernel@xxxxxxxxxxxxxxx
>Subject: Re: [PATCH] sysfs: Remove page boundary align limitation on sysfs_emit
>and sysfs_emit_at
>
>On Thu, Sep 09, 2021 at 07:48:38AM +0000, Yu, Lang wrote:
>> [Public]
>>
>>
>>
>> >-----Original Message-----
>> >From: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
>> >Sent: Thursday, September 9, 2021 2:36 PM
>> >To: Yu, Lang <Lang.Yu@xxxxxxx>
>> >Cc: Joe Perches <joe@xxxxxxxxxxx>; Rafael J . Wysocki
>> ><rafael@xxxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx
>> >Subject: Re: [PATCH] sysfs: Remove page boundary align limitation on
>> >sysfs_emit and sysfs_emit_at
>> >
>> >On Thu, Sep 09, 2021 at 06:22:54AM +0000, Yu, Lang wrote:
>> >> [Public]
>> >>
>> >>
>> >>
>> >> >-----Original Message-----
>> >> >From: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
>> >> >Sent: Thursday, September 9, 2021 2:08 PM
>> >> >To: Yu, Lang <Lang.Yu@xxxxxxx>
>> >> >Cc: Joe Perches <joe@xxxxxxxxxxx>; Rafael J . Wysocki
>> >> ><rafael@xxxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx
>> >> >Subject: Re: [PATCH] sysfs: Remove page boundary align limitation
>> >> >on sysfs_emit and sysfs_emit_at
>> >> >
>> >> >On Thu, Sep 09, 2021 at 05:52:23AM +0000, Yu, Lang wrote:
>> >> >> [Public]
>> >> >>
>> >> >>
>> >> >>
>> >> >> >-----Original Message-----
>> >> >> >From: Joe Perches <joe@xxxxxxxxxxx>
>> >> >> >Sent: Thursday, September 9, 2021 1:44 PM
>> >> >> >To: Yu, Lang <Lang.Yu@xxxxxxx>; Greg Kroah-Hartman
>> >> >> ><gregkh@xxxxxxxxxxxxxxxxxxx>; Rafael J . Wysocki
>> >> >> ><rafael@xxxxxxxxxx>;
>> >> >> >linux- kernel@xxxxxxxxxxxxxxx
>> >> >> >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@xxxxxxxxxxx> 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.
>> >> >
>> >> >Why not, what usage model can it not cover?
>> >>
>> >> Of course, we can modify driver code to obey sysfs_emit and
>> >> sysfs_emit_at
>> >rules or just use scnprintf in show functions.
>> >
>> >Great, please do.
>> >
>> >> Now that you introduced them, why not make them more flexible like
>> >> scnprintf
>> >family functions.
>> >
>> >Because that is not what they are for.
>> >
>> >> The page boundary align rule makes life hard and I don't like it :
>> >> ). Many thanks
>> >for your explanations!
>> >
>> >Then fix your sysfs files to not violate the sysfs rules.
>> >
>> >Again, which files are having problems and need to be fixed?  I will
>> >be glad to do this for you.
>>
>> Thanks. I can do it by myself instead of wasting your time... Many thanks!
>
>When doing so, please switch to using DEVICE_ATTR_RO() instead of the "open
>coded" DEVICE_ATTR() usage in the driver.  That way we all "know"
>that these are read-only attributes.

Many thanks for you reminders! Have a nice day!

Regards,
Lang
>
>thanks,
>
>greg k-h