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

From: Yu, Lang
Date: Thu Sep 09 2021 - 03:48:45 EST


[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!

Regards,
Lang

>thanks,
>
>greg k-h