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

[Greg Kroah-Hartman]

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.

thanks,

greg k-h