Re: Rules on how to use sysfs in userspace programs

From: Randy Dunlap
Date: Sun Jun 10 2007 - 12:56:48 EST

On Sun, 10 Jun 2007 10:02:00 -0400 Theodore Tso wrote:

> On Fri, Jun 08, 2007 at 01:36:37PM -0700, Greg KH wrote:
> > The kernel exported sysfs exports internal kernel implementation-details
> > and depends on internal kernel-structures and layout. It is agreed upon
> > kernel developers, that the Linux kernel does not provide a stable
> > internal API. As sysfs is a direct export of kernel internal
> > structures, the sysfs interface can't provide a stable interface too, it
> > may always change along with internal kernel changes.
> I want to step back and ask a very fundamental philosophical question
> --- who are the intended users of sysfs? If it exports an interface,
> part of which is known not be stable, except for backwards
> compatibility issues, why and to whom are we exporting them?

I agree that stepping back and asking questions is the right thing
to do here.

/proc also reports lots of kernel internals, yet it is required
to maintain compatibility.

ISTM that sysfs should allow additions, but only in ways that don't
break backwards compatibility. I.e., it should maintain backwards
compatible interfaces when new ones are added. Yeah, that means
leave some crud around. Yes, it may be as bad as /proc, but I've
said that for quite awhile.

> If the answer is "no one", then maybe modulo backwards compatibility
> issues, we should only export via sysfs those things that are
> guaranteed to be a stable interface. Or maybe there should be a mount
> option that filters out anything which isn't guaranteed to be stable,
> so that user programs can easily determine if they are using anything
> "bad" --- and then a year or two from now, we make that mount option
> the default, and require a mount option if people want to see the
> unstable bits of sysfs.
> Way back in 2003, Rusty's OLS keynote talked at great length about
> kernel design and interface similicity as a hallmark of good design:
> 1. Compiler/linker won't let you get it wrong.
> 2. Compiler will warn if you get it wrong.
> 3. The simplest use is the correct one.
> 4. The name tells you how to use it.
> 5. Do it right or it will break at runtime.
> 6. Follow common convention and you'll get it right.
> 7. Read the documentation and you'll get it right.
> 8. Read the implementation and you'll get it right.
> 9. Read the correct mailing list thread and you'll get it right.
> 10. Read the documentation and you'll get it wrong.
> 11. Follow common convention and you'll get it wrong.
> 12. Do it right and it will break at runtime.
> 13. The name tells you how not to use it.
> 14. The obvious use is wrong.
> 15. Compiler will warn if you get it right.
> 16. Compiler won't let you get it right.
> 17. It's impossible to get right.
> The documentation is a good step, but wouldn't it be better if we can
> prohibit (or warn) applications who are getting it wrong in the first
> place? Providing interfaces that aren't stable, and then requiring
> people to read documentation so they know not to use them, seems to be
> providing an attractive nuisance.

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