Re: [PATCH v3 0/4] Make sscanf() stricter

Petr Mladek
2023-06-20T13:34:17Z

On Thu 2023-06-15 14:23:59, Andy Shevchenko wrote:
> On Thu, Jun 15, 2023 at 08:06:46AM +0000, David Laight wrote:
> > From: Demi Marie Obenour
> > > Sent: 14 June 2023 21:09
>
> ...
>
> > > > What sort of formats and data are being used?
> > >
> > > Base-10 or base-16 integers, with whitespace never being valid.
> >
> > In which case sscanf() really isn't what you are looking for.
> >
> > > > The "%s" format terminates on whitespace.
> > > > Even stroul() (and friends) will skip leading whitespace.
> > >
> > > Yes, which is a reason that strto*l() are just broken IMO.
> >
> > They are not 'broken', that is what is useful most of the time.
> > The usual problem is that "020" is treated as octal.

I do not know how many users depend on this behavior. But I believe
that there are such users. And breaking compatibility with userspace
implementation would make more harm then good in this case.

> > > I’m trying to replace their uses in Xen with custom parsing code.
> >
> > Then write a custom parser :-)

Honestly, I dislike any sscanf() modification which have been suggested
so far:

+ %!d is not acceptable because it produces compiler errors

+ %d! is not acceptable because "use 64!" is a realistic string.
We could not be sure that "<number>!" will never be parsed
in kernel.

+ %d%[!] produces compiler error either. It is hard to parse by eyes.
Also the meaning of such a format would be far from obvious.

+ %pj or another %p modifiers would be hard to understand either.

Yes, we have %pe but I think that only few people really use it.
And it is kind of self-explanatory because it is typically
used together with ERR_PTR() and with variables called
"err" or "ret".

> Hmm... Usually we are against zillion implementations of the same with zillion
> bugs hidden (each buggy implementation with its own bugs).

I would really like to see the code depending on it. The cover letter
suggests that there already is a patch with such a custom parser.
I am sorry if it has already been mentioned. There were so many threads.

Sure, we do not want two full featured sscanf() implementations. But a
wrapper checking for leading whitespace and using kstrto<foo>
family does not sound too complex.

There should always be a good reason to introduce an incompatibility
between the kernel and the userspace implementation of a commonly
used API.

Best Regards,
Petr