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SubjectRe: copy_from_user() fixu
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> H. Peter Anvin writes:
> > >
> > > Well, being serious here, I have a library that is bug-free, but I
> > > can't depend on application writers to sustain my level of
> > > perfection. So, so help these lesser beings, I trap EFAULT in my
> > > library and yield a more meaningful diagnostic than a mere EFAULT or
> > > SEGV.
> >
> > Now you *really* convinced me that you're crazy.
>
> Why not insult me further while you're at it?

You claimed you have a library that's bug-free, and you're relying on
undefined behaviour. Clearly, your definition of "bug-free" is
something different than us "lesser beings" use.

> > Once again: if you're relying on EFAULT rather than SIGSEGV, your code
> > is broken, as you're relying on details of the libc/kernel interface.
> > I would argue that *IS* a bug in your "bug-free" library.
>
> I'm relying on what I've seen written in man pages for all varieties
> of Unix I've had access to.

Yes, it says that it's a PERMITTED return value, which is
fundamentally different from GUARANTEED.

> This abstract libc/kernel interface you refer to is an abstraction
> you've invented. It's not Unix practice. Unix practice is to return
> EFAULT on system calls. System calls are open(2), read(2), write(2)
> and similar.

I didn't invent it. It has been in every single Unix spec I've ever
read, and it's very explicit.

> > If you want to trap errors, you either have to sanitize the input, or
> > trap SIGSEGV.
>
> I can't sanitise the input: I don't know what pointer the application
> will pass. Trapping SEGV is a performance bugger: I have to install a
> signal handler before every pseudo-syscall and restore it afterwards
> (my library can't steal signals).

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