Re: var args in kernel?

[Jakub Jelinek]

On Mon, Nov 22, 2004 at 10:43:12AM +0100, Gerd Knorr wrote:
> On Fri, Nov 19, 2004 at 02:41:46PM -0700, Kevin P. Fleming wrote:
> > Gerd Knorr wrote:
> > >Yet another kobject bug.  It uses the varargs list twice in a illegal
> > >way.  That doesn't harm on i386 by pure luck, but blows things up on
> > >amd64 machines.  The patch below fixes it.
> > 
> > Is this safe? The normal glibc varargs implementation says you can't 
> > even call va_start on the same args list twice, you have to use va_copy 
> > to make a clone and then call va_start on that, _before_ you ever call 
> > va_start the first time.
> 
> Hmm, maybe.  I'm not sure who actually implements the varargs (gcc?
> Or glibc/kernel?) and whenever the above applies to the kernel as well
> or not ...

varargs is implemented entirely by GCC (GCC owns the stdarg.h header
and implements the builtin functions that are used in the va_* macros).

You can call va_start on the same args list twice, both:
void foo (int x, ...)
{
  va_list ap, ap2;
  va_start (ap, x);
  va_start (ap2, x);
...
  va_end (ap);
  va_end (ap2);
}
and
void foo (int x, ...)
{
  va_list ap;
  va_start (ap, x);
...
  va_end (ap);
  va_start (ap, x);
...
  va_end (ap);
}
are ok.  What you can't do is e.g.
  va_list ap;
  va_start (ap, x);
  bar (x, ap);
  bar (x, ap);
  va_end (ap);
(i.e. once you pass the ap to some other function, the only thing that you
have to do with it in the caller is va_end).

	Jakub
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/