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SubjectRe: select(0, ..) is valid ?
On Tue, 15 May 2007, Christoph Lameter wrote:
> On Tue, 15 May 2007, Andrew Morton wrote:
>
> > I _think_ we can just do
> >
> > --- a/fs/compat.c~a
> > +++ a/fs/compat.c
> > @@ -1566,9 +1566,13 @@ int compat_core_sys_select(int n, compat
> > */
> > ret = -ENOMEM;
> > size = FDS_BYTES(n);
> > - bits = kmalloc(6 * size, GFP_KERNEL);
> > - if (!bits)
> > - goto out_nofds;
> > + if (likely(size)) {
> > + bits = kmalloc(6 * size, GFP_KERNEL);
> > + if (!bits)
> > + goto out_nofds;
> > + } else {
> > + bits = NULL;
> > + }

It's interesting that compat_core_sys_select() shows this kmalloc(0)
failure but core_sys_select() does not. That's because core_sys_select()
avoids kmalloc by using a buffer on the stack for small allocations (and
0 sure is small). Shouldn't compat_core_sys_select() do just the same?
Or is SLUB going to be so efficient that doing so is a waste of time?

> > fds.in = (unsigned long *) bits;
> > fds.out = (unsigned long *) (bits + size);
> > fds.ex = (unsigned long *) (bits + 2*size);
> > _
> >
> > I mean, if that oopses then I'd be very interested in finding out why.
> >
> > But I'm starting to suspect that it would be better to permit kmalloc(0) in
> > slub. It depends on how many more of these things need fixing.
> >
> > otoh, a kmalloc(0) could be a sign of some buggy/inefficient/weird code, so
> > there's some value in forcing us to go look at all the callsites.
>
> Hmmm... We could have kmalloc(0) return a pointer to the zero page? That
> would catch any writers?

I don't think using the zero page that way would be at all safe:
there's probably configurations/architectures in which it is write
protected, but I don't believe that's a given at all.

But the principle is good: ERR_PTR(-MAX_ERRNO) should work,
that area up the top should always give a fault.
Hmm, but perhaps there are architectures on which it does not?

Hugh
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