[U-Boot] [PATCH] Prevent malloc with size 0

Marek Vasut
2012-04-02T14:42:30Z

Dear Joakim Tjernlund,

> Marek Vasut <marek.vasut@gmail.com> wrote on 2012/04/02 16:05:13:
> > Dear Joakim Tjernlund,
> >
> > > Hi Grame
> > >
> > > Graeme Russ <graeme.russ@gmail.com> wrote on 2012/04/02 09:17:44:
> > > > Hi Joakim,
> > > >
> > > > On Apr 2, 2012 4:55 PM, "Joakim Tjernlund"
> > > > <joakim.tjernlund@transmode.se>
> >
> > wrote:
> > > > > > Hi Marek,
> > > > > >
> > > > > > On Mon, Apr 2, 2012 at 1:36 PM, Marek Vasut
> > > > > > <marek.vasut@gmail.com>
> >
> > wrote:
> > > > > > > Dear Mike Frysinger,
> > > > > > >
> > > > > > >> On Sunday 01 April 2012 20:25:44 Graeme Russ wrote:
> > > > > > >> > b) The code calling malloc(0) is making a perfectly
> > > > > > >> > legitimate assumption
> > > > > > >> >
> > > > > > >> > based on how glibc handles malloc(0)
> > > > > > >>
> > > > > > >> not really. POSIX says malloc(0) is implementation defined
> > > > > > >> (so it may return a unique address, or it may return NULL).
> > > > > > >> no userspace code assuming malloc(0) will return non-NULL is
> > > > > > >> correct.
> > > > > > >
> > > > > > > Which is your implementation-defined ;-) But I have to agree
> > > > > > > with this one. So my vote is for returning NULL.
> > > > > >
> > > > > > Also, no userspace code assuming malloc(0) will return NULL is
> > > > > > correct
> > > > > >
> > > > > > Point being, no matter which implementation is chosen, it is up
> > > > > > to the caller to not assume that the choice that was made was,
> > > > > > in fact, the choice that was made.
> > > > > >
> > > > > > I.e. the behaviour of malloc(0) should be able to be changed on a
> > > > > > whim with no side-effects
> > > > > >
> > > > > > So I think I should change my vote to returning NULL for one
> > > > > > reason and one reason only - It is faster during run-time
> > > > >
> > > > > Then u-boot will be incompatible with both glibc and the linux
> > > > > kernel, it seems
> > > >
> > > > Forget aboug other implementations...
> > > > What matters is that the fact that the behaviour is undefined and it
> > > > is up to the caller to take that into account
> > >
> > > Well, u-boot borrows code from both kernel and user space so it would
> > > make sense if malloc(0) behaved the same. Especially for kernel code
> > > which tend to depend on the kernels impl.(just look at Scotts example)
> > >
> > > > > to me that any modern impl. of malloc(0) will return a non NULL
> > > > > ptr.
> > > > >
> > > > > It does need to be slower, just return ~0 instead, the kernel does
> > > > > something similar: if (!size)
> > > > >
> > > > > return ZERO_SIZE_PTR;
> > > >
> > > > That could work, but technically I don't think it complies as it is
> > > > not a pointer to allocated memory...
> > >
> > > It doesn't not have to be allocated memory, just a ptr != NULL which
> > > you can do free() on.
> >
> > But kernel has something mapped there to trap these pointers I believe.
>
> So? That only means that the kernel has extra protection if someone tries
> to deference such a ptr. You are not required to do that(nice to have
> though) You don have any protection for deferencing NULL either I think?

Can't GCC track it?

> Jocke

Best regards,
Marek Vasut