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

[Marek Vasut]

Dear Joakim Tjernlund,

> Marek Vasut <marek.vasut at gmail.com> wrote on 2012/04/02 17:23:03:
> > Dear Joakim Tjernlund,
> > 
> > > Marek Vasut <marek.vasut at gmail.com> wrote on 2012/04/02 16:42:30:
> > > > Dear Joakim Tjernlund,
> > > > 
> > > > > Marek Vasut <marek.vasut at gmail.com> wrote on 2012/04/02 16:05:13:
> > > > > > Dear Joakim Tjernlund,
> > > > > > 
> > > > > > > Hi Grame
> > > > > > > 
> > > > > > > Graeme Russ <graeme.russ at gmail.com> wrote on 2012/04/02 09:17:44:
> > > > > > > > Hi Joakim,
> > > > > > > > 
> > > > > > > > On Apr 2, 2012 4:55 PM, "Joakim Tjernlund"
> > > > > > > > <joakim.tjernlund at transmode.se>
> > > > > > 
> > > > > > wrote:
> > > > > > > > > > Hi Marek,
> > > > > > > > > > 
> > > > > > > > > > On Mon, Apr 2, 2012 at 1:36 PM, Marek Vasut
> > > > > > > > > > <marek.vasut at 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?
> > > 
> > > Track what? NULL ptrs? I don't think so. Possibly when you have a
> > > static NULL ptr but not in the general case.
> > 
> > Well of course.
> 
> What did you mean then with "Can't GCC track it?" then? Just a bad joke?

Never mind, didn't finish my train of thought.

> > > I am getting tired of this discussion now. I am just trying to tell you
> > > that no sane impl. of malloc() these days return NULL for malloc(0).
> > 
> > And I got your point. Though for u-boot, this would be the best solution
> > actually. Anyone who uses memory allocated by malloc(0) is insane.
> 
> No, you don't get the point. If you did you would not have have made the
> "insane" remark.

No, relying on malloc(0) returning something sane is messed up.

> > > Even
> > > though standards allow it they don't consider malloc(0) an error, glibc
> > > will not update errno in this case.
> > 
> > There's no errno in uboot I'm aware of ;-)
> 
> Just pointing out that malloc(0) is not an error even if malloc returns
> NULL in glibc/standards. malloc(0) represents the empty set, just like 0
> does in math and it is sometimes useful.
> 
>  Jocke

Best regards,
Marek Vasut