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

[Marek Vasut]

Dear Graeme Russ,

> Hi All
> 
> Here we go again ;)

Yay (polishing my flamethrower)!

> On Mon, Apr 2, 2012 at 12:21 AM, Marek Vasut <marek.vasut at gmail.com> wrote:
> > Dear Joakim Tjernlund,
> > 
> >> Marek Vasut <marek.vasut at gmail.com> wrote on 2012/04/01 16:01:56:
> >> > Dear Joakim Tjernlund,
> >> > 
> >> > > > Dear Mike Frysinger,
> >> > > > 
> >> > > > > On Thursday, October 21, 2010 17:10:31 Graeme Russ wrote:
> >> > > > > > On 22/10/10 06:51, Mike Frysinger wrote:
> >> > > > > > > have u-boot return an error.
> >> > > > > > 
> >> > > > > > Is NULL what you consider to be an error
> >> > > > > 
> >> > > > > yes
> >> > > > > 
> >> > > > > > Besides, is not free(NULL) valid (does nothing) as well?
> >> > > > > 
> >> > > > > yes, free(NULL) should work fine per POSIX
> >> > > > > -mike
> >> > > > 
> >> > > > Well then, this patch wasn't accepted yet and I consider it OK to
> >> > > > apply. Any objections?
> >> > > 
> >> > > There was a long debate on the list regarding this where I argued
> >> > > that malloc(0) should not be an error and malloc should return a
> >> > > ptr != NULL I guess that is why it hasn't been applied.
> >> > > 
> >> > >  Jocke
> >> > 
> >> > Ok, let's restart. Is there any objection why malloc(0) should not
> >> > return NULL in uboot?
> >> 
> >> Yes, read the thread to see why.
> > 
> > Well I did, that's why I have no objections to applying this patch
> > 
> >> > Is it coliding with any spec?
> >> 
> >> No, both are valid.
> 
> <quote author="Reinhard Meyer">
> Out of principle I would say that malloc(0) should return a non-NULL
> pointer of an area where exactly 0 bytes may be used. And, of course,
> free() of that area shall not fail or crash the system.
> </quote>
> 
> I'm wondering how exactly this would work - In theory, if you tried to
> access this pointer you should get a segv. But I suppose if you malloc(1)
> and try to access beyond the first byte there probably won't be a segv
> either....
> 
> So to review the facts:
> 
> - The original complaint was that malloc(0) corrupts the malloc data
>   structures, not that U-Boot's malloc(0) behaviour is non-standard
> - Both the malloc(0) returns NULL and malloc(0) returns a uniquely
>   free'able block of memory solutions are standard compliant
> - malloc(0) returning NULL may break code which, for the sake of code
>   simplicity, does not bother to check for zero-size before calling
>   malloc()

Well but you said malloc(0) corrupts the mallocator's data structures. Therefore 
malloc(0) used in code right now is broken anyway.

> - malloc(0) returning NULL may help to identify brain-dead use-cases

Agreed.

> 
> My vote:
> 
>         if ((long)bytes == 0) {
>                 DEBUG("Warning: malloc of zero block size\n");
>                 bytes = 1;

Well ... no, how can malloc(0) returning NULL break code that's already broken 
any more? It's silently roughing the mallocator structures up and it means the 
code is sitting on a ticking a-bomb anyway.

So we should add this like:

if (bytes == 0) {
	debug("You're sitting on a ticking A-Bomb doing this");
	return NULL;
} else if (bytes < 0) {
	return NULL;
}

>         } else if ((long)bytes < 0) {
>                 DEBUG("Error: malloc of negative block size\n");
>                 return 0;
>         }
> 
> Regards,
> 
> Graeme

Best regards,