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

[Joakim Tjernlund]

Reinhard Meyer <u-boot at emk-elektronik.de> wrote on 2010/10/22 09:18:02:
> 
> Dear Joakim Tjernlund,
> > Mike Frysinger <vapier at gentoo.org> wrote on 2010/10/21 21:51:53:
> >> On Thursday, October 21, 2010 07:45:10 Joakim Tjernlund wrote:
> >>> Wolfgang Denk wrote on 2010/10/21 13:32:54:
> >>>> Joakim Tjernlund you wrote:
> >>>>>> -  if ((long)bytes < 0) return 0;
> >>>>>> +  if ((long)bytes <= 0) return 0;
> >>>>> I think you should return some impossible ptr value =! NULL
> >>>>> Size 0 not really an error.
> >>>> It is legal for malloc() to return NULL in case of size==0,
> >>>> and for the sake of simplicity I recommend we do just that.
> >>> Yes, but not very useful. Glibc does not return NULL
> >> it is useful for malloc(0) == NULL.  the glibc behavior is downright 
> >> obnoxious.  we disable this for uClibc and dont see problems.  if 
> > anything, we 
> >> catch accidental programming mistakes which then get fixed.
> 
> My five cents:
> 
> > There is a value in having the possibility to express a
> > 0 bytes data set. Consider this simple example:
> > An app read lines from a file and mallocs each line read and builds an
> > array with malloced pointers. The last entry is a NULL ptr to
> > signal EOF. This breaks down for empty lines if malloc(0)
> > returns NULL. 
> 
> Your example is in the right way, but a bit flawed in its simplicity.
> Even empty lines need some form of information that they are of length 
zero,
> be it a 0x00 in the memory line itself (requiring malloc(length+1)) or 
that same
> information in a variable somewhere else:
> struct line {
>    byte *buf;
>    int length;
> } lines[...];

Thanks, this is better.

> As an (undercover) Mathematician:
> 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.
> 
> > Not to mention error handling, as I recall, a malloc(0) that returns 
NULL
> > does not set errno which screws error handling. One have to bend over
> > just to cope with this.
> 
> >> why exactly do you want malloc(0) to return valid memory ?  i would 
> > rather 
> >> have u-boot return an error.
> 
> In the case of u-boot, where a driver or whatever should never really 
need
> to allocate zero memory, such a programming error should be made obvious 
by
> at least a warning message.

Right, a driver probably doesn't need malloc(0) != NULL but some
command may.

You could have a DEBUG printout for 0 if you like though.

> 
> > Ideally it should return a ptr to invalid memory so you get a SEGV if 
you
> > try to defer the ptr but I take anything over a NULL ptr.
> 
> Makes sense only if any access outside of any allocated memory would 
behave
> the same, otherwise this is a special case again.