Messages in this thread | | | Date | Wed, 15 Feb 2012 15:17:57 -0500 | From | Nick Bowler <> | Subject | Re: Uninline kcalloc |
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On 2012-02-14 15:24 -0600, Christoph Lameter wrote: > On Tue, 14 Feb 2012, Nick Bowler wrote: > > > On 2012-02-14 13:37 -0600, Christoph Lameter wrote: > > > This patch still preserves kcalloc. But note that if kcalloc returns NULL > > > then multiple conditions may have caused it. One is that the array is > > > simply too large. The other may be that such an allocation is not possible > > > due to fragmentation. > > [...] > > > +static inline long calculate_array_size(size_t n, size_t size) > > > +{ > > > + if (size != 0 && n > ULONG_MAX / size) > > > + > > > + return 0; > > > > This isn't right. The above tests whether or not the result of the > > multiplication will not be representable in an 'unsigned long'... > > Yes and so does the current kcalloc.
Well, the current kcalloc doesn't assign the result to a signed long. However, it does assign the result to a size_t, which makes one wonder why it's not testing against SIZE_MAX. If size_t has the same range as unsigned long on all architectures, then this confusion doesn't matter, but is that actually the case?
> > > + return n * size; > > > > but then the result is assigned to a (signed) long, which may overflow > > if it's greater than LONG_MAX. > > That can happen?
Yes, because LONG_MAX (the maximum value of your return type) is strictly less than ULONG_MAX (what you test against). It's not hard to pick input numbers that multiply to something between LONG_MAX and ULONG_MAX, which will cause your function to return a negative value (standard C leaves the result of such a conversion implementation- defined, but I'll assume for now that it works this way for everything that compiles Linux).
Admittedly, your kcalloc change then assigns this negative value to a size_t, which will result in the correct positive value assuming SIZE_MAX == ULONG_MAX, but that's gratuitously roundabout.
[...] > > [...] > > > void *kcalloc(size_t n, size_t size, gfp_t flags) > > > { > > > - if (size != 0 && n > ULONG_MAX / size) > > > - return NULL; > > > - return __kmalloc(n * size, flags | __GFP_ZERO); > > > + size_t s = calculate_array_size(n, size); > > > + > > > + if (s) > > > + return kzalloc(s, flags); > > > + > > > + return NULL; > > > } > > > > This hunk changes the behaviour of kcalloc if either of the two size parameters > > is 0. > > You want ZERO_PTR returns? > > NULL is one permissible return value of calloc() if size == 0. So we are > now deviating from user space conventions.
Sort of. While standard C leaves it implementation-defined whether successful zero-sized allocations are possible, all sane implementations let them succeed. Hence, portable C apps need to handle 0 as a special case, because there are insane implementations out there. There's no reason for the kernel to be one of them.
Regardless, this was still a (presumably unintentional) change from the previous behaviour.
Cheers, -- Nick Bowler, Elliptic Technologies (http://www.elliptictech.com/)
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