Messages in this thread | | | Date | Wed, 15 Feb 2012 15:24:27 -0500 | From | Nick Bowler <> | Subject | Re: Uninline kcalloc |
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On 2012-02-15 15:17 -0500, Nick Bowler wrote: > 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.
Of course, right after I send this I realize that we do not appear to define SIZE_MAX in the kernel. So s/SIZE_MAX/((size_t)-1)/g.
> 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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