full-disclosure - Re: Linux Kernel sctp_setsockopt() Integer Overflow

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Message-ID: <Pine.LNX.4.58.0405152238470.5783@twin.jikos.cz>
From: jikos at jikos.cz (Jirka Kosina)
Subject: Re: Linux Kernel sctp_setsockopt() Integer
 Overflow

On Sat, 15 May 2004, Michael Tokarev wrote:

> But kmalloc(0) will return NULL, and the whole setsockopt
> will finish with errno set to ENOMEM.
> 
> From 2.4 mm/slab.c:
> void * kmalloc (size_t size, int flags)
> {
>         cache_sizes_t *csizep = cache_sizes;
> 
>         for (; csizep->cs_size; csizep++) {
>                 if (size > csizep->cs_size)
>                         continue;
>                 return __kmem_cache_alloc(flags & GFP_DMA ?
>                          csizep->cs_dmacachep : csizep->cs_cachep, flags);
>         }
>         return NULL;
> }

How did you come from the above snippet of the code to the idea that
kmalloc(0) returns NULL?

It allocates the number of bytes equal to the closest larger value of
cache_sizes->cs_size entries ... so on typical system this would be 
something like 32 or 64 bytes, depending on the page size (see 
include/linux/kmalloc_sizes.h) ... and of course returns pointer to this 
data, which is definitely not NULL.

-- 
JiKos.