Messages in this thread | | | Date | Mon, 27 Oct 2014 19:46:33 +0300 | From | Andrey Ryabinin <> | Subject | Re: [PATCH] netlink: don't copy over empty attribute data |
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On 10/27/2014 05:42 PM, Sasha Levin wrote: > On 10/26/2014 10:03 PM, David Miller wrote: >> From: Sasha Levin <sasha.levin@oracle.com> >> Date: Sun, 26 Oct 2014 19:32:42 -0400 >> >>> How so? GCC states clearly that you should *never* pass a NULL >>> pointer there: >>> >>> "The pointers passed to memmove (and similar functions in <string.h>) must >>> be non-null even when nbytes==0" (https://gcc.gnu.org/gcc-4.9/porting_to.html). >>> >>> Even if it doesn't dereference it, it can break somehow in a subtle way. Leaving >>> the kernel code assuming that gcc (or any other compiler) would always behave >>> the same in a situation that shouldn't occur. >> >> Show me a legal way in which one could legally dereference the pointer >> when length is zero, and I'll entertain this patch. > > The moment you've triggered an undefined behaviour you have GCC license to > dereference anything it wants. GCC would be well within it's rights > dereferencing a NULL "from". > > They even state it clearly in that GCC 4.9 porting guide I've linked above: > > """ > Calling copy(p, NULL, 0) can therefore deference a null pointer and crash. > > The example above needs to be fixed to avoid the invalid memmove call, for example: > > > if (nbytes != 0) > memmove (dest, src, nbytes); > """ >
In example from link null ptr deref could happen because GCC will optimize away null pointer check after memmove():
int copy (int* dest, int* src, size_t nbytes) { memmove (dest, src, nbytes); if (src != NULL) <---- GCC will eliminate this check because src can't be null. return *src; <-- NULL ptr deref return 0; }
Even though GCC and C standard treats such code ( memmove(dest, NULL, 0); ) as invalid, it probably will not crash in linux kernel case, because that kind of optimization disabled via -fno-delete-null-pointer-checks option.
> > Thanks, > Sasha >
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