Re: [PATCH v2 5/7] malloc: Verify the integrity of mmapped chunks in calloc.

[Florian Weimer <fweimer at redhat dot com>]

* Florian Weimer:

> On 11/07/2017 04:27 PM, Istvan Kurucsai wrote:
>>      * malloc/malloc.c (__libc_calloc): Check mmapped chunks.
>> ---
>>   malloc/malloc.c | 9 +++++++++
>>   1 file changed, 9 insertions(+)
>>
>> diff --git a/malloc/malloc.c b/malloc/malloc.c
>> index 8e48952..5eb661e 100644
>> --- a/malloc/malloc.c
>> +++ b/malloc/malloc.c
>> @@ -3447,6 +3447,15 @@ __libc_calloc (size_t n, size_t elem_size)
>>     /* Two optional cases in which clearing not necessary */
>>     if (chunk_is_mmapped (p))
>>       {
>> +      size_t pagesize = GLRO (dl_pagesize);
>> +      INTERNAL_SIZE_T offset = prev_size (p);
>> +      INTERNAL_SIZE_T size = chunksize (p);
>> +      uintptr_t block = (uintptr_t) p - offset;
>> +      size_t total_size = offset + size;
>> +      if (__glibc_unlikely ((block | total_size) & (pagesize - 1)) != 0
>> +          || __glibc_unlikely (!powerof2 ((uintptr_t) mem & (pagesize - 1))))
>> +        malloc_printerr ("calloc(): invalid mmapped chunk");
>> +
>>         if (__builtin_expect (perturb_byte, 0))
>>           return memset (mem, 0, sz);
>
> Sorry, I don't understand the powerof2 check.  Could you elaborate?

I believe this is similar to the mremap_chunk case DJ reviewed.

Thanks,
Florian