[libcamera-devel] [PATCH v2 2/2] libcamera: ipa_module: Fix implicit sign-extension in elfSection

[Umang Jain]

Hi Laurent,

On 6/5/20 10:11 PM, Laurent Pinchart wrote:
> Hi Umang,
>
> Thank you for the patch.
>
> On Fri, Jun 05, 2020 at 03:09:16PM +0000, Umang Jain wrote:
>> Given how the elfSection() is used, the sub-expression
>>         (idx * eHdr->e_shentsize)
>> has effectively two (16 bits, unsigned) operands.
>> The sub-expression is promoted to type int (32 bits, signed) for
>> multiplication and then added to eHdr->e_shoff, which is uint32_t
>> on 32-bit platforms and uint64_t on 64-bit platforms. Since eHdr->e_shoff
>> is unsigned, the integer conversion rules dictates that the other signed
>> operand(i.e. the resultant of aforementioned sub-expression) will be
>> converted to unsigned type too. This causes sign-extension for both of
>> the above operands to match eHdr->e_shoff's type and should be avoided.
>>
>> The solution is to explicitly cast one of the operands of the
>> sub-expression with unsigned int type. Hence, the other operand will be
>> integer promoted and the resultant will also be of unsigned int type,
>> not requiring to bother about a sign-extension.
>>
>> Reported-by: Coverity CID=280008
>> Reported-by: Coverity CID=280009
>> Reported-by: Coverity CID=280010
>> Signed-off-by: Umang Jain <email at uajain.com>
>> Reviewed-by: Kieran Bingham <kieran.bingham at ideasonboard.com>
>> ---
>>   src/libcamera/ipa_module.cpp | 3 ++-
>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/src/libcamera/ipa_module.cpp b/src/libcamera/ipa_module.cpp
>> index f54dd8b..d79151d 100644
>> --- a/src/libcamera/ipa_module.cpp
>> +++ b/src/libcamera/ipa_module.cpp
>> @@ -93,7 +93,8 @@ ElfW(Shdr) *elfSection(Span<uint8_t> elf, ElfW(Ehdr) *eHdr, unsigned int idx)
>>   	if (idx >= eHdr->e_shnum)
>>   		return nullptr;
>>   
>> -	off_t offset = eHdr->e_shoff + idx * eHdr->e_shentsize;
>> +	off_t offset = eHdr->e_shoff + idx *
>> +				       static_cast<uint32_t>(eHdr->e_shentsize);
> Is this needed now that idx is an unsigned int and not an uint16_t
> anymore ?

Should I assume `unsigned int` will be 32-bit minimum? If yes, then you 
are right.
Well, I have not heard "16-bit platforms" in recent years but I am not 
sure if there is

any obscurity out there in wild.

>
>>   	return elfPointer<ElfW(Shdr)>(elf, offset);
>>   }