[libcamera-devel] [PATCH v2] libcamera: media_device: prevent sign extension on casts

[Kieran Bingham]

Hi Laurent,

On 18/02/2020 17:33, Laurent Pinchart wrote:
> Hi Kieran,
> 
> On Tue, Feb 18, 2020 at 05:07:41PM +0000, Kieran Bingham wrote:
>> On 18/02/2020 15:17, Laurent Pinchart wrote:
>>> On Tue, Feb 18, 2020 at 02:18:39PM +0000, Kieran Bingham wrote:
>>>> The conversion of pointers to integers is implementation defined and
>>>> differs between g++ and clang++ when utilising a uint64_t type.
>>>>
>>>>     #include <iostream>
>>>>
>>>>     int main(int argc, char **argv)
>>>>     {
>>>>         void *ptr = reinterpret_cast<void *>(0xf1234567);
>>>>         uint64_t u64 = reinterpret_cast<uint64_t>(ptr);
>>>> 	uint64_t uint64 = reinterpret_cast<uintptr_t>(ptr);
>>>>
>>>>         std::cout << "ptr " << ptr
>>>> 		  << " u64 0x" << std::hex << u64
>>>> 		  << " uint64 0x" << std::hex << uint64
>>>
>>> Maybe " ptr -> u64 0x" and " ptr -> uintptr_t -> u64 0x" ?
>>>
>>>> 		  << std::endl;
>>>>
>>>> 	return 0;
>>>>     }
>>>
>>> That's a weird mix of tabs and spaces :-)
>>
>> Agh, I indented the left hand items by 4 to indent the whole code block
>> for the commit message.
>>
>> Indenting the whole lot by a tab fits, but it looks a bit weird and too
>> far to the right ... but I don't think it's important.
> 
> No big deal, no. I was however referring to lines starting with
> "uint64_t uint64" and "return" that seem to have a different combination
> of spaces and tabs than the other ones.
> 
>>>> When compiled with g++ produces the following unexpected output:
>>>
>>> s/with g++/with g++ for a 32-bit platform/
>>
>> No, I don't think that's true.
>> This issue affects a 64-bit platform just the same.
>>
>> If a pointer is passed through here which happens to be
>> 0x00000000f1234567, it will be incorrect when it gets to the kernel.
>>
>> (or any pointer with leading zeros up to a set bit 32...)
> 
> Really ? Testing the above code on x86-64 without -m32 prints
> 
> ptr 0xf1234567 u64 0xf1234567 uint64 0xf1234567
> 
> for me.

Ah, I /may/ have been building the snippit code on the target, when I
thought I had done it on the host... :-D

I.e. I hadn't specified -m32 'explicitly' ... ahem...

So you are correct, this is only an issue when compiled for 32 bit systems.

--
Kieran

> 
>>>>   ptr 0xf1234567 u64 0xfffffffff1234567 uint64 0xf1234567
>>>>
>>>> The standards states:
>>>>
>>>> "A pointer can be explicitly converted to any integral type large enough
>>>> to hold all values of its type. The mapping function is
>>>> implementation-defined. [Note: It is intended to be unsurprising to
>>>> those who know the addressing structure of the underlying machine. — end
>>>> note]"
>>>>
>>>> And as such the g++ implementation appears to be little more surprising
>>>> than expected in this situation.
>>>>
>>>> The MediaDevice passes pointers to the kernel via the struct
>>>> media_v2_topology in which pointers are cast using a uint64 type (__u64),
>>>> which is affected by the sign extension described above when BIT(32) is
>>>> set and causes an invalid address to be given to the kernel.
>>>>
>>>> Ensure that we cast using uintptr_t which is not affected by the sign
>>>> extension issue.
>>>>
>>>> Signed-off-by: Kieran Bingham <kieran.bingham at ideasonboard.com>
>>>
>>> Reviewed-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
>>>
>>>> ---
>>>> v2:
>>>>  - Expand commit message to explain the underlying issue.
>>>>  - include stdint.h
>>>>  - use uintptr_t instead of std::uintptr_t
>>>>
>>>>  src/libcamera/media_device.cpp | 9 +++++----
>>>>  1 file changed, 5 insertions(+), 4 deletions(-)
>>>>
>>>> diff --git a/src/libcamera/media_device.cpp b/src/libcamera/media_device.cpp
>>>> index fad475b9ac76..0d6b5efd9e7a 100644
>>>> --- a/src/libcamera/media_device.cpp
>>>> +++ b/src/libcamera/media_device.cpp
>>>> @@ -9,6 +9,7 @@
>>>>  
>>>>  #include <errno.h>
>>>>  #include <fcntl.h>
>>>> +#include <stdint.h>
>>>>  #include <string>
>>>>  #include <string.h>
>>>>  #include <sys/ioctl.h>
>>>> @@ -236,10 +237,10 @@ int MediaDevice::populate()
>>>>  	 */
>>>>  	while (true) {
>>>>  		topology.topology_version = 0;
>>>> -		topology.ptr_entities = reinterpret_cast<__u64>(ents);
>>>> -		topology.ptr_interfaces = reinterpret_cast<__u64>(interfaces);
>>>> -		topology.ptr_links = reinterpret_cast<__u64>(links);
>>>> -		topology.ptr_pads = reinterpret_cast<__u64>(pads);
>>>> +		topology.ptr_entities = reinterpret_cast<uintptr_t>(ents);
>>>> +		topology.ptr_interfaces = reinterpret_cast<uintptr_t>(interfaces);
>>>> +		topology.ptr_links = reinterpret_cast<uintptr_t>(links);
>>>> +		topology.ptr_pads = reinterpret_cast<uintptr_t>(pads);
>>>>  
>>>>  		ret = ioctl(fd_, MEDIA_IOC_G_TOPOLOGY, &topology);
>>>>  		if (ret < 0) {
> 

-- 
Regards
--
Kieran