libcamera: media_device: prevent sign extension on casts

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
			  << " ptr -> u64 0x" << std::hex << u64
			  << " ptr -> uintptr_t -> u64 0x" << std::hex << uint64
			  << std::endl;

		return 0;
	}

When compiled with g++ for a 32-bit platform produces the following
unexpected output:

  ptr 0xf1234567 ptr -> u64 0xfffffffff1234567 ptr -> uintptr_t -> u64 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@ideasonboard.com>

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>
@@ -231,10 +232,10 @@ 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) {