Commit f518d466 authored Dec 26, 2024 by Isaac J. Manjarres Committed by Isaac Manjarres Jan 08, 2025

ashmem: Only use memfd if the kernel has ashmem-memfd compat support



There are still old applications that have not migrated to using
libcutils, and allocate shared memory buffers by directly allocating
them from "/dev/ashmem", and then using the ashmem ioctl commands
to operate on the buffer.

This works well on a system where all shared memory buffers are ashmem
buffers. However, in a system where there could be a mix of ashmem and
memfd buffers (e.g. if sys.use_memfd == true), this could be a problem
as ashmem ioctl commands on memfds will fail if the kernel does not
have support for ashmem-memfd compatibility.

Therefore, check for ashmem-memfd compatiblity before deciding if memfd
is usable.

Bug: 111903542
Change-Id: I760e57b31b53cdd4cfbf94cb35a6fb47b089bb1b
Signed-off-by: Isaac J. Manjarres <isaacmanjarres@google.com>

parent 20f62340

libcutils/ashmem-dev.cpp

+10 −0

Original line number	Diff line number	Diff line
		@@ -132,6 +132,16 @@ static bool __has_memfd_support() {
		return false;
		}

		/*
		* Ensure that the kernel supports ashmem ioctl commands on memfds. If not,
		* fall back to using ashmem.
		*/
		if (TEMP_FAILURE_RETRY(ioctl(fd, ASHMEM_SET_SIZE, getpagesize())) < 0) {
		ALOGE("ioctl(ASHMEM_SET_SIZE) failed: %s, no ashmem-memfd compat support.\n",
		strerror(errno));
		return false;
		}

		if (debug_log) {
		ALOGD("memfd: device has memfd support, using it\n");
		}