On Tue, Oct 9, 2018 at 3:20 PM, Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> wrote: > Android uses ashmem for sharing memory regions. We are looking forward > to migrating all usecases of ashmem to memfd so that we can possibly > remove the ashmem driver in the future from staging while also > benefiting from using memfd and contributing to it. Note staging drivers > are also not ABI and generally can be removed at anytime. > > One of the main usecases Android has is the ability to create a region > and mmap it as writeable, then drop its protection for "future" writes > while keeping the existing already mmap'ed writeable-region active. > This allows us to implement a usecase where receivers of the shared > memory buffer can get a read-only view, while the sender continues to > write to the buffer. See CursorWindow in Android for more details: > https://developer.android.com/reference/android/database/CursorWindow > > This usecase cannot be implemented with the existing F_SEAL_WRITE seal. > To support the usecase, this patch adds a new F_SEAL_FS_WRITE seal which > prevents any future mmap and write syscalls from succeeding while > keeping the existing mmap active. The following program shows the seal > working in action: > > int main() { > int ret, fd; > void *addr, *addr2, *addr3, *addr1; > ret = memfd_create_region("test_region", REGION_SIZE); > printf("ret=%d\n", ret); > fd = ret; > > // Create map > addr = mmap(0, REGION_SIZE, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0); > if (addr == MAP_FAILED) > printf("map 0 failed\n"); > else > printf("map 0 passed\n"); > > if ((ret = write(fd, "test", 4)) != 4) > printf("write failed even though no fs-write seal " > "(ret=%d errno =%d)\n", ret, errno); > else > printf("write passed\n"); > > addr1 = mmap(0, REGION_SIZE, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0); > if (addr1 == MAP_FAILED) > perror("map 1 prot-write failed even though no seal\n"); > else > printf("map 1 prot-write passed as expected\n"); > > ret = fcntl(fd, F_ADD_SEALS, F_SEAL_FS_WRITE); > if (ret == -1) > printf("fcntl failed, errno: %d\n", errno); > else > printf("fs-write seal now active\n"); > > if ((ret = write(fd, "test", 4)) != 4) > printf("write failed as expected due to fs-write seal\n"); > else > printf("write passed (unexpected)\n"); > > addr2 = mmap(0, REGION_SIZE, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0); > if (addr2 == MAP_FAILED) > perror("map 2 prot-write failed as expected due to seal\n"); > else > printf("map 2 passed\n"); > > addr3 = mmap(0, REGION_SIZE, PROT_READ, MAP_SHARED, fd, 0); > if (addr3 == MAP_FAILED) > perror("map 3 failed\n"); > else > printf("map 3 prot-read passed as expected\n"); > } > > The output of running this program is as follows: > ret=3 > map 0 passed > write passed > map 1 prot-write passed as expected > fs-write seal now active > write failed as expected due to fs-write seal > map 2 prot-write failed as expected due to seal > : Permission denied > map 3 prot-read passed as expected > > Note: This seal will also prevent growing and shrinking of the memfd. > This is not something we do in Android so it does not affect us, however > I have mentioned this behavior of the seal in the manpage. > > Cc: jreck@xxxxxxxxxx > Cc: john.stultz@xxxxxxxxxx > Cc: tkjos@xxxxxxxxxx > Cc: gregkh@xxxxxxxxxxxxxxxxxxx > Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> Reviewed-by: John Stultz <john.stultz@xxxxxxxxxx> thanks -john
