On Wed, Nov 21, 2018 at 6:27 PM Andrew Morton akpm@linux-foundation.org wrote:
On Tue, 20 Nov 2018 13:13:35 -0800 Joel Fernandes joel@joelfernandes.org wrote:
I am Ok with whatever Andrew wants to do, if it is better to squash it with the original, then I can do that and send another patch.
From experience, Andrew will food in fixups on request :)
Andrew, could you squash this patch into the one titled ("mm: Add an F_SEAL_FUTURE_WRITE seal to memfd")?
Sure.
I could of course queue them separately but I rarely do so - I don't think that the intermediate development states are useful in the infinite-term, and I make them available via additional Link: tags in the changelog footers anyway.
I think that the magnitude of these patches is such that John Stultz's Reviewed-by is invalidated, so this series is now in the "unreviewed" state.
So can we have a re-review please? For convenience, here's the folded-together [1/1] patch, as it will go to Linus.
From: "Joel Fernandes (Google)" joel@joelfernandes.org Subject: mm: Add an F_SEAL_FUTURE_WRITE seal to memfd
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 add protection against making any "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 documentation 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_FUTURE_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:
#include <stdio.h> #include <errno.h> #include <sys/mman.h> #include <linux/memfd.h> #include <linux/fcntl.h> #include <asm/unistd.h> #include <unistd.h> #define F_SEAL_FUTURE_WRITE 0x0010 #define REGION_SIZE (5 * 1024 * 1024)
int memfd_create_region(const char *name, size_t size) { int ret; int fd = syscall(__NR_memfd_create, name, MFD_ALLOW_SEALING); if (fd < 0) return fd; ret = ftruncate(fd, size); if (ret < 0) { close(fd); return ret; } return fd; }
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 future-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_FUTURE_WRITE | F_SEAL_GROW | F_SEAL_SHRINK); if (ret == -1) printf("fcntl failed, errno: %d\n", errno); else printf("future-write seal now active\n"); if ((ret = write(fd, "test", 4)) != 4) printf("write failed as expected due to future-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 future-write seal now active write failed as expected due to future-write seal map 2 prot-write failed as expected due to seal : Permission denied map 3 prot-read passed as expected
[joel@joelfernandes.org: make F_SEAL_FUTURE_WRITE seal more robust] Link: http://lkml.kernel.org/r/20181120052137.74317-1-joel@joelfernandes.org Link: http://lkml.kernel.org/r/20181108041537.39694-1-joel@joelfernandes.org Signed-off-by: Joel Fernandes (Google) joel@joelfernandes.org Cc: John Stultz john.stultz@linaro.org Cc: John Reck jreck@google.com Cc: Todd Kjos tkjos@google.com Cc: Greg Kroah-Hartman gregkh@linuxfoundation.org Cc: Christoph Hellwig hch@infradead.org Cc: Al Viro viro@zeniv.linux.org.uk Cc: Daniel Colascione dancol@google.com Cc: J. Bruce Fields bfields@fieldses.org Cc: Jeff Layton jlayton@kernel.org Cc: Khalid Aziz khalid.aziz@oracle.com Cc: Lei Yang Lei.Yang@windriver.com Cc: Marc-Andr Lureau marcandre.lureau@redhat.com Cc: Mike Kravetz mike.kravetz@oracle.com Cc: Minchan Kim minchan@kernel.org Cc: Shuah Khan shuah@kernel.org Cc: Valdis Kletnieks valdis.kletnieks@vt.edu Cc: Andy Lutomirski luto@kernel.org Cc: Jann Horn jannh@google.com Signed-off-by: Andrew Morton akpm@linux-foundation.org
--- a/include/uapi/linux/fcntl.h~mm-add-an-f_seal_future_write-seal-to-memfd +++ a/include/uapi/linux/fcntl.h @@ -41,6 +41,7 @@ #define F_SEAL_SHRINK 0x0002 /* prevent file from shrinking */ #define F_SEAL_GROW 0x0004 /* prevent file from growing */ #define F_SEAL_WRITE 0x0008 /* prevent writes */ +#define F_SEAL_FUTURE_WRITE 0x0010 /* prevent future writes while mapped */ /* (1U << 31) is reserved for signed error codes */
/* --- a/mm/memfd.c~mm-add-an-f_seal_future_write-seal-to-memfd +++ a/mm/memfd.c @@ -131,7 +131,8 @@ static unsigned int *memfd_file_seals_pt #define F_ALL_SEALS (F_SEAL_SEAL | \ F_SEAL_SHRINK | \ F_SEAL_GROW | \
F_SEAL_WRITE)
F_SEAL_WRITE | \F_SEAL_FUTURE_WRITE)static int memfd_add_seals(struct file *file, unsigned int seals) { --- a/fs/hugetlbfs/inode.c~mm-add-an-f_seal_future_write-seal-to-memfd +++ a/fs/hugetlbfs/inode.c @@ -530,7 +530,7 @@ static long hugetlbfs_punch_hole(struct inode_lock(inode);
/* protected by i_mutex */
if (info->seals & F_SEAL_WRITE) {
if (info->seals & (F_SEAL_WRITE | F_SEAL_FUTURE_WRITE)) { inode_unlock(inode); return -EPERM; }--- a/mm/shmem.c~mm-add-an-f_seal_future_write-seal-to-memfd +++ a/mm/shmem.c @@ -2119,6 +2119,23 @@ out_nomem:
static int shmem_mmap(struct file *file, struct vm_area_struct *vma) {
struct shmem_inode_info *info = SHMEM_I(file_inode(file));/** New PROT_READ and MAP_SHARED mmaps are not allowed when "future
PROT_WRITE, perhaps?
* write" seal active.*/if ((vma->vm_flags & VM_SHARED) && (vma->vm_flags & VM_WRITE) &&(info->seals & F_SEAL_FUTURE_WRITE))return -EPERM;/** Since the F_SEAL_FUTURE_WRITE seals allow for a MAP_SHARED read-only* mapping, take care to not allow mprotect to revert protections.*/if (info->seals & F_SEAL_FUTURE_WRITE)vma->vm_flags &= ~(VM_MAYWRITE);
This might all be clearer as:
if (info->seals & F_SEAL_FUTURE_WRITE) { if (vma->vm_flags ...) return -EPERM; vma->vm_flags &= ~VM_MAYWRITE; }
with appropriate comments inserted.