On Fri, Nov 09, 2018 at 10:06:31PM +0100, Jann Horn wrote:

+linux-api for API addition
+hughd as FYI since this is somewhat related to mm/shmem

On Fri, Nov 9, 2018 at 9:46 PM Joel Fernandes (Google)
[off-list ref] wrote:

quoted

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.

Please CC linux-api@ on patches like this. If you had done that, I
might have criticized your v1 patch instead of your v3 patch...

Ok, will do from next time.

quoted

The following program shows the seal
working in action:

[...]

quoted

Cc: jreck@google.com
Cc: john.stultz@linaro.org
Cc: tkjos@google.com
Cc: gregkh@linuxfoundation.org
Cc: hch@infradead.org
Reviewed-by: John Stultz <redacted>
Signed-off-by: Joel Fernandes (Google) <redacted>
---

[...]

quoted

diff --git a/mm/memfd.c b/mm/memfd.c
index 2bb5e257080e..5ba9804e9515 100644
--- a/mm/memfd.c
+++ b/mm/memfd.c

[...]

quoted

@@ -219,6 +220,25 @@ static int memfd_add_seals(struct file *file, unsigned int seals)
                }
        }

+       if ((seals & F_SEAL_FUTURE_WRITE) &&
+           !(*file_seals & F_SEAL_FUTURE_WRITE)) {
+               /*
+                * The FUTURE_WRITE seal also prevents growing and shrinking
+                * so we need them to be already set, or requested now.
+                */
+               int test_seals = (seals | *file_seals) &
+                                (F_SEAL_GROW | F_SEAL_SHRINK);
+
+               if (test_seals != (F_SEAL_GROW | F_SEAL_SHRINK)) {
+                       error = -EINVAL;
+                       goto unlock;
+               }
+
+               spin_lock(&file->f_lock);
+               file->f_mode &= ~(FMODE_WRITE | FMODE_PWRITE);
+               spin_unlock(&file->f_lock);
+       }

So you're fiddling around with the file, but not the inode? How are
you preventing code like the following from re-opening the file as
writable?

$ cat memfd.c
#define _GNU_SOURCE
#include <unistd.h>
#include <sys/syscall.h>
#include <printf.h>
#include <fcntl.h>
#include <err.h>
#include <stdio.h>

int main(void) {
  int fd = syscall(__NR_memfd_create, "testfd", 0);
  if (fd == -1) err(1, "memfd");
  char path[100];
  sprintf(path, "/proc/self/fd/%d", fd);
  int fd2 = open(path, O_RDWR);
  if (fd2 == -1) err(1, "reopen");
  printf("reopen successful: %d\n", fd2);
}
$ gcc -o memfd memfd.c
$ ./memfd
reopen successful: 4

Great catch and this is indeed an issue :-(. I verified it too.

That aside: I wonder whether a better API would be something that
allows you to create a new readonly file descriptor, instead of
fiddling with the writability of an existing fd.

Android usecases cannot deal with a new fd number because it breaks the
continuity of having the same old fd, as Dan also pointed out.

Also such API will have the same issues you brought up?

thanks,

 - Joel