On Wed, Nov 07, 2018 at 08:15:36PM -0800, Joel Fernandes (Google) 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 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: > [...] > 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 > > Cc: jreck@xxxxxxxxxx > Cc: john.stultz@xxxxxxxxxx > Cc: tkjos@xxxxxxxxxx > Cc: gregkh@xxxxxxxxxxxxxxxxxxx > Cc: hch@xxxxxxxxxxxxx > Reviewed-by: John Stultz <john.stultz@xxxxxxxxxx> > Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> > --- > v1->v2: No change, just added selftests to the series. manpages are > ready and I'll submit them once the patches are accepted. > > v2->v3: Updated commit message to have more support code (John Stultz) > Renamed seal from F_SEAL_FS_WRITE to F_SEAL_FUTURE_WRITE > (Christoph Hellwig) > Allow for this seal only if grow/shrink seals are also > either previous set, or are requested along with this seal. > (Christoph Hellwig) > Added locking to synchronize access to file->f_mode. > (Christoph Hellwig) Christoph, do the patches look Ok to you now? If so, then could you give an Acked-by or Reviewed-by tag? Thanks a lot, - Joel > include/uapi/linux/fcntl.h | 1 + > mm/memfd.c | 22 +++++++++++++++++++++- > 2 files changed, 22 insertions(+), 1 deletion(-) > > diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h > index 6448cdd9a350..a2f8658f1c55 100644 > --- a/include/uapi/linux/fcntl.h > +++ b/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 */ > > /* > diff --git a/mm/memfd.c b/mm/memfd.c > index 2bb5e257080e..5ba9804e9515 100644 > --- a/mm/memfd.c > +++ b/mm/memfd.c > @@ -150,7 +150,8 @@ static unsigned int *memfd_file_seals_ptr(struct file *file) > #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) > { > @@ -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); > + } > + > *file_seals |= seals; > error = 0; > > -- > 2.19.1.930.g4563a0d9d0-goog
