Re: [f2fs-dev] [PATCH] f2fs: disallow mmap write and data-modifying fallocate on atomic files

[Daeho Jeong]

On Fri, Oct 9, 2026 at 1:35 AM Chao Yu <chao@xxxxxxxxxx> wrote:
>
> On 10/3/26 01:17, Daeho Jeong wrote:
> > From: Daeho Jeong <daehojeong@xxxxxxxxxx>
> >
> > Stores through a shared writable mapping and fallocate with
> > FALLOC_FL_PUNCH_HOLE, FALLOC_FL_ZERO_RANGE, FALLOC_FL_COLLAPSE_RANGE or
> > FALLOC_FL_INSERT_RANGE do not go through ->write_begin, so they bypass
> > the COW inode and modify the original inode directly. Reject them while
> > the file is in atomic write mode, as fallocate already does for pinned
> > and compressed files. A write fault gets SIGBUS and fallocate gets
> > -EOPNOTSUPP.
> >
> > fallocate without these flags only preallocates blocks and stays
> > allowed.
> >
> > Fixes: 3db1de0e582c ("f2fs: change the current atomic write way")
> > Signed-off-by: Daeho Jeong <daehojeong@xxxxxxxxxx>
> > ---
> >  fs/f2fs/file.c | 10 ++++++++--
> >  1 file changed, 8 insertions(+), 2 deletions(-)
> >
> > diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
> > index ef4d218e694..56c686b8580 100644
> > --- a/fs/f2fs/file.c
> > +++ b/fs/f2fs/file.c
> > @@ -140,6 +140,10 @@ static vm_fault_t f2fs_vm_page_mkwrite(struct vm_fault *vmf)
> >               return VM_FAULT_SIGBUS;
> >       }
> >
> > +     /* mmap stores bypass the COW inode and would break atomicity */
> > +     if (f2fs_is_atomic_file(inode))
> > +             return VM_FAULT_SIGBUS;
>
> Will we suffer race issue due to the check w/o lock?
>
> Thanks,

Hi Chao,

Yes, there is a race with f2fs_ioc_start_atomic_write().

page_mkwrite cannot take inode_lock, because the lock order is i_rwsem
-> mmap_lock (e.g. fiemap copies to user under inode_lock). So v2 will
use invalidate_lock instead:

f2fs_ioc_start_atomic_write() holds filemap_invalidate_lock() from the
flush until FI_ATOMIC_FILE is set.
f2fs_vm_page_mkwrite() rechecks the flag after
filemap_invalidate_lock_shared(), which it holds until the folio is
dirtied.

If you are okay with this, I'll send v2.

Thanks,

>
> > +
> >       if (is_inode_flag_set(inode, FI_COMPRESS_RELEASED)) {
> >               err = -EIO;
> >               goto out;
> > @@ -2113,9 +2117,11 @@ static long f2fs_fallocate(struct file *file, int mode,
> >
> >       /*
> >        * Pinned file should not support partial truncation since the block
> > -      * can be used by applications.
> > +      * can be used by applications. Atomic files should not either, since
> > +      * these modify the original inode directly and break atomicity.
> >        */
> > -     if ((f2fs_compressed_file(inode) || f2fs_is_pinned_file(inode)) &&
> > +     if ((f2fs_compressed_file(inode) || f2fs_is_pinned_file(inode) ||
> > +          f2fs_is_atomic_file(inode)) &&
> >               (mode & (FALLOC_FL_PUNCH_HOLE | FALLOC_FL_COLLAPSE_RANGE |
> >                       FALLOC_FL_ZERO_RANGE | FALLOC_FL_INSERT_RANGE))) {
> >               ret = -EOPNOTSUPP;
>