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

[Daeho Jeong <daeho43@gmail.com>]

From: Daeho Jeong <daehojeong@google.com>

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@google.com>
---
 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;
+
 	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;
-- 
2.56.0.rc1.315.gc6ed9934b7-goog