[PATCH v5 4/4] smb/client: fix incorrect nlink returned by fstat()

From: ChenXiaoSong <[email protected]>

Reproducer:

  1. mount -t cifs //${server_ip}/export /mnt
  2. touch /mnt/file1; ln /mnt/file1 /mnt/file2; ln /mnt/file1 /mnt/file3
  3. C program: int fd = open("/mnt/file1", O_RDONLY);
  4. C program: struct stat stbuf; fstat(fd, &stbuf);
                stbuf.st_nlink is always 1, should be 3

Setting `unknown_nlink` to true in `SMB2_open()` triggers the
`CIFS_FATTR_UNKNOWN_NLINK` flag in `cifs_open_info_to_fattr()`,
which safely preserves the existing i_nlink in
`cifs_nlink_fattr_to_inode()`.

See the detailed procedure below:

  path_openat
    open_last_lookups
      lookup_open
        atomic_open
          cifs_atomic_open // dir->i_op->atomic_open
            cifs_lookup
              cifs_get_inode_info
                cifs_get_fattr
                  smb2_query_path_info // server->ops->query_path_info
                    smb2_compound_op
                      SMB2_open_init
                      case SMB2_OP_QUERY_INFO
                      SMB2_query_info_init(FILE_ALL_INFORMATION,)
                  cifs_open_info_to_fattr
                    fattr->cf_nlink = le32_to_cpu(info->NumberOfLinks)
                update_inode_info
                  cifs_iget
                    cifs_fattr_to_inode
                      cifs_nlink_fattr_to_inode
                        set_nlink(inode, fattr->cf_nlink)
    do_open
      vfs_open
        do_dentry_open
          cifs_open
            cifs_nt_open
              smb2_open_file // server->ops->open
                SMB2_open
                  buf->unknown_nlink = true
              cifs_get_inode_info
                cifs_get_fattr
                  cifs_open_info_to_fattr
                    if (data->unknown_nlink) // true
                    fattr->cf_flags |= CIFS_FATTR_UNKNOWN_NLINK
                update_inode_info
                  cifs_fattr_to_inode
                    cifs_nlink_fattr_to_inode
                      if (fattr->cf_flags & CIFS_FATTR_UNKNOWN_NLINK) // true
                      return // do not modify nlink

Signed-off-by: ChenXiaoSong <[email protected]>
---
 fs/smb/client/cifsglob.h | 1 +
 fs/smb/client/inode.c    | 2 ++
 fs/smb/client/smb2pdu.c  | 1 +
 3 files changed, 4 insertions(+)

diff --git a/fs/smb/client/cifsglob.h b/fs/smb/client/cifsglob.h
index ea1c5eb2356a..0454f69630ae 100644
--- a/fs/smb/client/cifsglob.h
+++ b/fs/smb/client/cifsglob.h
@@ -250,6 +250,7 @@ struct cifs_open_info_data {
 	bool adjust_tz;
 	bool reparse_point;
 	bool contains_posix_file_info;
+	bool unknown_nlink;
 	struct {
 		/* ioctl response buffer */
 		struct {
diff --git a/fs/smb/client/inode.c b/fs/smb/client/inode.c
index 1dbcfd163ff0..4711886d7b4b 100644
--- a/fs/smb/client/inode.c
+++ b/fs/smb/client/inode.c
@@ -909,6 +909,8 @@ static void cifs_open_info_to_fattr(struct cifs_fattr *fattr,
 	struct cifs_tcon *tcon = cifs_sb_master_tcon(cifs_sb);
 
 	memset(fattr, 0, sizeof(*fattr));
+	if (data->unknown_nlink)
+		fattr->cf_flags |= CIFS_FATTR_UNKNOWN_NLINK;
 	fattr->cf_cifsattrs = le32_to_cpu(info->Attributes);
 	if (info->DeletePending)
 		fattr->cf_flags |= CIFS_FATTR_DELETE_PENDING;
diff --git a/fs/smb/client/smb2pdu.c b/fs/smb/client/smb2pdu.c
index 5707d76a8647..d4d4c510412c 100644
--- a/fs/smb/client/smb2pdu.c
+++ b/fs/smb/client/smb2pdu.c
@@ -3380,6 +3380,7 @@ SMB2_open(const unsigned int xid, struct cifs_open_parms *oparms, __le16 *path,
 		file_info->EndOfFile = rsp->EndofFile;
 		file_info->Attributes = rsp->FileAttributes;
 		file_info->NumberOfLinks = cpu_to_le32(1);
+		buf->unknown_nlink = true;
 		file_info->DeletePending = 0; /* successful open = not delete pending */
 	}
 
-- 
2.54.0