[PATCH v1 1/3] vfs: Add inode_sgid_strip() api

[Yang Xu <xuyang2018.jy@fujitsu.com>]

inode_sgid_strip() function is used to strip S_ISGID mode
when creat/open/mknod file.

Suggested-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Yang Xu <xuyang2018.jy@fujitsu.com>
---
 fs/inode.c         | 12 ++++++++++++
 include/linux/fs.h |  3 +++
 2 files changed, 15 insertions(+)

diff --git a/fs/inode.c b/fs/inode.c
index 63324df6fa27..1f964e7f9698 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -2405,3 +2405,15 @@ struct timespec64 current_time(struct inode *inode)
 	return timestamp_truncate(now, inode);
 }
 EXPORT_SYMBOL(current_time);
+
+void inode_sgid_strip(struct user_namespace *mnt_userns, struct inode *dir,
+		      umode_t *mode)
+{
+	if ((dir && dir->i_mode & S_ISGID) &&
+		(*mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP) &&
+		!S_ISDIR(*mode) &&
+		!in_group_p(i_gid_into_mnt(mnt_userns, dir)) &&
+		!capable_wrt_inode_uidgid(mnt_userns, dir, CAP_FSETID))
+		*mode &= ~S_ISGID;
+}
+EXPORT_SYMBOL(inode_sgid_strip);
diff --git a/include/linux/fs.h b/include/linux/fs.h
index e2d892b201b0..639c830ad797 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1921,6 +1921,9 @@ extern long compat_ptr_ioctl(struct file *file, unsigned int cmd,
 void inode_init_owner(struct user_namespace *mnt_userns, struct inode *inode,
 		      const struct inode *dir, umode_t mode);
 extern bool may_open_dev(const struct path *path);
+void inode_sgid_strip(struct user_namespace *mnt_userns, struct inode *dir,
+		      umode_t *mode);
+
 
 /*
  * This is the "filldir" function type, used by readdir() to let
-- 
2.27.0