[PATCH v2 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.

Reviewed-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Signed-off-by: Yang Xu <xuyang2018.jy@fujitsu.com>
---
 fs/inode.c         | 18 ++++++++++++++++++
 include/linux/fs.h |  3 ++-
 2 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/fs/inode.c b/fs/inode.c
index 9d9b422504d1..d63264998855 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -2405,3 +2405,21 @@ 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))
+		return;
+	if ((*mode & (S_ISGID | S_IXGRP)) != (S_ISGID | S_IXGRP))
+		return;
+	if (S_ISDIR(*mode))
+		return;
+	if (in_group_p(i_gid_into_mnt(mnt_userns, dir)))
+		return;
+	if (capable_wrt_inode_uidgid(mnt_userns, dir, CAP_FSETID))
+		return;
+
+	*mode &= ~S_ISGID;
+}
+EXPORT_SYMBOL(inode_sgid_strip);
diff --git a/include/linux/fs.h b/include/linux/fs.h
index bbde95387a23..94d94219fe7c 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1897,7 +1897,8 @@ 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
  * the kernel specify what kind of dirent layout it wants to have.
-- 
2.27.0