Require X_OK on path ancestors and mountpoint gates via
inode_checkpathperm(), and use inode_checkperm() for inode mode checks.
inode_checkpathperm() takes the inode tree read lock (or INODE_CHECK_LOCKED
when the caller already holds it) and can fold a non-zero amode check onto
the target inode in the same call.
When CONFIG_FS_PERMISSION is disabled, inode_checkperm/inode_checkpathperm
are empty macros returning 0 so there is no stub .text cost.
Skip mountpoint R/W open-mode checks in inode_checkopenperm(); keep
traversal separate from open. Add an optional mountpt_operations.permission
hook (tmpfs when CONFIG_FS_PERMISSION) for in-volume DAC without making the
VFS mount-crossing path depend on it.
Signed-off-by: Abhishek Mishra <mishra.abhishek2808@gmail.com>
08afc5f2cf