[PATCH 5.10 0/5] kernfs: backport locking and concurrency improvement

[Qingfang Deng <dqfext@gmail.com>]

KCSAN reports concurrent accesses to inode->i_mode:

==================================================================
BUG: KCSAN: data-race in generic_permission / kernfs_iop_permission

write to 0xffffffe001129590 of 2 bytes by task 2477 on cpu 1:
 kernfs_iop_permission+0x72/0x1a0
 link_path_walk.part.0.constprop.0+0x348/0x420
 path_openat+0xee/0x10f0
 do_filp_open+0xaa/0x160
 do_sys_openat2+0x252/0x380
 sys_openat+0x4c/0xa0
 ret_from_syscall+0x0/0x2

read to 0xffffffe001129590 of 2 bytes by task 3902 on cpu 3:
 generic_permission+0x26/0x120
 kernfs_iop_permission+0x150/0x1a0
 link_path_walk.part.0.constprop.0+0x348/0x420
 path_lookupat+0x58/0x280
 filename_lookup+0xae/0x1f0
 user_path_at_empty+0x3a/0x70
 vfs_statx+0x82/0x170
 __do_sys_newfstatat+0x36/0x70
 sys_newfstatat+0x2e/0x50
 ret_from_syscall+0x0/0x2

Reported by Kernel Concurrency Sanitizer on:
CPU: 3 PID: 3902 Comm: ls Not tainted 5.10.104+ #0
==================================================================

kernfs_iop_permission+0x72/0x1a0:

kernfs_refresh_inode at fs/kernfs/inode.c:174
 169 	
 170 	static void kernfs_refresh_inode(struct kernfs_node *kn, struct inode *inode)
 171 	{
 172 		struct kernfs_iattrs *attrs = kn->iattr;
 173 	
>174<		inode->i_mode = kn->mode;
 175 		if (attrs)
 176 			/*
 177 			 * kernfs_node has non-default attributes get them from
 178 			 * persistent copy in kernfs_node.
 179 			 */

(inlined by) kernfs_iop_permission at fs/kernfs/inode.c:285
 280 			return -ECHILD;
 281 	
 282 		kn = inode->i_private;
 283 	
 284 		mutex_lock(&kernfs_mutex);
>285<		kernfs_refresh_inode(kn, inode);
 286 		mutex_unlock(&kernfs_mutex);
 287 	
 288 		return generic_permission(inode, mask);
 289 	}
 290 	

generic_permission+0x26/0x120:

acl_permission_check at fs/namei.c:298
 293 	 * Note that the POSIX ACL check cares about the MAY_NOT_BLOCK bit,
 294 	 * for RCU walking.
 295 	 */
 296 	static int acl_permission_check(struct inode *inode, int mask)
 297 	{
>298<		unsigned int mode = inode->i_mode;
 299 	
 300 		/* Are we the owner? If so, ACL's don't matter */
 301 		if (likely(uid_eq(current_fsuid(), inode->i_uid))) {
 302 			mask &= 7;
 303 			mode >>= 6;

(inlined by) generic_permission at fs/namei.c:353
 348 		int ret;
 349 	
 350 		/*
 351 		 * Do the basic permission checks.
 352 		 */
>353<		ret = acl_permission_check(inode, mask);
 354 		if (ret != -EACCES)
 355 			return ret;
 356 	
 357 		if (S_ISDIR(inode->i_mode)) {
 358 			/* DACs are overridable for directories */

Backport the series from 5.15 to fix the concurrency bug.
https://lore.kernel.org/all/162642752894.63632.5596341704463755308.stgit@web.messagingengine.com

Ian Kent (5):
  kernfs: add a revision to identify directory node changes
  kernfs: use VFS negative dentry caching
  kernfs: switch kernfs to use an rwsem
  kernfs: use i_lock to protect concurrent inode updates
  kernfs: dont call d_splice_alias() under kernfs node lock

 fs/kernfs/dir.c             | 153 ++++++++++++++++++++----------------
 fs/kernfs/file.c            |   4 +-
 fs/kernfs/inode.c           |  26 +++---
 fs/kernfs/kernfs-internal.h |  24 +++++-
 fs/kernfs/mount.c           |  12 +--
 fs/kernfs/symlink.c         |   4 +-
 include/linux/kernfs.h      |   7 +-
 7 files changed, 138 insertions(+), 92 deletions(-)

-- 
2.43.0