Re: kernel crash in mknod

[Al Viro <viro@zeniv.linux.org.uk>]

On Sun, Mar 24, 2024 at 12:00:15AM -0500, Steve French wrote:
> Anyone else seeing this kernel crash in do_mknodat (I see it with a
> simple "mkfifo" on smb3 mount).  I started seeing this in 6.9-rc (did
> not see it in 6.8).   I did not see it with the 3/12/23 mainline
> (early in the 6.9-rc merge Window) but I do see it in the 3/22 build
> so it looks like the regression was introduced by:

	FWIW, successful ->mknod() is allowed to return 0 and unhash
dentry, rather than bothering with lookups.  So commit in question
is bogus - lack of error does *NOT* mean that you have struct inode
existing, let alone attached to dentry.  That kind of behaviour
used to be common for network filesystems more than just for ->mknod(),
the theory being "if somebody wants to look at it, they can bloody
well pay the cost of lookup after dcache miss".

Said that, the language in D/f/vfs.rst is vague as hell and is very easy
to misread in direction of "you must instantiate".

Thankfully, there's no counterpart with mkdir - *there* it's not just
possible, it's inevitable in some cases for e.g. nfs.

What the hell is that hook doing in non-S_IFREG cases, anyway?  Move it
up and be done with it...

diff --git a/fs/namei.c b/fs/namei.c
index ceb9ddf8dfdd..821fe0e3f171 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -4050,6 +4050,8 @@ static int do_mknodat(int dfd, struct filename *name, umode_t mode,
 		case 0: case S_IFREG:
 			error = vfs_create(idmap, path.dentry->d_inode,
 					   dentry, mode, true);
+			if (!error)
+				error = security_path_post_mknod(idmap, dentry);
 			break;
 		case S_IFCHR: case S_IFBLK:
 			error = vfs_mknod(idmap, path.dentry->d_inode,
@@ -4061,10 +4063,6 @@ static int do_mknodat(int dfd, struct filename *name, umode_t mode,
 			break;
 	}
 
-	if (error)
-		goto out2;
-
-	security_path_post_mknod(idmap, dentry);
 out2:
 	done_path_create(&path, dentry);
 	if (retry_estale(error, lookup_flags)) {