linux-kernel - Re: [PATCH 01/19] VFS: introduce vfs_mkdir

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Message-ID: <20250207194538.GE1977892@ZenIV>
Date: Fri, 7 Feb 2025 19:45:38 +0000
From: Al Viro <viro@...iv.linux.org.uk>
To: NeilBrown <neilb@...e.de>
Cc: Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jeff Layton <jlayton@...nel.org>,
	Dave Chinner <david@...morbit.com>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 01/19] VFS: introduce vfs_mkdir_return()

On Thu, Feb 06, 2025 at 04:42:38PM +1100, NeilBrown wrote:
> vfs_mkdir() does not guarantee to make the child dentry positive on
> success.  It may leave it negative and then the caller needs to perform a
> lookup to find the target dentry.
> 
> This patch introduced vfs_mkdir_return() which performs the lookup if
> needed so that this code is centralised.
> 
> This prepares for a new inode operation which will perform mkdir and
> returns the correct dentry.

* Calling conventions stink; make it _consume_ dentry reference and
return dentry reference or ERR_PTR().  Callers will be happier that way
(check it).

* Calling conventions should be documented in commit message *and* in
D/f/porting

* devpts, nfs4recover and xfs might as well convert (not going to hit
the "need a lookup" case anyway)

* that 
+                       /* Need a "const" pointer.  We know d_name is const
+                        * because we hold an exclusive lock on i_rwsem
+                        * in d_parent.
+                        */
+                       const struct qstr *d_name = (void*)&dentry->d_name;
+                       d = lookup_dcache(d_name, dentry->d_parent, 0);
+                       if (!d)
+                               d = __lookup_slow(d_name, dentry->d_parent, 0);
doesn't need a cast.  C is perfectly fine with
	T *x = foo();
	const T *y = x;

You are not allowed to _strip_ qualifiers; adding them is fine.
Same reason why you are allowed to pass char * to strlen() without
any casts whatsoever.

Comment re stability is fine; the cast is pure WTF material.