link_path_walk(): be careful when failing with ENOTDIR

Al Viro · Al Viro · commit 97242f99a013 · 2015-08-01T20:18:38.000-04:00
In RCU mode we might end up with dentry evicted just we check
that it's a directory.  In such case we should return ECHILD
rather than ENOTDIR, so that pathwalk would be retries in non-RCU
mode.

Breakage had been introduced in commit b18825a - prior to that
we were looking at nd-&gt;inode, which had been fetched before
verifying that -&gt;d_seq was still valid.  That form of check
would only be satisfied if at some point the pathname prefix
would indeed have resolved to a non-directory.  The fix consists
of checking -&gt;d_seq after we'd run into a non-directory dentry,
and failing with ECHILD in case of mismatch.

Note that all branches since 3.12 have that problem...

Signed-off-by: Al Viro &lt;viro@zeniv.linux.org.uk&gt;
diff --git a/fs/namei.c b/fs/namei.c
@@ -1954,8 +1954,13 @@ static int link_path_walk(const char *name, struct nameidata *nd)
 				continue;
 			}
 		}
-		if (unlikely(!d_can_lookup(nd->path.dentry)))
+		if (unlikely(!d_can_lookup(nd->path.dentry))) {
+			if (nd->flags & LOOKUP_RCU) {
+				if (unlazy_walk(nd, NULL, 0))
+					return -ECHILD;
+			}
 			return -ENOTDIR;
+		}
 	}
 }
 

Original file line number	Diff line number	Diff line change
`@@ -1954,8 +1954,13 @@ static int link_path_walk(const char name, struct nameidata nd)`
`1954`	`1954`	`continue;`
`1955`	`1955`	`}`
`1956`	`1956`	`}`
`1957`		`- if (unlikely(!d_can_lookup(nd->path.dentry)))`
	`1957`	`+ if (unlikely(!d_can_lookup(nd->path.dentry))) {`
	`1958`	`+ if (nd->flags & LOOKUP_RCU) {`
	`1959`	`+ if (unlazy_walk(nd, NULL, 0))`
	`1960`	`+ return -ECHILD;`
	`1961`	`+ }`
`1958`	`1962`	`return -ENOTDIR;`
	`1963`	`+ }`
`1959`	`1964`	`}`
`1960`	`1965`	`}`
`1961`	`1966`