[PATCH v4 43/45] namei: initialize parameters passed to step_into()

[Alexander Potapenko]

Under certain circumstances initialization of `unsigned seq` and
`struct inode *inode` passed into step_into() may be skipped.
In particular, if the call to lookup_fast() in walk_component()
returns NULL, and lookup_slow() returns a valid dentry, then the
`seq` and `inode` will remain uninitialized until the call to
step_into() (see [1] for more info).

Right now step_into() does not use these uninitialized values,
yet passing uninitialized values to functions is considered undefined
behavior (see [2]). To fix that, we initialize `seq` and `inode` at
definition.

[1] https://github.com/ClangBuiltLinux/linux/issues/1648#issuecomment-1146608063
[2] https://lore.kernel.org/linux-toolchains/CAHk-=whjz3wO8zD+itoerphWem+JZz4uS3myf6u1Wd6epGRgmQ@xxxxxxxxxxxxxx/

Cc: Evgenii Stepanov <eugenis@xxxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Marco Elver <elver@xxxxxxxxxx>
Cc: Nathan Chancellor <nathan@xxxxxxxxxx>
Cc: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Segher Boessenkool <segher@xxxxxxxxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Vitaly Buka <vitalybuka@xxxxxxxxxx>
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: linux-toolchains@xxxxxxxxxxxxxxx
Signed-off-by: Alexander Potapenko <glider@xxxxxxxxxx>
---
Link: https://linux-review.googlesource.com/id/I94d4e8cc1f0ecc7174659e9506ce96aaf2201d0a
---
 fs/namei.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index 1f28d3f463c3b..6b39dfd3b41bc 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1995,8 +1995,8 @@ static const char *handle_dots(struct nameidata *nd, int type)
 static const char *walk_component(struct nameidata *nd, int flags)
 {
 	struct dentry *dentry;
-	struct inode *inode;
-	unsigned seq;
+	struct inode *inode = NULL;
+	unsigned seq = 0;
 	/*
 	 * "." and ".." are special - ".." especially so because it has
 	 * to be able to know about the current root directory and
@@ -3393,8 +3393,8 @@ static const char *open_last_lookups(struct nameidata *nd,
 	struct dentry *dir = nd->path.dentry;
 	int open_flag = op->open_flag;
 	bool got_write = false;
-	unsigned seq;
-	struct inode *inode;
+	unsigned seq = 0;
+	struct inode *inode = NULL;
 	struct dentry *dentry;
 	const char *res;
 
-- 
2.37.0.rc0.161.g10f37bed90-goog