[PATCH] [RFC] Initialization of unused function parameters

[Alexander Potapenko <glider@google.com>]

Consider the patch below, where 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() correctly handles this and does not use the
uninitialized values, but explicitly initializing them will guarantee
their sanity in the case of future changes to step_into().

The bigger question I want to raise here is whether we want to
discourage passing uninitialized variables to functions in the kernel
altogether. While this is not required by the C standard (see the whole
discussion at [2]), KMSAN can help with detecting cases where uninits
are passed to functions (there is a more aggressive
-fsanitize-memory-param-retval compiler option for that). This is
somewhat similar to -Wmaybe-uninitialized, but works dynamically and has
no false positives.

Would initializing function parameters be a net benefit to the kernel?

[1] https://github.com/ClangBuiltLinux/linux/issues/1648#issuecomment-1146608063
[2] https://github.com/ClangBuiltLinux/linux/issues/1648

Cc: Evgenii Stepanov <eugenis@google.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Marco Elver <elver@google.com>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vitaly Buka <vitalybuka@google.com>
Cc: linux-kernel@vger.kernel.org
Cc: linux-toolchains@vger.kernel.org
Signed-off-by: Alexander Potapenko <glider@google.com>
---
 fs/namei.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index 509657fdf4f56..dde370338f5d6 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2001,8 +2001,8 @@ 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
@@ -3349,8 +3349,8 @@ 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;
 
-- 
2.36.1.476.g0c4daa206d-goog