[PATCH v4 2/3] seq_file: Fix clang warning for NULL pointer arithmetic

[Kees Cook <keescook@xxxxxxxxxxxx>]

From: Arnd Bergmann <arnd@xxxxxxxx>

Clang points out that adding something to NULL is not allowed in
standard C:

fs/kernfs/file.c:127:15: warning: performing pointer arithmetic on a
null pointer has undefined behavior [-Wnull-pointer-arithmetic]
                return NULL + !*ppos;
                       ~~~~ ^
fs/seq_file.c:529:14: warning: performing pointer arithmetic on a
null pointer has undefined behavior [-Wnull-pointer-arithmetic]
        return NULL + (*pos == 0);

Rephrase the code to be extra explicit about the valid, giving them
named SEQ_OPEN_EOF and SEQ_OPEN_SINGLE definitions.  The instance in
kernfs was copied from single_start, so fix both at once.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Fixes: c2b19daf6760 ("sysfs, kernfs: prepare read path for kernfs")
Reviewed-by: Christoph Hellwig <hch@xxxxxx>
Reviewed-by: Nathan Chancellor <natechancellor@xxxxxxxxx>
Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20201028151202.3074398-1-arnd@xxxxxxxxxx
---
 fs/kernfs/file.c         | 9 ++++++---
 fs/seq_file.c            | 5 ++++-
 include/linux/seq_file.h | 6 ++++++
 3 files changed, 16 insertions(+), 4 deletions(-)

diff --git a/fs/kernfs/file.c b/fs/kernfs/file.c
index c75719312147..721bcbc1d4d0 100644
--- a/fs/kernfs/file.c
+++ b/fs/kernfs/file.c
@@ -122,10 +122,13 @@ static void *kernfs_seq_start(struct seq_file *sf, loff_t *ppos)
 		return next;
 	} else {
 		/*
-		 * The same behavior and code as single_open().  Returns
-		 * !NULL if pos is at the beginning; otherwise, NULL.
+		 * The same behavior and code as single_open().  Continues
+		 * if pos is at the beginning; otherwise, NULL.
 		 */
-		return NULL + !*ppos;
+		if (*ppos)
+			return NULL;
+
+		return SEQ_OPEN_SINGLE;
 	}
 }
 
diff --git a/fs/seq_file.c b/fs/seq_file.c
index cb11a34fb871..1b5bd95d0a48 100644
--- a/fs/seq_file.c
+++ b/fs/seq_file.c
@@ -542,7 +542,10 @@ EXPORT_SYMBOL(seq_dentry);
 
 static void *single_start(struct seq_file *p, loff_t *pos)
 {
-	return NULL + (*pos == 0);
+	if (*pos)
+		return NULL;
+
+	return SEQ_OPEN_SINGLE;
 }
 
 static void *single_next(struct seq_file *p, void *v, loff_t *pos)
diff --git a/include/linux/seq_file.h b/include/linux/seq_file.h
index b83b3ae3c877..51c870765bfd 100644
--- a/include/linux/seq_file.h
+++ b/include/linux/seq_file.h
@@ -37,6 +37,12 @@ struct seq_operations {
 
 #define SEQ_SKIP 1
 
+/*
+ * op->start must return a non-NULL pointer for single_open(),
+ * this is used when we don't care about the specific value.
+ */
+#define SEQ_OPEN_SINGLE ((void *)1)
+
 /**
  * seq_has_overflowed - check if the buffer has overflowed
  * @m: the seq_file handle
-- 
2.25.1