Implement conditional logic in order to replace NULL pointer arithmetic.

The use of NULL pointer arithmetic was pointed out by clang with the
following warning:

fs/kernfs/file.c:128:15: warning: performing pointer arithmetic on a
null pointer has undefined behavior [-Wnull-pointer-arithmetic]
                return NULL + !*ppos;
                       ~~~~ ^
fs/seq_file.c:559:14: warning: performing pointer arithmetic on a
null pointer has undefined behavior [-Wnull-pointer-arithmetic]
        return NULL + (*pos == 0);

Signed-off-by: Maíra Canal <maira.canal@usp.br>
---
V1 -> V2:
- Use SEQ_START_TOKEN instead of open-coding it
- kernfs_seq_start call single_start instead of open-coding it
---
 fs/kernfs/file.c         | 7 +------
 fs/seq_file.c            | 5 +++--
 include/linux/seq_file.h | 1 +
 3 files changed, 5 insertions(+), 8 deletions(-)

diff --git a/fs/kernfs/file.c b/fs/kernfs/file.c
index 9414a7a60a9f..7aefaca876a0 100644
--- a/fs/kernfs/file.c
+++ b/fs/kernfs/file.c
@@ -120,13 +120,8 @@ static void *kernfs_seq_start(struct seq_file *sf, loff_t *ppos)
 		if (next == ERR_PTR(-ENODEV))
 			kernfs_seq_stop_active(sf, next);
 		return next;
-	} else {
-		/*
-		 * The same behavior and code as single_open().  Returns
-		 * !NULL if pos is at the beginning; otherwise, NULL.
-		 */
-		return NULL + !*ppos;
 	}
+	return single_start(sf, ppos);
 }
 
 static void *kernfs_seq_next(struct seq_file *sf, void *v, loff_t *ppos)
diff --git a/fs/seq_file.c b/fs/seq_file.c
index f8e1f4ee87ff..bb2b15d3f1c5 100644
--- a/fs/seq_file.c
+++ b/fs/seq_file.c
@@ -554,10 +554,11 @@ int seq_dentry(struct seq_file *m, struct dentry *dentry, const char *esc)
 }
 EXPORT_SYMBOL(seq_dentry);
 
-static void *single_start(struct seq_file *p, loff_t *pos)
+void *single_start(struct seq_file *p, loff_t *pos)
 {
-	return NULL + (*pos == 0);
+	return *pos ? NULL : SEQ_START_TOKEN;
 }
+EXPORT_SYMBOL(single_start);
 
 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 88cc16444b43..60820ab511d2 100644
--- a/include/linux/seq_file.h
+++ b/include/linux/seq_file.h
@@ -162,6 +162,7 @@ int seq_dentry(struct seq_file *, struct dentry *, const char *);
 int seq_path_root(struct seq_file *m, const struct path *path,
 		  const struct path *root, const char *esc);
 
+void *single_start(struct seq_file *, loff_t *);
 int single_open(struct file *, int (*)(struct seq_file *, void *), void *);
 int single_open_size(struct file *, int (*)(struct seq_file *, void *), void *, size_t);
 int single_release(struct inode *, struct file *);
-- 
2.34.1

On Mon, Jan 31, 2022 at 02:56:13PM -0300, Maíra Canal wrote:
> +EXPORT_SYMBOL(single_start);

kernfs is a 'bool', which means it can't be a module, so there's
no need to EXPORT this symbol.

Em seg., 31 de jan. de 2022 às 14:59, Matthew Wilcox
<willy@infradead.org> escreveu:
>
> On Mon, Jan 31, 2022 at 02:56:13PM -0300, Maíra Canal wrote:
> > +EXPORT_SYMBOL(single_start);
>
> kernfs is a 'bool', which means it can't be a module, so there's
> no need to EXPORT this symbol.

Thank you for the feedback! Still learning...