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Subject[PATCH v2] seq_file: fix NULL pointer arithmetic warning
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
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