linux-kernel - [PATCH] seq_file: fix clang warning for NULL pointer arithmetic

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Message-Id: <20201026215321.3894419-1-arnd@kernel.org>
Date:   Mon, 26 Oct 2020 22:52:56 +0100
From:   Arnd Bergmann <arnd@...nel.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Tejun Heo <tj@...nel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Nathan Chancellor <natechancellor@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Arnd Bergmann <arnd@...db.de>, Amir Goldstein <amir73il@...il.com>,
        Jan Kara <jack@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        clang-built-linux@...glegroups.com
Subject: [PATCH] seq_file: fix clang warning for NULL pointer arithmetic

From: Arnd Bergmann <arnd@...db.de>

Clang points out that adding something to NULL is notallowed
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 function to do the same thing without triggering that
warning. Linux already relies on a specific binary representation
of NULL, so it makes no real difference here. 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")
Signed-off-by: Arnd Bergmann <arnd@...db.de>
---
 fs/kernfs/file.c | 2 +-
 fs/seq_file.c    | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/kernfs/file.c b/fs/kernfs/file.c
index f277d023ebcd..b55e6ef4d677 100644
--- a/fs/kernfs/file.c
+++ b/fs/kernfs/file.c
@@ -124,7 +124,7 @@ static void *kernfs_seq_start(struct seq_file *sf, loff_t *ppos)
 		 * The same behavior and code as single_open().  Returns
 		 * !NULL if pos is at the beginning; otherwise, NULL.
 		 */
-		return NULL + !*ppos;
+		return (void *)(uintptr_t)!*ppos;
 	}
 }
 
diff --git a/fs/seq_file.c b/fs/seq_file.c
index 31219c1db17d..d456468eb934 100644
--- a/fs/seq_file.c
+++ b/fs/seq_file.c
@@ -526,7 +526,7 @@ EXPORT_SYMBOL(seq_dentry);
 
 static void *single_start(struct seq_file *p, loff_t *pos)
 {
-	return NULL + (*pos == 0);
+	return (void *)(uintptr_t)(*pos == 0);
 }
 
 static void *single_next(struct seq_file *p, void *v, loff_t *pos)
-- 
2.27.0