From: Arnd Bergmann arnd@arndb.de
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@lst.de Reviewed-by: Nathan Chancellor natechancellor@gmail.com Signed-off-by: Arnd Bergmann arnd@arndb.de Signed-off-by: Kees Cook keescook@chromium.org Link: https://lore.kernel.org/r/20201028151202.3074398-1-arnd@kernel.org --- 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