| Message ID | 20201026215321.3894419-1-arnd@kernel.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | seq_file: fix clang warning for NULL pointer arithmetic | expand |
On Mon, Oct 26, 2020 at 10:52:56PM +0100, Arnd Bergmann wrote: > From: Arnd Bergmann <arnd@arndb.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@arndb.de> Reviewed-by: Nathan Chancellor <natechancellor@gmail.com> > --- > 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 >
> 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; Yikes. This is just horrible, why bnot the completely obvious: if (ops->seq_start) { ... return next; } if (*ppos) return NULL; return ppos; /* random cookie */ > static void *single_start(struct seq_file *p, loff_t *pos) > { > - return NULL + (*pos == 0); > + return (void *)(uintptr_t)(*pos == 0); Same here.
On Tue, Oct 27, 2020 at 11:45 AM Christoph Hellwig <hch@infradead.org> wrote: > > > 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; > > Yikes. This is just horrible, why bnot the completely obvious: > > if (ops->seq_start) { > ... > return next; > } > > if (*ppos) > return NULL; > return ppos; /* random cookie */ I was trying to not change the behavior, but I guess we can do better than either the original version mine. Not sure I'd call your version 'obvious' either though, at least it was immediately clear to me that returning an unrelated pointer here is the right thing to do (it works, since it is guaranteed to be neither NULL nor an error pointer and it is never dereferenced, but it's still odd). I'd rather define something like #define SEQ_OPEN_SINGLE (void *)1ul and return that here. I'll send a patch doing that, let me know what you think. Arnd
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)