[PATCH] Btrfs: fix use of uninitialized err variable

[PATCH] Btrfs: fix use of uninitialized err variable

[Filipe David Borba Manana]

>From the compiler:

fs/btrfs/file.c: In function ‘prepare_pages.isra.18’:
fs/btrfs/file.c:1265:6: warning: ‘err’ may be used uninitialized in this function [-Wuninitialized]

Signed-off-by: Filipe David Borba Manana <fdmanana@gmail.com>
---
 fs/btrfs/file.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index 740ae8c..35bf838 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -1244,7 +1244,7 @@ static noinline int prepare_pages(struct inode *inode, struct page **pages,
 	int i;
 	unsigned long index = pos >> PAGE_CACHE_SHIFT;
 	gfp_t mask = btrfs_alloc_write_mask(inode->i_mapping);
-	int err;
+	int err = 0;
 	int faili;
 
 	for (i = 0; i < num_pages; i++) {
-- 
1.7.9.5

Re: [PATCH] Btrfs: fix use of uninitialized err variable

[David Sterba]

On Fri, Dec 13, 2013 at 07:39:34PM +0000, Filipe David Borba Manana wrote:
> >From the compiler:
> 
> fs/btrfs/file.c: In function ‘prepare_pages.isra.18’:
> fs/btrfs/file.c:1265:6: warning: ‘err’ may be used uninitialized in this function [-Wuninitialized]

My gcc 4.8.1 does not see this warning, nor do I while inspecting the
souces in current next-master.

Re: [PATCH] Btrfs: fix use of uninitialized err variable

[Filipe David Manana]

On Mon, Dec 16, 2013 at 2:34 PM, David Sterba <dsterba@suse.cz> wrote:
> On Fri, Dec 13, 2013 at 07:39:34PM +0000, Filipe David Borba Manana wrote:
>> >From the compiler:
>>
>> fs/btrfs/file.c: In function ‘prepare_pages.isra.18’:
>> fs/btrfs/file.c:1265:6: warning: ‘err’ may be used uninitialized in this function [-Wuninitialized]
>
> My gcc 4.8.1 does not see this warning, nor do I while inspecting the
> souces in current next-master.

Here it's gcc 4.6.3.



-- 
Filipe David Manana,

"Reasonable men adapt themselves to the world.
 Unreasonable men adapt the world to themselves.
 That's why all progress depends on unreasonable men."

Re: [PATCH] Btrfs: fix use of uninitialized err variable

[David Sterba]

On Mon, Dec 16, 2013 at 05:03:25PM +0000, Filipe David Manana wrote:
> On Mon, Dec 16, 2013 at 2:34 PM, David Sterba <dsterba@suse.cz> wrote:
> > On Fri, Dec 13, 2013 at 07:39:34PM +0000, Filipe David Borba Manana wrote:
> >> >From the compiler:
> >>
> >> fs/btrfs/file.c: In function ‘prepare_pages.isra.18’:
> >> fs/btrfs/file.c:1265:6: warning: ‘err’ may be used uninitialized in this function [-Wuninitialized]
> >
> > My gcc 4.8.1 does not see this warning, nor do I while inspecting the
> > souces in current next-master.
> 
> Here it's gcc 4.6.3.

I've seen that some versions of gcc produce bogus warnings of that sort
and manual review is needed, but I haven't found a code path that would
lead to uninitialized use of err.

The warning points to

1259                 if (i == 0)
1260                         err = prepare_uptodate_page(pages[i], pos,
1261                                                     force_uptodate);
1262                 if (i == num_pages - 1)
1263                         err = prepare_uptodate_page(pages[i],
1264                                                     pos + write_bytes, false);
1265                 if (err) {
^^^^

1266                         page_cache_release(pages[i]);
1267                         faili = i - 1;
1268                         goto fail;
1269                 }

But the loop starts from i = 0 and the variable is initialized before
the check. So ti's gcc that does not see that, not a real error.

Re: [PATCH] Btrfs: fix use of uninitialized err variable

[Filipe David Manana]

On Tue, Dec 17, 2013 at 3:27 PM, David Sterba <dsterba@suse.cz> wrote:
> On Mon, Dec 16, 2013 at 05:03:25PM +0000, Filipe David Manana wrote:
>> On Mon, Dec 16, 2013 at 2:34 PM, David Sterba <dsterba@suse.cz> wrote:
>> > On Fri, Dec 13, 2013 at 07:39:34PM +0000, Filipe David Borba Manana wrote:
>> >> >From the compiler:
>> >>
>> >> fs/btrfs/file.c: In function ‘prepare_pages.isra.18’:
>> >> fs/btrfs/file.c:1265:6: warning: ‘err’ may be used uninitialized in this function [-Wuninitialized]
>> >
>> > My gcc 4.8.1 does not see this warning, nor do I while inspecting the
>> > souces in current next-master.
>>
>> Here it's gcc 4.6.3.
>
> I've seen that some versions of gcc produce bogus warnings of that sort
> and manual review is needed, but I haven't found a code path that would
> lead to uninitialized use of err.
>
> The warning points to
>
> 1259                 if (i == 0)
> 1260                         err = prepare_uptodate_page(pages[i], pos,
> 1261                                                     force_uptodate);
> 1262                 if (i == num_pages - 1)
> 1263                         err = prepare_uptodate_page(pages[i],
> 1264                                                     pos + write_bytes, false);
> 1265                 if (err) {
> ^^^^
>
> 1266                         page_cache_release(pages[i]);
> 1267                         faili = i - 1;
> 1268                         goto fail;
> 1269                 }
>
> But the loop starts from i = 0 and the variable is initialized before
> the check. So ti's gcc that does not see that, not a real error.

Right, my intention was to silence a compiler warning. Should have
made it more explicit in the commit message title.




-- 
Filipe David Manana,

"Reasonable men adapt themselves to the world.
 Unreasonable men adapt the world to themselves.
 That's why all progress depends on unreasonable men."