clang warning: fs/ext4/mballoc.c, line 4178, column 6
Branch condition evaluates to a garbage value.

err is uninitialized and will be judged when it enters the
loop first time and the condition "!ext4_sb_block_valid()"
is true. Although this can't make problems now, it's better
to correct it.

Signed-off-by: Su Hui <suhui@nfschina.com>
---
 fs/ext4/mballoc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 21b903fe546e..769000c970b0 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -4084,7 +4084,7 @@ void ext4_mb_mark_bb(struct super_block *sb, ext4_fsblk_t block,
 	struct ext4_sb_info *sbi = EXT4_SB(sb);
 	ext4_group_t group;
 	ext4_grpblk_t blkoff;
-	int i, err;
+	int i, err = 0;
 	int already;
 	unsigned int clen, clen_changed, thisgrp_len;
 
-- 
2.30.2

On Sun, Jul 23, 2023 at 10:46 PM Su Hui <suhui@nfschina.com> wrote:
>
> clang warning: fs/ext4/mballoc.c, line 4178, column 6
> Branch condition evaluates to a garbage value.

Specifically this is clang's static analysis; not the compiler itself.

>
> err is uninitialized and will be judged when it enters the
> loop first time and the condition "!ext4_sb_block_valid()"
> is true. Although this can't make problems now, it's better
> to correct it.
>
> Signed-off-by: Su Hui <suhui@nfschina.com>

Hi Su,
Thanks for the patch!  I see what the warning is getting at;

If `len <= 0` then `err` is never initialized, then is used at line
4178 (that is UB).

Would you mind sending a v2 with the commit message updated to reflect
the above points?  I'd be happy to sign-off on that.

> ---
>  fs/ext4/mballoc.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index 21b903fe546e..769000c970b0 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -4084,7 +4084,7 @@ void ext4_mb_mark_bb(struct super_block *sb, ext4_fsblk_t block,
>         struct ext4_sb_info *sbi = EXT4_SB(sb);
>         ext4_group_t group;
>         ext4_grpblk_t blkoff;
> -       int i, err;
> +       int i, err = 0;
>         int already;
>         unsigned int clen, clen_changed, thisgrp_len;
>
> --
> 2.30.2
>
>


-- 
Thanks,
~Nick Desaulniers

On Mon, Jul 24, 2023 at 10:19:02AM -0700, Nick Desaulniers wrote:
> On Sun, Jul 23, 2023 at 10:46 PM Su Hui <suhui@nfschina.com> wrote:
> >
> > clang warning: fs/ext4/mballoc.c, line 4178, column 6
> > Branch condition evaluates to a garbage value.
> 
> Specifically this is clang's static analysis; not the compiler itself.
> 
> >
> > err is uninitialized and will be judged when it enters the
> > loop first time and the condition "!ext4_sb_block_valid()"
> > is true. Although this can't make problems now, it's better
> > to correct it.
> >
> > Signed-off-by: Su Hui <suhui@nfschina.com>
> 
> Hi Su,
> Thanks for the patch!  I see what the warning is getting at;
> 
> If `len <= 0` then `err` is never initialized, then is used at line
> 4178 (that is UB).
> 
> Would you mind sending a v2 with the commit message updated to reflect
> the above points?  I'd be happy to sign-off on that.

The commit message talks about !ext4_sb_block_valid() on the first
iteration through the loop and not that len <= 0.

But either way, brelse(NULL) is a no-op.

regards,
dan carpenter

On Mon, Jul 24, 2023 at 10:19:02AM -0700, Nick Desaulniers wrote:
> >
> > err is uninitialized and will be judged when it enters the
> > loop first time and the condition "!ext4_sb_block_valid()"
> > is true. Although this can't make problems now, it's better
> > to correct it.
> >
> > Signed-off-by: Su Hui <suhui@nfschina.com>
> 
> Hi Su,
> Thanks for the patch!  I see what the warning is getting at;
> 
> If `len <= 0` then `err` is never initialized, then is used at line
> 4178 (that is UB).
> 
> Would you mind sending a v2 with the commit message updated to reflect
> the above points?  I'd be happy to sign-off on that.

Fortunately, as near as I can tell, ext4_mb_mark_bb() should never be
called with len <= 0.  It might be possible to trick ext4 via a
corrupted file system --- I'd have to take a closer look at that, but
fortunately, in the case where len <= 0, bitmap_bh will be NULL, so
regardless of whether err is 0, or some garbage non-zero value,
brelse(NULL) is a no-op.

So while it's good to avoid the clang warning, but the fact that it
might be possible for err to be a "garbage value" shouldn't be causing
any problem.

Cheers,

						- Ted

On 2023/7/25 04:57, Theodore Ts'o wrote:
> On Mon, Jul 24, 2023 at 10:19:02AM -0700, Nick Desaulniers wrote:
>>> err is uninitialized and will be judged when it enters the
>>> loop first time and the condition "!ext4_sb_block_valid()"
>>> is true. Although this can't make problems now, it's better
>>> to correct it.
>>>
>>> Signed-off-by: Su Hui <suhui@nfschina.com>
>> Hi Su,
>> Thanks for the patch!  I see what the warning is getting at;
>>
>> If `len <= 0` then `err` is never initialized, then is used at line
>> 4178 (that is UB).
>>
>> Would you mind sending a v2 with the commit message updated to reflect
>> the above points?  I'd be happy to sign-off on that.
> Fortunately, as near as I can tell, ext4_mb_mark_bb() should never be
> called with len <= 0.  It might be possible to trick ext4 via a
> corrupted file system --- I'd have to take a closer look at that, but
> fortunately, in the case where len <= 0, bitmap_bh will be NULL, so
> regardless of whether err is 0, or some garbage non-zero value,
> brelse(NULL) is a no-op.
> So while it's good to avoid the clang warning, but the fact that it
> might be possible for err to be a "garbage value" shouldn't be causing
> any problem.
Maybe it can make the code more robust and clearer. I will send v2 patch 
soon.
Thanks for your reply!

Su Hui

>
> Cheers,
>
> 						- Ted