[PATCH] ext4: extents: Remove unnecessary ‘NULL’ values from ablocks

[PATCH] ext4: extents: Remove unnecessary ‘NULL’ values from ablocks

[Li zeming]

ablocks is assigned first, so it does not need to initialize the
assignment.

Signed-off-by: Li zeming <zeming@nfschina.com>
---
 fs/ext4/extents.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index 4ab96f01a6f31..caace8c3fd3c1 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -1061,7 +1061,7 @@ static int ext4_ext_split(handle_t *handle, struct inode *inode,
 	int i = at, k, m, a;
 	ext4_fsblk_t newblock, oldblock;
 	__le32 border;
-	ext4_fsblk_t *ablocks = NULL; /* array of allocated blocks */
+	ext4_fsblk_t *ablocks; /* array of allocated blocks */
 	gfp_t gfp_flags = GFP_NOFS;
 	int err = 0;
 	size_t ext_size = 0;
-- 
2.18.2

Re: [PATCH] ext4: extents: Remove unnecessary ‘NULL’ values from ablocks

[Theodore Ts'o]

On Tue, Apr 02, 2024 at 10:48:04AM +0800, Li zeming wrote:
> ablocks is assigned first, so it does not need to initialize the
> assignment.

That's technically true, but the compiler is perfectly capable of
optimizing it out.  So it's harmless, and removing it does make the
code a bit more fragile, since it needs to be set so that the cleanup
code doesn't accidentally dereference an uninitialized pointer.

Cheers,

					- Ted

Re: [PATCH] ext4: extents: Remove unnecessary ‘NULL’ values from ablocks

[Andreas Dilger]

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On Apr 1, 2024, at 8:48 PM, Li zeming <zeming@nfschina.com> wrote:
> 
> ablocks is assigned first, so it does not need to initialize the
> assignment.

While it is true that "ablocks" is currently set before use,
this is happening a long way away from the variable declaration
and also "ablocks" is used after the "cleanup:" label error case:

cleanup:
        if (bh) {
                if (buffer_locked(bh))
                        unlock_buffer(bh);
                brelse(bh);
        }

        if (err) {
                /* free all allocated blocks in error case */
                for (i = 0; i < depth; i++) {
                        if (!ablocks[i])
                                continue;
                        ext4_free_blocks(handle, inode, NULL, ablocks[i], 1,
                                         EXT4_FREE_BLOCKS_METADATA);
                }
        }
        kfree(ablocks);

So there is definitely a risk that a code change in the future
would introduce hard-to-debug problems, crashes, or even just
spurious static code analysis warnings.  My recommendation would
be to keep this 1-cycle local variable initialization in place
rather than spend hours or days trying to debug and fix a crash
here in the future.

Cheers, Andreas

> 
> Signed-off-by: Li zeming <zeming@nfschina.com>
> ---
> fs/ext4/extents.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
> index 4ab96f01a6f31..caace8c3fd3c1 100644
> --- a/fs/ext4/extents.c
> +++ b/fs/ext4/extents.c
> @@ -1061,7 +1061,7 @@ static int ext4_ext_split(handle_t *handle, struct inode *inode,
> 	int i = at, k, m, a;
> 	ext4_fsblk_t newblock, oldblock;
> 	__le32 border;
> -	ext4_fsblk_t *ablocks = NULL; /* array of allocated blocks */
> +	ext4_fsblk_t *ablocks; /* array of allocated blocks */
> 	gfp_t gfp_flags = GFP_NOFS;
> 	int err = 0;
> 	size_t ext_size = 0;
> --
> 2.18.2
> 


Cheers, Andreas






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