[PATCH] btrfs: Use sizeof directly instead of a constant variable

[PATCH] btrfs: Use sizeof directly instead of a constant variable

[Nikolay Borisov]

The kernel would like to have all stack VLA usage removed[1].
Unfortunately using an integer constant variable as the size of an
array is still considered a VLA. Instead let's use directly sizeof(var)
which removes the VLA usage. Use the occasion to remove csum_size
altogether and use sizeof() also for the size passed to memcmp

[1]: https://lkml.org/lkml/2018/3/7/621

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
---
 fs/btrfs/disk-io.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 3809d6d66f6a..6247162e334a 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -403,8 +403,7 @@ static int btrfs_check_super_csum(struct btrfs_fs_info *fs_info,
 
 	if (csum_type == BTRFS_CSUM_TYPE_CRC32) {
 		u32 crc = ~(u32)0;
-		const int csum_size = sizeof(crc);
-		char result[csum_size];
+		char result[sizeof(crc)];
 
 		/*
 		 * The super_block structure does not span the whole
@@ -415,7 +414,7 @@ static int btrfs_check_super_csum(struct btrfs_fs_info *fs_info,
 				crc, BTRFS_SUPER_INFO_SIZE - BTRFS_CSUM_SIZE);
 		btrfs_csum_final(crc, result);
 
-		if (memcmp(raw_disk_sb, result, csum_size))
+		if (memcmp(raw_disk_sb, result, sizeof(crc)))
 			ret = 1;
 	}
 
-- 
2.7.4

Re: [PATCH] btrfs: Use sizeof directly instead of a constant variable

[David Sterba]

On Mon, Mar 12, 2018 at 06:52:28PM +0200, Nikolay Borisov wrote:
> The kernel would like to have all stack VLA usage removed[1].
> Unfortunately using an integer constant variable as the size of an
> array is still considered a VLA. Instead let's use directly sizeof(var)
> which removes the VLA usage. Use the occasion to remove csum_size
> altogether and use sizeof() also for the size passed to memcmp

The point of csum_size here is to define once, use everywhere, so we
don't have to opencode sizeof. To avoid the 'not really a VLA', we can
rewrite that.

> 
> [1]: https://lkml.org/lkml/2018/3/7/621
> 
> Signed-off-by: Nikolay Borisov <nborisov@suse.com>
> ---
>  fs/btrfs/disk-io.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
> index 3809d6d66f6a..6247162e334a 100644
> --- a/fs/btrfs/disk-io.c
> +++ b/fs/btrfs/disk-io.c
> @@ -403,8 +403,7 @@ static int btrfs_check_super_csum(struct btrfs_fs_info *fs_info,
>  
>  	if (csum_type == BTRFS_CSUM_TYPE_CRC32) {
>  		u32 crc = ~(u32)0;
> -		const int csum_size = sizeof(crc);
> -		char result[csum_size];
> +		char result[sizeof(crc)];
>  
>  		/*
>  		 * The super_block structure does not span the whole
> @@ -415,7 +414,7 @@ static int btrfs_check_super_csum(struct btrfs_fs_info *fs_info,
>  				crc, BTRFS_SUPER_INFO_SIZE - BTRFS_CSUM_SIZE);
>  		btrfs_csum_final(crc, result);
>  
> -		if (memcmp(raw_disk_sb, result, csum_size))
> +		if (memcmp(raw_disk_sb, result, sizeof(crc)))

sizeof(result) would be better here as it's clear what we're going to
memcpy, while 'crc' is define somewhere above.

>  			ret = 1;
>  	}
>  
> -- 
> 2.7.4
> 
> --
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[PATCH v2] btrfs: Use sizeof directly instead of a constant variable

[Nikolay Borisov]

The kernel would like to have all stack VLA usage removed[1].
Unfortunately using an integer constant variable as the size of an
array is still considered a VLA. Instead let's use directly sizeof(var)
which removes the VLA usage. Use the occasion to remove csum_size
altogether and use sizeof() also for the size passed to memcmp

[1]: https://lkml.org/lkml/2018/3/7/621

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
---

V2: 
 * Use sizeof(result) in memcmp to make it more clear what we are comparing. 
 fs/btrfs/disk-io.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 3809d6d66f6a..04834cb65272 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -403,8 +403,7 @@ static int btrfs_check_super_csum(struct btrfs_fs_info *fs_info,
 
 	if (csum_type == BTRFS_CSUM_TYPE_CRC32) {
 		u32 crc = ~(u32)0;
-		const int csum_size = sizeof(crc);
-		char result[csum_size];
+		char result[sizeof(crc)];
 
 		/*
 		 * The super_block structure does not span the whole
@@ -415,7 +414,7 @@ static int btrfs_check_super_csum(struct btrfs_fs_info *fs_info,
 				crc, BTRFS_SUPER_INFO_SIZE - BTRFS_CSUM_SIZE);
 		btrfs_csum_final(crc, result);
 
-		if (memcmp(raw_disk_sb, result, csum_size))
+		if (memcmp(raw_disk_sb, result, sizeof(result)))
 			ret = 1;
 	}
 
-- 
2.7.4

Re: [PATCH v2] btrfs: Use sizeof directly instead of a constant variable

[David Sterba]

On Tue, Mar 13, 2018 at 10:26:06AM +0200, Nikolay Borisov wrote:
> The kernel would like to have all stack VLA usage removed[1].
> Unfortunately using an integer constant variable as the size of an
> array is still considered a VLA. Instead let's use directly sizeof(var)
> which removes the VLA usage. Use the occasion to remove csum_size
> altogether and use sizeof() also for the size passed to memcmp
> 
> [1]: https://lkml.org/lkml/2018/3/7/621
> 
> Signed-off-by: Nikolay Borisov <nborisov@suse.com>
> ---
> 
> V2: 
>  * Use sizeof(result) in memcmp to make it more clear what we are comparing. 

Reviewed-by: David Sterba <dsterba@suse.com>

Added to next, thanks.