linux-ext4 - Re: [PATCH] Remove VLAIS usage from JBD2 code

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Message-ID: <20121030191307.GC19576@blackbox.djwong.org>
Date:	Tue, 30 Oct 2012 12:13:07 -0700
From:	"Darrick J. Wong" <darrick.wong@...cle.com>
To:	Behan Webster <behanw@...verseincode.com>
Cc:	tytso@....edu, linux-ext4@...r.kernel.org,
	Mark Charlebois <charlebm@...il.com>
Subject: Re: [PATCH] Remove VLAIS usage from JBD2 code

On Tue, Oct 30, 2012 at 02:40:04PM -0400, Behan Webster wrote:
> From: Mark Charlebois <charlebm@...il.com>
> 
> The use of variable length arrays in structs (VLAIS) in the Linux Kernel code
> precludes the use of compilers which don't implement VLAIS (for instance the
> Clang compiler). Since ctx is always a 32-bit CRC, hard coding a size of 4
> bytes accomplishes the same thing without the use of VLAIS. This is the same
> technique already employed in fs/ext4/ext4.h
> 
> Signed-off-by: Mark Charlebois <charlebm@...il.com>
> Signed-off-by: Behan Webster <behanw@...verseincode.com>
> ---
>  include/linux/jbd2.h |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/jbd2.h b/include/linux/jbd2.h
> index 3efc43f..efcbdfc 100644
> --- a/include/linux/jbd2.h
> +++ b/include/linux/jbd2.h
> @@ -1308,7 +1308,7 @@ static inline u32 jbd2_chksum(journal_t *journal, u32 crc,
>  {
>  	struct {
>  		struct shash_desc shash;
> -		char ctx[crypto_shash_descsize(journal->j_chksum_driver)];
> +		char ctx[4];

I wonder if this code ought to have a defensive programming check such as
BUG_ON(crypto_shash_descsize(journal->j_chksum_driver) > 4) here just in case
we ever decide to support a checksum function that is bigger than 32 bits?

At this stage of the game I doubt we'll be adding larger checksums to
ext4/jbd2, but I wouldn't want to rely on remembering this detail if we ever
do want to change it.  I guess we could hide the check behind CONFIG_JBD_DEBUG
if we're concerned about slowing down the hot path.

The same comment applies to Ted's patch ("ext4: remove dynamic array size in
ext4_chksum()") back in July.

<shrug> Otherwise,

Acked-by: Darrick J. Wong <darrick.wong@...cle.com>

--D
>  	} desc;
>  	int err;
>  
> -- 
> 1.7.9.5
> 
> --
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