linux-kernel - [PATCH RFC 6/6] btrfs: LLVMLinux: Remove VLAIS

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Date:	Tue,  2 Sep 2014 15:32:33 -0700
From:	behanw@...verseincode.com
To:	agk@...hat.com, clm@...com, davem@...emloft.net,
	dm-devel@...hat.com, fabf@...net.be, herbert@...dor.apana.org.au,
	jbacik@...com, snitzer@...hat.com
Cc:	akpm@...ux-foundation.org, james.l.morris@...cle.com,
	john.johansen@...onical.com, linux-btrfs@...r.kernel.org,
	linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-raid@...r.kernel.org, linux-security-module@...r.kernel.org,
	neilb@...e.de, serge@...lyn.com, torvalds@...ux-foundation.org,
	Vinícius Tinti <viniciustinti@...il.com>,
	Jan-Simon Möller <dl9pf@....de>,
	Behan Webster <behanw@...verseincode.com>,
	Mark Charlebois <charlebm@...il.com>
Subject: [PATCH RFC 6/6] btrfs: LLVMLinux: Remove VLAIS

From: Vinícius Tinti <viniciustinti@...il.com>

Replaced the use of a Variable Length Array In Struct (VLAIS) with a C99
compliant equivalent. This is the original VLAIS struct.

struct {
	struct shash_desc shash;
	char ctx[crypto_shash_descsize(tfm)];
} desc;

This patch instead allocates the appropriate amount of memory using an char
array.

The new code can be compiled with both gcc and clang.

struct shash_desc contains a flexible array member member ctx declared with
CRYPTO_MINALIGN_ATTR, so sizeof(struct shash_desc) aligns the beginning
of the array declared after struct shash_desc with long long.

No trailing padding is required because it is not a struct type that can
be used in an array.

The CRYPTO_MINALIGN_ATTR is required so that desc is aligned with long long
as would be the case for a struct containing a member with
CRYPTO_MINALIGN_ATTR.

Signed-off-by: Jan-Simon Möller <dl9pf@....de>
Signed-off-by: Behan Webster <behanw@...verseincode.com>
Signed-off-by: Vinícius Tinti <viniciustinti@...il.com>
Signed-off-by: Mark Charlebois <charlebm@...il.com>
---
 fs/btrfs/hash.c | 18 +++++++++---------
 1 file changed, 9 insertions(+), 9 deletions(-)

diff --git a/fs/btrfs/hash.c b/fs/btrfs/hash.c
index 85889aa..2fdacda 100644
--- a/fs/btrfs/hash.c
+++ b/fs/btrfs/hash.c
@@ -33,18 +33,18 @@ void btrfs_hash_exit(void)
 
 u32 btrfs_crc32c(u32 crc, const void *address, unsigned int length)
 {
-	struct {
-		struct shash_desc shash;
-		char ctx[crypto_shash_descsize(tfm)];
-	} desc;
+	char desc[sizeof(struct shash_desc)
+		+ crypto_shash_descsize(tfm)] CRYPTO_MINALIGN_ATTR;
+	struct shash_desc *shash = (struct shash_desc *)desc;
+	u32 *ctx = (u32 *)shash_desc_ctx(shash);
 	int err;
 
-	desc.shash.tfm = tfm;
-	desc.shash.flags = 0;
-	*(u32 *)desc.ctx = crc;
+	shash->tfm = tfm;
+	shash->flags = 0;
+	*ctx = crc;
 
-	err = crypto_shash_update(&desc.shash, address, length);
+	err = crypto_shash_update(shash, address, length);
 	BUG_ON(err);
 
-	return *(u32 *)desc.ctx;
+	return *ctx;
 }
-- 
1.9.1

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