[PATCH] Removing the use of VLAIS from the Linux Kernel

[PATCH] Removing the use of VLAIS from the Linux Kernel

[Behan Webster]

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). The LLVMLinux Project is working towards the ability of
providing the Linux kernel developer the choice of using the Clang compiler
toolchain. This is a part of a series of patches which remove the use of VLAIS
from crypto code, dm-crypt, jbd2, libcrc32c, netfilter, and usb gadget. Other
patches to allow Clang to be used will follow.

Jan-Simon Möller (1):
  Remove VLAIS usage from dm-crypt

 drivers/md/dm-crypt.c |   22 +++++++++++-----------
 1 file changed, 11 insertions(+), 11 deletions(-)

-- 
1.7.9.5

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[PATCH] Remove VLAIS usage from dm-crypt

[Behan Webster]

From: Jan-Simon Möller <dl9pf@gmx.de>

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

Patch from series at
http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20120507/142707.html
by PaX Team.

Signed-off-by: Jan-Simon Möller <dl9pf@gmx.de>
Cc: pageexec@freemail.hu
Signed-off-by: Behan Webster <behanw@converseincode.com>
---
 drivers/md/dm-crypt.c |   22 +++++++++++-----------
 1 file changed, 11 insertions(+), 11 deletions(-)

diff --git a/drivers/md/dm-crypt.c b/drivers/md/dm-crypt.c
index bbf459b..8b724ea 100644
--- a/drivers/md/dm-crypt.c
+++ b/drivers/md/dm-crypt.c
@@ -525,29 +525,29 @@ static int crypt_iv_lmk_one(struct crypt_config *cc, u8 *iv,
 			    u8 *data)
 {
 	struct iv_lmk_private *lmk = &cc->iv_gen_private.lmk;
-	struct {
-		struct shash_desc desc;
-		char ctx[crypto_shash_descsize(lmk->hash_tfm)];
-	} sdesc;
+	char sdesc[sizeof(struct shash_desc)
+		+ crypto_shash_descsize(lmk->hash_tfm)
+		+ CRYPTO_MINALIGN] CRYPTO_MINALIGN_ATTR;
+	struct shash_desc *desc = (struct shash_desc *)sdesc;
 	struct md5_state md5state;
 	u32 buf[4];
 	int i, r;
 
-	sdesc.desc.tfm = lmk->hash_tfm;
-	sdesc.desc.flags = CRYPTO_TFM_REQ_MAY_SLEEP;
+	desc->tfm = lmk->hash_tfm;
+	desc->flags = CRYPTO_TFM_REQ_MAY_SLEEP;
 
-	r = crypto_shash_init(&sdesc.desc);
+	r = crypto_shash_init(desc);
 	if (r)
 		return r;
 
 	if (lmk->seed) {
-		r = crypto_shash_update(&sdesc.desc, lmk->seed, LMK_SEED_SIZE);
+		r = crypto_shash_update(desc, lmk->seed, LMK_SEED_SIZE);
 		if (r)
 			return r;
 	}
 
 	/* Sector is always 512B, block size 16, add data of blocks 1-31 */
-	r = crypto_shash_update(&sdesc.desc, data + 16, 16 * 31);
+	r = crypto_shash_update(desc, data + 16, 16 * 31);
 	if (r)
 		return r;
 
@@ -556,12 +556,12 @@ static int crypt_iv_lmk_one(struct crypt_config *cc, u8 *iv,
 	buf[1] = cpu_to_le32((((u64)dmreq->iv_sector >> 32) & 0x00FFFFFF) | 0x80000000);
 	buf[2] = cpu_to_le32(4024);
 	buf[3] = 0;
-	r = crypto_shash_update(&sdesc.desc, (u8 *)buf, sizeof(buf));
+	r = crypto_shash_update(desc, (u8 *)buf, sizeof(buf));
 	if (r)
 		return r;
 
 	/* No MD5 padding here */
-	r = crypto_shash_export(&sdesc.desc, &md5state);
+	r = crypto_shash_export(desc, &md5state);
 	if (r)
 		return r;
 
-- 
1.7.9.5

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Re: [PATCH] Remove VLAIS usage from dm-crypt

[Milan Broz]

On 10/30/2012 07:15 PM, Behan Webster wrote:
> From: Jan-Simon Möller <dl9pf@gmx.de>
> 
> 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). This patch instead allocates the appropriate amount of memory
> using an char array.

The dmcrypt code should use the same code practices as crypto API.

Apparently, your approach was not accepted there

http://article.gmane.org/gmane.linux.kernel/1386451
http://article.gmane.org/gmane.linux.kernel.cryptoapi/7993

> +	char sdesc[sizeof(struct shash_desc)
> +		+ crypto_shash_descsize(lmk->hash_tfm)
> +		+ CRYPTO_MINALIGN] CRYPTO_MINALIGN_ATTR;
> +	struct shash_desc *desc = (struct shash_desc *)sdesc;

I would like to see kenrel compilable by Clang but obfuscating the code
this way is perhaps not the ideal way.

Until it is accepted in crypto layer, NACK.

Milan
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