Re: [PATCH] lib/bch: Remove VLA usage

[Ivan Djelic <ivan.djelic@parrot.com>]

On Tue, May 29, 2018 at 03:42:07PM -0700, Kees Cook wrote:
> In the quest to remove all stack VLA usage from the kernel[1], this removes
> the on-stack working buffers in favor of pre-allocated working buffers
> (which were already used in other places). Since these routines must
> already be serialized (since they work on bch->ecc_buf), adding the usage
> of bch->ecc_work would be similarly safe. Additionally, since "max m" is
> only 15, this was adjusted to just use a fixed size array in those cases.

Hi Kees,

Using an on-stack buffer instead of a pre-allocated buffer was done initially
for performance reasons.  For "usual" (m,t) values (for instance m=13, t=4),
there is a huge performance difference between the on-stack buffer version and
the kmalloc version. I didn't investigate the reason for this, but I ran a
quick benchmark on my PC:

little-endian, type sizes: int=4 long=8 longlong=8
cpu: Intel(R) Core(TM) i5 CPU         650  @ 3.20GHz
calibration: iter=4.9143µs niter=2034 nsamples=200 m=13 t=4

  Buffer allocation |  Encoding throughput (Mbit/s)
---------------------------------------------------
 on-stack, VLA      |   3988
 on-stack, fixed    |   4494
 kmalloc            |   1967

The first line shows the performance of the current code, using a VLA.
The second line shows the performance when r[] is allocated on the stack with
a fixed, constant size (the maximum allowed value).
The third line shows the performance when r is a pre-allocated working buffer.

In fact, when using a pre-allocated buffer there is no need to introduce 'ecc_work':
you can directly point 'r' to bch->ecc_buf and remove memcpy() surrounding the
'while (mlen--)' loop. Everything happens inside the 'bch->ecc_buf' buffer.
But with a big performance penalty. Looks like declaring a temporary buffer on the
stack to store ECC values allows GCC to do a better job at optimizing the loop.

So rather than introducing 'ecc_work', I suggest we compute the maximum allowed
size for r[] and use that:

sizeof(r) = sizeof(uint32_t)*(l+1)
l+1 = BCH_ECC_WORDS(bch) = DIV_ROUND_UP(m*t, 32)

We also know that:

m*t < 2^m - 1 (ECC maximum size)

therefore:

l+1 < DIV_ROUND_UP(2^m - 1, 32) < 2^(m-5)

So instead of 'uint32_t r[l+1]' we could declare 'uint32_t r[1 << (BCH_MAX_M-5)]'.
And replace 'sizeof(r)' with 'sizeof(*bch->ecc_buf)*(l+1)' in memset/memcpy calls.
In practice the actual maximum size of r[] is (1 << (15-5))*sizeof(uint32_t) = 4096 bytes.

What do you think ?
--
Ivan
 




> [1] https://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=qPXydAacU1RqZWA@mail.gmail.com
> 
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
> This is directed at linux-mtd because it's the only user of this library
> and it's how it originally entered the kernel tree...
> ---
>  include/linux/bch.h |  4 ++--
>  lib/bch.c           | 27 +++++++++++++++------------
>  2 files changed, 17 insertions(+), 14 deletions(-)
> 
> diff --git a/include/linux/bch.h b/include/linux/bch.h
> index 295b4ef153bb..4d46e6a73319 100644
> --- a/include/linux/bch.h
> +++ b/include/linux/bch.h
> @@ -39,7 +39,7 @@
>   * @a_log_tab:  Galois field GF(2^m) log lookup table
>   * @mod8_tab:   remainder generator polynomial lookup tables
>   * @ecc_buf:    ecc parity words buffer
> - * @ecc_buf2:   ecc parity words buffer
> + * @ecc_work:   ecc parity words working buffer
>   * @xi_tab:     GF(2^m) base for solving degree 2 polynomial roots
>   * @syn:        syndrome buffer
>   * @cache:      log-based polynomial representation buffer
> @@ -57,7 +57,7 @@ struct bch_control {
>  	uint16_t       *a_log_tab;
>  	uint32_t       *mod8_tab;
>  	uint32_t       *ecc_buf;
> -	uint32_t       *ecc_buf2;
> +	uint32_t       *ecc_work;
>  	unsigned int   *xi_tab;
>  	unsigned int   *syn;
>  	int            *cache;
> diff --git a/lib/bch.c b/lib/bch.c
> index bc89dfe4d1b3..f14eac93ecc4 100644
> --- a/lib/bch.c
> +++ b/lib/bch.c
> @@ -78,10 +78,12 @@
>  #define GF_M(_p)               (CONFIG_BCH_CONST_M)
>  #define GF_T(_p)               (CONFIG_BCH_CONST_T)
>  #define GF_N(_p)               ((1 << (CONFIG_BCH_CONST_M))-1)
> +#define BCH_MAX_M              (CONFIG_BCH_CONST_M)
>  #else
>  #define GF_M(_p)               ((_p)->m)
>  #define GF_T(_p)               ((_p)->t)
>  #define GF_N(_p)               ((_p)->n)
> +#define BCH_MAX_M              15
>  #endif
>  
>  #define BCH_ECC_WORDS(_p)      DIV_ROUND_UP(GF_M(_p)*GF_T(_p), 32)
> @@ -187,7 +189,7 @@ void encode_bch(struct bch_control *bch, const uint8_t *data,
>  	const unsigned int l = BCH_ECC_WORDS(bch)-1;
>  	unsigned int i, mlen;
>  	unsigned long m;
> -	uint32_t w, r[l+1];
> +	uint32_t w;
>  	const uint32_t * const tab0 = bch->mod8_tab;
>  	const uint32_t * const tab1 = tab0 + 256*(l+1);
>  	const uint32_t * const tab2 = tab1 + 256*(l+1);
> @@ -198,7 +200,7 @@ void encode_bch(struct bch_control *bch, const uint8_t *data,
>  		/* load ecc parity bytes into internal 32-bit buffer */
>  		load_ecc8(bch, bch->ecc_buf, ecc);
>  	} else {
> -		memset(bch->ecc_buf, 0, sizeof(r));
> +		memset(bch->ecc_work, 0, bch->ecc_bytes);
>  	}
>  
>  	/* process first unaligned data bytes */
> @@ -215,7 +217,7 @@ void encode_bch(struct bch_control *bch, const uint8_t *data,
>  	mlen  = len/4;
>  	data += 4*mlen;
>  	len  -= 4*mlen;
> -	memcpy(r, bch->ecc_buf, sizeof(r));
> +	memcpy(bch->ecc_work, bch->ecc_buf, bch->ecc_bytes);
>  
>  	/*
>  	 * split each 32-bit word into 4 polynomials of weight 8 as follows:
> @@ -229,6 +231,8 @@ void encode_bch(struct bch_control *bch, const uint8_t *data,
>  	 * xxxxxxxx  yyyyyyyy  zzzzzzzz  tttttttt  mod g = r0^r1^r2^r3
>  	 */
>  	while (mlen--) {
> +		uint32_t *r = bch->ecc_work;
> +
>  		/* input data is read in big-endian format */
>  		w = r[0]^cpu_to_be32(*pdata++);
>  		p0 = tab0 + (l+1)*((w >>  0) & 0xff);
> @@ -241,7 +245,7 @@ void encode_bch(struct bch_control *bch, const uint8_t *data,
>  
>  		r[l] = p0[l]^p1[l]^p2[l]^p3[l];
>  	}
> -	memcpy(bch->ecc_buf, r, sizeof(r));
> +	memcpy(bch->ecc_buf, bch->ecc_work, bch->ecc_bytes);
>  
>  	/* process last unaligned bytes */
>  	if (len)
> @@ -434,7 +438,7 @@ static int solve_linear_system(struct bch_control *bch, unsigned int *rows,
>  {
>  	const int m = GF_M(bch);
>  	unsigned int tmp, mask;
> -	int rem, c, r, p, k, param[m];
> +	int rem, c, r, p, k, param[BCH_MAX_M];
>  
>  	k = 0;
>  	mask = 1 << m;
> @@ -1009,10 +1013,10 @@ int decode_bch(struct bch_control *bch, const uint8_t *data, unsigned int len,
>  		}
>  		/* load received ecc or assume it was XORed in calc_ecc */
>  		if (recv_ecc) {
> -			load_ecc8(bch, bch->ecc_buf2, recv_ecc);
> +			load_ecc8(bch, bch->ecc_work, recv_ecc);
>  			/* XOR received and calculated ecc */
>  			for (i = 0, sum = 0; i < (int)ecc_words; i++) {
> -				bch->ecc_buf[i] ^= bch->ecc_buf2[i];
> +				bch->ecc_buf[i] ^= bch->ecc_work[i];
>  				sum |= bch->ecc_buf[i];
>  			}
>  			if (!sum)
> @@ -1114,7 +1118,7 @@ static int build_deg2_base(struct bch_control *bch)
>  {
>  	const int m = GF_M(bch);
>  	int i, j, r;
> -	unsigned int sum, x, y, remaining, ak = 0, xi[m];
> +	unsigned int sum, x, y, remaining, ak = 0, xi[BCH_MAX_M];
>  
>  	/* find k s.t. Tr(a^k) = 1 and 0 <= k < m */
>  	for (i = 0; i < m; i++) {
> @@ -1254,7 +1258,6 @@ struct bch_control *init_bch(int m, int t, unsigned int prim_poly)
>  	struct bch_control *bch = NULL;
>  
>  	const int min_m = 5;
> -	const int max_m = 15;
>  
>  	/* default primitive polynomials */
>  	static const unsigned int prim_poly_tab[] = {
> @@ -1270,7 +1273,7 @@ struct bch_control *init_bch(int m, int t, unsigned int prim_poly)
>  		goto fail;
>  	}
>  #endif
> -	if ((m < min_m) || (m > max_m))
> +	if ((m < min_m) || (m > BCH_MAX_M))
>  		/*
>  		 * values of m greater than 15 are not currently supported;
>  		 * supporting m > 15 would require changing table base type
> @@ -1300,7 +1303,7 @@ struct bch_control *init_bch(int m, int t, unsigned int prim_poly)
>  	bch->a_log_tab = bch_alloc((1+bch->n)*sizeof(*bch->a_log_tab), &err);
>  	bch->mod8_tab  = bch_alloc(words*1024*sizeof(*bch->mod8_tab), &err);
>  	bch->ecc_buf   = bch_alloc(words*sizeof(*bch->ecc_buf), &err);
> -	bch->ecc_buf2  = bch_alloc(words*sizeof(*bch->ecc_buf2), &err);
> +	bch->ecc_work  = bch_alloc(words*sizeof(*bch->ecc_work), &err);
>  	bch->xi_tab    = bch_alloc(m*sizeof(*bch->xi_tab), &err);
>  	bch->syn       = bch_alloc(2*t*sizeof(*bch->syn), &err);
>  	bch->cache     = bch_alloc(2*t*sizeof(*bch->cache), &err);
> @@ -1349,7 +1352,7 @@ void free_bch(struct bch_control *bch)
>  		kfree(bch->a_log_tab);
>  		kfree(bch->mod8_tab);
>  		kfree(bch->ecc_buf);
> -		kfree(bch->ecc_buf2);
> +		kfree(bch->ecc_work);
>  		kfree(bch->xi_tab);
>  		kfree(bch->syn);
>  		kfree(bch->cache);
> -- 
> 2.17.0
> 
> 
> -- 
> Kees Cook
> Pixel Security