lib/bch: Remove VLA usage
Subsystems (2)
PATCH v2 1/1 lib/bch: Remove VLA usage
In the quest to remove all stack VLA usage from the kernel[1], this allocates a fixed size stack array to cover the range needed for bch. This was done instead of a preallocation on the SLAB due to performance reasons, shown by Ivan Djelic:
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
So this change actually improves performance too, it seems.
The resulting stack allocation can get rather large; without CONFIG_BCH_CONST_PARAMS, it will allocate 4096 bytes, which trips the stack size checking:
lib/bch.c: In function ‘encode_bch’:lib/bch.c:261:1: warning: the frame size of 4432 bytes is larger than 2048 bytes [-Wframe-larger-than=]
Even the default case for "allmodconfig" (with CONFIG_BCH_CONST_M=14 and CONFIG_BCH_CONST_T=4) would have started throwing a warning:
lib/bch.c: In function ‘encode_bch’:lib/bch.c:261:1: warning: the frame size of 2288 bytes is larger than 2048 bytes [-Wframe-larger-than=]
But this is how large it's always been; it was just hidden from the checker because it was a VLA. So the Makefile has been adjusted to silence this warning for anything smaller than 4500 bytes, which should provide room for normal cases, but still low enough to catch any future pathological situations.
[1] https://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=qPXydAacU1RqZWA@mail.gmail.com
---
v2: switch to fixed-size stack array
--- lib/Makefile | 1 + lib/bch.c | 23 +++++++++++++++-------- 2 files changed, 16 insertions(+), 8 deletions(-)
lib/Makefile +1 -0
@@ -121,6 +121,7 @@ obj-$(CONFIG_ZLIB_INFLATE) += zlib_inflate/ obj-$(CONFIG_ZLIB_DEFLATE) += zlib_deflate/ obj-$(CONFIG_REED_SOLOMON) += reed_solomon/ obj-$(CONFIG_BCH) += bch.o +CFLAGS_bch.o := $(call cc-option,-Wframe-larger-than=4500) obj-$(CONFIG_LZO_COMPRESS) += lzo/ obj-$(CONFIG_LZO_DECOMPRESS) += lzo/ obj-$(CONFIG_LZ4_COMPRESS) += lz4/
lib/bch.c +15 -8
@@ -78,15 +78,22 @@#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_MAX_T (((1 << BCH_MAX_M) - 1) / BCH_MAX_M) +#define BCH_ECC_WORDS (_p) DIV_ROUND_UP(GF_M(_p)*GF_T(_p), 32)#define BCH_ECC_BYTES (_p) DIV_ROUND_UP(GF_M(_p)*GF_T(_p), 8) +#define BCH_ECC_MAX_WORDS DIV_ROUND_UP(BCH_MAX_M * BCH_MAX_T, 32) +#define BCH_ECC_MAX_BYTES DIV_ROUND_UP(BCH_MAX_M * BCH_MAX_T, 8) +#ifndef dbg #define dbg(_fmt, args...) do {} while (0)#endif
@@ -187,7 +194,8 @@ 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, r[BCH_ECC_MAX_WORDS ]; +const size_t r_bytes =BCH_ECC_BYTES (bch);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 +206,7 @@ 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_buf,0 , r_bytes); }/* process first unaligned data bytes */
@@ -215,7 +223,7 @@ 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 (r, bch->ecc_buf, r_bytes);/* * split each 32-bit word into 4 polynomials of weight 8 as follows:
@@ -241,7 +249,7 @@ 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, r, r_bytes);/* process last unaligned bytes */ if (len)
@@ -434,7 +442,7 @@ 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;
@@ -1114,7 +1122,7 @@ 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 +1262,6 @@ 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 +1277,7 @@ 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
Signature · 6 lines
2.17.0 -- Kees Cook Pixel Security
Quoted 42 lines · Kees Cook
In the quest to remove all stack VLA usage from the kernel[1], this allocates a fixed size stack array to cover the range needed for bch. This was done instead of a preallocation on the SLAB due to performance reasons, shown by Ivan Djelic:
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=4Buffer allocation | Encoding throughput (Mbit/s)
---------------------------------------------------on-stack, VLA | 3988 on-stack, fixed | 4494 kmalloc | 1967So this change actually improves performance too, it seems.
The resulting stack allocation can get rather large; without
CONFIG_BCH_CONST_PARAMS, it will allocate 4096 bytes, which trips the stack size checking:
lib/bch.c: In function ‘encode_bch’:lib/bch.c:261:1: warning: the frame size of 4432 bytes is larger than 2048 bytes [-Wframe-larger-than=]Even the default case for "allmodconfig" (with
CONFIG_BCH_CONST_M=14andCONFIG_BCH_CONST_T=4) would have started throwing a warning:
lib/bch.c: In function ‘encode_bch’:lib/bch.c:261:1: warning: the frame size of 2288 bytes is larger than 2048 bytes [-Wframe-larger-than=]But this is how large it's always been; it was just hidden from the checker because it was a VLA. So the
Makefilehas been adjusted to silence this warning for anything smaller than 4500 bytes, which should provide room for normal cases, but still low enough to catch any future pathological situations.[1] https://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=qPXydAacU1RqZWA@mail.gmail.com
Signed-off-by Kees Cook <[email protected]>---
v2: switch to fixed-size stack array
---
(...)
Quoted 10 lines
#define
BCH_ECC_WORDS(_p)DIV_ROUND_UP(GF_M(_p)*GF_T(_p), 32)
#defineBCH_ECC_BYTES(_p)DIV_ROUND_UP(GF_M(_p)*GF_T(_p), 8)lib/bch.c +3
-0+#define BCH_ECC_MAX_WORDS DIV_ROUND_UP(BCH_MAX_M * BCH_MAX_T, 32) +#define BCH_ECC_MAX_BYTES DIV_ROUND_UP(BCH_MAX_M * BCH_MAX_T, 8) +#ifndef dbg #define dbg(_fmt, args...) do {} while (0)#endif
lib/bch.c void encode_bch(struct bch_control *bch, const uint8_t *data, · line 198
Quoted 6 lines
lib/bch.c +2
-1@@ -187,7 +194,8 @@ 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, r[BCH_ECC_MAX_WORDS ]; +const size_t r_bytes =BCH_ECC_BYTES (bch);
Here 'r_bytes' is too small, because BCH_ECC_BYTES(bch) != BCH_ECC_WORDS(bch)*sizeof(uint32_t).
It should be:
const size_t r_bytes = BCH_ECC_WORDS(bch)*sizeof(uint32_t);
or an equivalent like:
const size_t r_bytes = (l+1)*sizeof(*bch->ecc_buf);
The rest of the patch seems fine to me.
BR,
--
Ivan