On Wed, May 30, 2018 at 02:22:06PM -0700, Kees Cook wrote:
> 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=qpxydaacu1rq...@mail.gmail.com
>
> Signed-off-by: Kees Cook <[email protected]>
> ---
> v2: switch to fixed-size stack array
> ---
(...)
> #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);
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
