Messages in this thread | | | Date | Thu, 31 May 2018 10:52:21 +0200 | From | Ivan Djelic <> | Subject | Re: [PATCH v2] lib/bch: Remove VLA usage |
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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=qPXydAacU1RqZWA@mail.gmail.com > > Signed-off-by: Kees Cook <keescook@chromium.org> > --- > 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
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