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2018-10-12lib/bch: fix possible stack overrunArnd Bergmann
The previous patch introduced very large kernel stack usage and a Makefile change to hide the warning about it. From what I can tell, a number of things went wrong here: - The BCH_MAX_T constant was set to the maximum value for 'n', not the maximum for 't', which is much smaller. - The stack usage is actually larger than the entire kernel stack on some architectures that can use 4KB stacks (m68k, sh, c6x), which leads to an immediate overrun. - The justification in the patch description claimed that nothing changed, however that is not the case even without the two points above: the configuration is machine specific, and most boards never use the maximum BCH_ECC_WORDS() length but instead have something much smaller. That maximum would only apply to machines that use both the maximum block size and the maximum ECC strength. The largest value for 't' that I could find is '32', which in turn leads to a 60 byte array instead of 2048 bytes. Making it '64' for future extension seems also worthwhile, with 120 bytes for the array. Anything larger won't fit into the OOB area on NAND flash. With that changed, the warning can be enabled again. Only linux-4.19+ contains the breakage, so this is only needed as a stable backport if it does not make it into the release. Fixes: 02361bc77888 ("lib/bch: Remove VLA usage") Reported-by: Ard Biesheuvel <ard.biesheuvel@linaro.org> Cc: stable@vger.kernel.org Signed-off-by: Arnd Bergmann <arnd@arndb.de> Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
2018-06-22lib/bch: Remove VLA usageKees 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=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> Reviewed-by: Ivan Djelic <ivan.djelic@parrot.com> Tested-by: Ivan Djelic <ivan.djelic@parrot.com> Acked-by: Boris Brezillon <boris.brezillon@bootlin.com> Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
2011-03-11lib: add shared BCH ECC libraryIvan Djelic
This is a new software BCH encoding/decoding library, similar to the shared Reed-Solomon library. Binary BCH (Bose-Chaudhuri-Hocquenghem) codes are widely used to correct errors in NAND flash devices requiring more than 1-bit ecc correction; they are generally better suited for NAND flash than RS codes because NAND bit errors do not occur in bursts. Latest SLC NAND devices typically require at least 4-bit ecc protection per 512 bytes block. This library provides software encoding/decoding, but may also be used with ASIC/SoC hardware BCH engines to perform error correction. It is being currently used for this purpose on an OMAP3630 board (4bit/8bit HW BCH). It has also been used to decode raw dumps of NAND devices with on-die BCH ecc engines (e.g. Micron 4bit ecc SLC devices). Latest NAND devices (including SLC) can exhibit high error rates (typically a dozen or more bitflips per hour during stress tests); in order to minimize the performance impact of error correction, this library implements recently developed algorithms for fast polynomial root finding (see bch.c header for details) instead of the traditional exhaustive Chien root search; a few performance figures are provided below: Platform: arm926ejs @ 468 MHz, 32 KiB icache, 16 KiB dcache BCH ecc : 4-bit per 512 bytes Encoding average throughput: 250 Mbits/s Error correction time (compared with Chien search): average worst average (Chien) worst (Chien) ---------------------------------------------------------- 1 bit 8.5 µs 11 µs 200 µs 383 µs 2 bit 9.7 µs 12.5 µs 477 µs 728 µs 3 bit 18.1 µs 20.6 µs 758 µs 1010 µs 4 bit 19.5 µs 23 µs 1028 µs 1280 µs In the above figures, "worst" is meant in terms of error pattern, not in terms of cache miss / page faults effects (not taken into account here). The library has been extensively tested on the following platforms: x86, x86_64, arm926ejs, omap3630, qemu-ppc64, qemu-mips. Signed-off-by: Ivan Djelic <ivan.djelic@parrot.com> Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>