linux-kernel - Re: [PATCH] LZ4: compression/decompression signedness mismatch

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Message-ID: <20130718211806.GB2394@swordfish>
Date:	Fri, 19 Jul 2013 00:18:07 +0300
From:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:	Kyungsik Lee <kyungsik.lee@....com>
Cc:	Yann Collet <yann.collet.73@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] LZ4: compression/decompression signedness mismatch

Hello,

On (07/12/13 12:48), Sergey Senozhatsky wrote:
> On (07/12/13 11:28), Yann Collet wrote:
> >    The reference implementation, hosted at :�
> >    [1]https://code.google.com/p/lz4/
> >    only proposes char* (signed) types as part of the interface contract.
> >    I would recommend to keep it that way, to remain consistent.
> >    Regards
> 
> Crypto lz4 accepts u8 * for both compression and decompression:
> 
>  lz4_compress_crypto(struct crypto_tfm *tfm, const u8 *src,
>                             unsigned int slen, u8 *dst, unsigned int *dlen)
> 
>  lz4_decompress_crypto(struct crypto_tfm *tfm, const u8 *src,
>                        unsigned int slen, u8 *dst, unsigned int *dlen)
> 
> 
> Internally LZ4 may cast unsigned char* to signed char*, the same way you
> already do with compression:
> 
> 	int lz4_compress(const unsigned char *src, size_t src_len,
> 			unsigned char *dst, size_t *dst_len, void *wrkmem)
> 
> 		calls:
> 			lz4_compressctx(void *ctx,
> 				const char *source, char *dest,
> 				int isize, int maxoutputsize)
> 


+ lib/decompress_unlz4.c
 STATIC int INIT decompress(unsigned char *buf, int in_len,
				int(*fill)(void*, unsigned int),
				int(*flush)(void*, unsigned int),
				unsigned char *output,
				int *posp,
				void(*error)(char *x)

> 
> At the moment API is a bit misaligned: unsiged char* for compression and signed char* for
> decompression.
> 
> 
> My 'real word' use case is, suppose:
> 
> 	struct foo {
> 	[..]
> 		int (*compress)(const unsigned char *src, size_t src_len,
> 				unsigned char *dst, size_t *dst_len, void *wrkmem);
> 		int (*decompress)(const unsigned char *src, size_t src_len,
> 				unsigned char *dst, size_t *dst_len);
> 	};
> 
> 
> and (for example) module also provides sysfs attribute, so user can switch select
> LZO or LZ4 compressions depending of his needs:
> 
> 	->compress = lzo1x_1_compress;
> 	->decompress = lzo1x_decompress_safe;
> 
> to
> 	->compress = lz4_compress;
> 	->decompress = lz4_decompress_unknownoutputsize;
> 
> 
> the last one produces unneccessary compilation warning.
> 

did you guys have a chance to review the patch? it does not change
implementation/internals, just decompression exported symbols.


	-ss

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