Re: [PATCH] LZ4: compression/decompression signedness mismatch

Sergey Senozhatsky
2013-07-12T09:49:01Z

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)

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.

-ss

> 2013/7/12 Sergey Senozhatsky <[2]sergey.senozhatsky@gmail.com>
>
> LZ4 compression and decompression functions require different
> in signedness input/output parameters: unsigned char for
> compression and signed char for decompression.
>
> Change decompression API to require unsigned char.
>
> Signed-off-by: Sergey Senozhatsky <[3]sergey.senozhatsky@gmail.com>
>
> ---
>
> �include/linux/lz4.h � � �| 8 ++++----
> �lib/lz4/lz4_decompress.c | 8 ++++----
> �2 files changed, 8 insertions(+), 8 deletions(-)
>
> diff --git a/include/linux/lz4.h b/include/linux/lz4.h
> index d21c13f..c13f0bc 100644
> --- a/include/linux/lz4.h
> +++ b/include/linux/lz4.h
> @@ -67,8 +67,8 @@ int lz4hc_compress(const unsigned char *src, size_t
> src_len,
> � * � � note : �Destination buffer must be already allocated.
> � * � � � � � � slightly faster than lz4_decompress_unknownoutputsize()
> � */
> -int lz4_decompress(const char *src, size_t *src_len, char *dest,
> - � � � � � � � size_t actual_dest_len);
> +int lz4_decompress(unsigned const char *src, size_t *src_len,
> + � � � � � � � unsigned char *dest, size_t actual_dest_len);
>
> �/*
> � * lz4_decompress_unknownoutputsize()
> @@ -82,6 +82,6 @@ int lz4_decompress(const char *src, size_t *src_len,
> char *dest,
> � * � � � � � � � Error if return (< 0)
> � * � � note : �Destination buffer must be already allocated.
> � */
> -int lz4_decompress_unknownoutputsize(const char *src, size_t src_len,
> - � � � � � � � char *dest, size_t *dest_len);
> +int lz4_decompress_unknownoutputsize(unsigned const char *src, size_t
> src_len,
> + � � � � � � � unsigned char *dest, size_t *dest_len);
> �#endif
> diff --git a/lib/lz4/lz4_decompress.c b/lib/lz4/lz4_decompress.c
> index d3414ea..7ceda1f 100644
> --- a/lib/lz4/lz4_decompress.c
> +++ b/lib/lz4/lz4_decompress.c
> @@ -283,8 +283,8 @@ _output_error:
> � � � � return (int) (-(((char *) ip) - source));
> �}
>
> -int lz4_decompress(const char *src, size_t *src_len, char *dest,
> - � � � � � � � size_t actual_dest_len)
> +int lz4_decompress(unsigned const char *src, size_t *src_len,
> + � � � � � � � unsigned char *dest, size_t actual_dest_len)
> �{
> � � � � int ret = -1;
> � � � � int input_len = 0;
> @@ -302,8 +302,8 @@ exit_0:
> �EXPORT_SYMBOL_GPL(lz4_decompress);
> �#endif
>
> -int lz4_decompress_unknownoutputsize(const char *src, size_t src_len,
> - � � � � � � � char *dest, size_t *dest_len)
> +int lz4_decompress_unknownoutputsize(unsigned const char *src, size_t
> src_len,
> + � � � � � � � unsigned char *dest, size_t *dest_len)
> �{
> � � � � int ret = -1;
> � � � � int out_len = 0;
>
> References
>
> Visible links
> 1. https://code.google.com/p/lz4/
> 2. mailto:sergey.senozhatsky@gmail.com
> 3. mailto:sergey.senozhatsky@gmail.com