lib: crc32: add functionality to combine two crc32{, c}s in GF(2)

This patch adds a combinator to merge two or more crc32{,c}s
into a new one. This is useful for checksum computations of
fragmented skbs that use crc32/crc32c as checksums.

The arithmetics for combining both in the GF(2) was taken and
slightly modified from zlib. Only passing two crcs is insufficient
as two crcs and the length of the second piece is needed for
merging. The code is made generic, so that only polynomials
need to be passed for crc32_le resp. crc32c_le.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 40 insertions, 0 deletions

diff --git a/include/linux/crc32.h b/include/linux/crc32.h
index 68267b64bb98..7d275c4fc011 100644
--- a/include/linux/crc32.h
+++ b/include/linux/crc32.h
@@ -11,8 +11,48 @@
 extern u32  crc32_le(u32 crc, unsigned char const *p, size_t len);
 extern u32  crc32_be(u32 crc, unsigned char const *p, size_t len);
 
+/**
+ * crc32_le_combine - Combine two crc32 check values into one. For two
+ *                    sequences of bytes, seq1 and seq2 with lengths len1
+ *                    and len2, crc32_le() check values were calculated
+ *                    for each, crc1 and crc2.
+ *
+ * @crc1: crc32 of the first block
+ * @crc2: crc32 of the second block
+ * @len2: length of the second block
+ *
+ * Return: The crc32_le() check value of seq1 and seq2 concatenated,
+ *         requiring only crc1, crc2, and len2. Note: If seq_full denotes
+ *         the concatenated memory area of seq1 with seq2, and crc_full
+ *         the crc32_le() value of seq_full, then crc_full ==
+ *         crc32_le_combine(crc1, crc2, len2) when crc_full was seeded
+ *         with the same initializer as crc1, and crc2 seed was 0. See
+ *         also crc32_combine_test().
+ */
+extern u32  crc32_le_combine(u32 crc1, u32 crc2, size_t len2);
+
 extern u32  __crc32c_le(u32 crc, unsigned char const *p, size_t len);
 
+/**
+ * __crc32c_le_combine - Combine two crc32c check values into one. For two
+ *                       sequences of bytes, seq1 and seq2 with lengths len1
+ *                       and len2, __crc32c_le() check values were calculated
+ *                       for each, crc1 and crc2.
+ *
+ * @crc1: crc32c of the first block
+ * @crc2: crc32c of the second block
+ * @len2: length of the second block
+ *
+ * Return: The __crc32c_le() check value of seq1 and seq2 concatenated,
+ *         requiring only crc1, crc2, and len2. Note: If seq_full denotes
+ *         the concatenated memory area of seq1 with seq2, and crc_full
+ *         the __crc32c_le() value of seq_full, then crc_full ==
+ *         __crc32c_le_combine(crc1, crc2, len2) when crc_full was
+ *         seeded with the same initializer as crc1, and crc2 seed
+ *         was 0. See also crc32c_combine_test().
+ */
+extern u32  __crc32c_le_combine(u32 crc1, u32 crc2, size_t len2);
+
 #define crc32(seed, data, length)  crc32_le(seed, (unsigned char const *)(data), length)
 
 /*