[PATCH 1/2] Helper macros used for replacing the use of VLAIS

[Behan Webster]

The use of variable length arrays in structs (VLAIS) in the Linux Kernel code
precludes the use of compilers which don't implement VLAIS (for instance the
Clang compiler). This new header file contains macros which can be used to
calculate the size and offset of variables in an allocated buffer of memory
taking into account alignment issues.

Signed-off-by: Behan Webster <behanw@xxxxxxxxxxxxxxxxxx>
---
 include/linux/valign.h |   87 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 87 insertions(+)
 create mode 100644 include/linux/valign.h

diff --git a/include/linux/valign.h b/include/linux/valign.h
new file mode 100644
index 0000000..b39381b
--- /dev/null
+++ b/include/linux/valign.h
@@ -0,0 +1,87 @@
+/*
+ * Variable alignment macros used to break up a larger chunk of memory into
+ * smaller variables. Meant to be used to replace the use of Variable Length
+ * Arrays In Structures (VLAIS)
+ *
+ *  Copyright (C) 2012 Behan Webster <behanw@xxxxxxxxxxxxxxxxx>
+ */
+
+#ifndef _VALIGN_H_
+#define _VALIGN_H_
+
+/**
+ * truncalign() - Align a memory address by truncation
+ * @num:	Address or size to align
+ * @padwidth:	Number of byte upon which to align
+ *
+ * Truncate an address or size to a particular memory alignment.
+ * Used by truncalign().
+ */
+#define truncalign(num, padwidth) ((long)(num) & ~((padwidth)-1))
+
+/**
+ * padalign() - Align a memory address by padding
+ * @num:	Address or size to align
+ * @padwidth:	Number of byte upon which to align
+ *
+ * Pad out an address or size to a particular memory alignment
+ * Used by paddedsize() and paddedstart().
+ */
+#define padalign(num, padwidth) \
+	truncalign((long)(num) + ((padwidth)-1), padwidth)
+
+/**
+ * paddedsize() - Calculate the size of an chunk of aligned memory
+ * @offset:	Unaligned offset to the start of the chunk size being calculated
+ * @num:	The number of variables in the array of "type" (can be 1)
+ * @type:	The type of variables in the array
+ * @nexttype:	The type of the next variable in the large piece of memory
+ *
+ * Calculate the size that a variable (or array) will take as a part of a
+ * larger piece of memory.  Takes into account a potentially unaligned offset
+ * into the larger piece of allocated memory, the alignment of the variable
+ * type, and the alignement of the type of the variable to be used after that.
+ *
+ * Example: size_t l = paddedsize(1, 2, short, int);
+ *
+ * The example above would give you a padded size of 6 bytes: 2x 16-bit shorts,
+ * starting at 2 bytes into the buffer (the offset of 1 byte being padded out
+ * to 2 bytes) followed by 2 bytes of padding so that the next type (a 32-bit
+ * int) would be 32-bit aligned. looking like this:
+ *
+ *   0: O.SS SS.. iiii
+ *        \-----/ <-- 2 shorts + 2 bytes of padding = size of 6 bytes
+ *
+ * O = The offset
+ * . = Padding bytes
+ * S = 2 shorts
+ * i = int which will theoretically be next
+ */
+#define paddedsize(offset, num, type, nexttype) (padalign((offset) \
+	+ (num) * sizeof(type), __alignof__(nexttype)) - (offset))
+
+/**
+ * paddedstart() - Calculate the start of a chunk of aligned memory
+ * @ptr:	Pointer from which to calculate the start of the chunk
+ * @offset:	Offset from the ptr to the start of the chunk being calculated
+ * @type:	The type of variable in the chunk of memory
+ *
+ * Calculate the start address of a variable based on the offset from an
+ * address, aligned based on the type of the variable specified.
+ *
+ * Example: char *data = kmalloc(size, GFP_KERNEL);
+ *          long *var = paddedstart(data, 12, long);
+ *
+ * The example above on a 64-bit machine would return the equivalent of
+ * &buffer[16] since a long needs to be 8 byte aligned.
+ *
+ *   0: OOOO OOOO OOOO .... LLLL LLLL
+ *                          ^ <-- The start address of the long
+ * O = The offset
+ * . = Padding bytes
+ * L = The long
+ */
+#define paddedstart(ptr, offset, type) \
+	(type *)padalign((long)(ptr)+(offset), __alignof__(type))
+
+#endif
-- 
1.7.9.5

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