utils: add helper functions useful for allocating a ring buffer

This creates a mapping with twice the size of the allocated memory. The
second half of that mapping points at the same memory as the first half.

This is useful for ring buffers, because any read starting in the first
half can overflow into the second half as long as the read size is
smaller than the size of the memory area.

Signed-off-by: Felix Fietkau <nbd@nbd.name>

utils.c

@@ -16,10 +16,11 @@
  * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  */
 
-#include "utils.h"
+#include <sys/mman.h>
 #include <stdarg.h>
 #include <stdlib.h>
 #include <stdio.h>
+#include "utils.h"
 
 #define foreach_arg(_arg, _addr, _len, _first_addr, _first_len) \
 	for (_addr = (_first_addr), _len = (_first_len); \
@@ -103,3 +104,48 @@ out:
 }
 
 #endif
+
+void *cbuf_alloc(unsigned int order)
+{
+	char path[] = "/tmp/cbuf-XXXXXX";
+	unsigned long size = cbuf_size(order);
+	void *ret = NULL;
+	int fd;
+
+	fd = mkstemp(path);
+	if (fd < 0)
+		return NULL;
+
+	if (unlink(path))
+		goto close;
+
+	if (ftruncate(fd, cbuf_size(order)))
+		goto close;
+
+#ifndef MAP_ANONYMOUS
+#define MAP_ANONYMOUS MAP_ANON
+#endif
+
+	ret = mmap(NULL, size * 2, PROT_NONE, MAP_ANON | MAP_PRIVATE, -1, 0);
+	if (ret == MAP_FAILED) {
+		ret = NULL;
+		goto close;
+	}
+
+	if (mmap(ret, size, PROT_READ | PROT_WRITE, MAP_FIXED | MAP_SHARED,
+		 fd, 0) != ret ||
+	    mmap(ret + size, size, PROT_READ | PROT_WRITE,
+		 MAP_FIXED | MAP_SHARED, fd, 0) != ret + size) {
+		munmap(ret, size * 2);
+		ret = NULL;
+	}
+
+close:
+	close(fd);
+	return ret;
+}
+
+void cbuf_free(void *ptr, unsigned int order)
+{
+	munmap(ptr, cbuf_size(order) * 2);
+}

utils.h

@@ -23,6 +23,7 @@
 #include <sys/time.h>
 #include <stdint.h>
 #include <stdbool.h>
+#include <unistd.h>
 #include <time.h>
 
 /*
@@ -192,4 +193,23 @@ int b64_decode(const void *src, void *dest, size_t dest_len);
 #define B64_ENCODE_LEN(_len)	((((_len) + 2) / 3) * 4 + 1)
 #define B64_DECODE_LEN(_len)	(((_len) / 4) * 3 + 1)
 
+static inline unsigned int cbuf_order(unsigned int x)
+{
+	return 32 - __builtin_clz(x - 1);
+}
+
+static inline unsigned long cbuf_size(int order)
+{
+	unsigned long page_size = sysconf(_SC_PAGESIZE);
+	unsigned long ret = 1ULL << order;
+
+	if (ret < page_size)
+		ret = page_size;
+
+	return ret;
+}
+
+void *cbuf_alloc(unsigned int order);
+void cbuf_free(void *ptr, unsigned int order);
+
 #endif