util: Add read_file() function for reading sysfs files

Add a function that can read a complete, unknown size file for
reading entry point files from sysfs.  This function models its signature
on the mem_chunk() funtion, so it also allocates memory that the caller
needs to free.  The files that we are interested in reading are very small,
and have a known upper bound on the size.  The EINTR handling is based
on the myread() function.

Contributed by Roy Franz.

Author: jdelvare

CHANGELOG

@@ -1,3 +1,8 @@
+2015-04-21  Roy Franz  <[email protected]>
+
+	* util.c, util.h: Add utility function read_file, which reads an
+	  entire binary file into a buffer.
+
 2015-04-20  Jean Delvare  <[email protected]>
 
 	* biosdecode.c: Add support for the _SM3_ entry point, as defined in

util.c

@@ -88,6 +88,61 @@ int checksum(const u8 *buf, size_t len)
 	return (sum == 0);
 }
 
+/*
+ * Reads all of file, up to max_len bytes.
+ * A buffer of max_len bytes is allocated by this function, and
+ * needs to be freed by the caller.
+ * This provides a similar usage model to mem_chunk()
+ *
+ * Returns pointer to buffer of max_len bytes, or NULL on error
+ *
+ */
+void *read_file(size_t max_len, const char *filename)
+{
+	int fd;
+	size_t r2 = 0;
+	ssize_t r;
+	u8 *p;
+
+	/*
+	 * Don't print error message on missing file, as we will try to read
+	 * files that may or may not be present.
+	 */
+	if ((fd = open(filename, O_RDONLY)) == -1)
+	{
+		if (errno != ENOENT)
+			perror(filename);
+		return(NULL);
+	}
+
+	if ((p = malloc(max_len)) == NULL)
+	{
+		perror("malloc");
+		return NULL;
+	}
+
+	do
+	{
+		r = read(fd, p + r2, max_len - r2);
+		if (r == -1)
+		{
+			if (errno != EINTR)
+			{
+				close(fd);
+				perror(filename);
+				free(p);
+				return NULL;
+			}
+		}
+		else
+			r2 += r;
+	}
+	while (r != 0);
+
+	close(fd);
+	return p;
+}
+
 /*
  * Copy a physical memory chunk into a memory buffer.
  * This function allocates memory.

util.h

@@ -25,6 +25,7 @@
 #define ARRAY_SIZE(x) (sizeof(x)/sizeof((x)[0]))
 
 int checksum(const u8 *buf, size_t len);
+void *read_file(size_t len, const char *filename);
 void *mem_chunk(size_t base, size_t len, const char *devmem);
 int write_dump(size_t base, size_t len, const void *data, const char *dumpfile, int add);
 u64 u64_range(u64 start, u64 end);