[PATCH RFC 2/3] libfile: implement read_fd counterpart to read_file

[Ahmad Fatoum <a.fatoum@pengutronix.de>]

Files opened with O_TMPFILE have no name, so read_file can't be used
with them. Therefore add a read_fd function, which slurps all a file's
contents into a buffer.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
---
 include/libfile.h |  2 ++
 lib/libfile.c     | 44 ++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 46 insertions(+)

diff --git a/include/libfile.h b/include/libfile.h
index a353ccfa9ea9..423e7ffec5b7 100644
--- a/include/libfile.h
+++ b/include/libfile.h
@@ -12,6 +12,8 @@ char *read_file_line(const char *fmt, ...);
 
 void *read_file(const char *filename, size_t *size);
 
+void *read_fd(int fd, size_t *size);
+
 int read_file_2(const char *filename, size_t *size, void **outbuf,
 		loff_t max_size);
 
diff --git a/lib/libfile.c b/lib/libfile.c
index e53ba08415a2..c257baaa2733 100644
--- a/lib/libfile.c
+++ b/lib/libfile.c
@@ -306,6 +306,50 @@ void *read_file(const char *filename, size_t *size)
 }
 EXPORT_SYMBOL(read_file);
 
+/**
+ * read_fd - read from a file descriptor to an allocated buffer
+ * @filename:  The file descriptor to read
+ * @size:      After successful return contains the size of the file
+ *
+ * This function reads a file descriptor from offset 0 until EOF to an
+ * allocated buffer.
+ *
+ * Return: The buffer containing the file or NULL on failure
+ */
+void *read_fd(int fd, size_t *out_size)
+{
+	off_t off;
+	ssize_t ret;
+	size_t size;
+	void *buf;
+
+	off = lseek(fd, SEEK_CUR, 0);
+	if (off >= 0) {
+		size = off;
+		off = lseek(fd, SEEK_SET, 0);
+	}
+	if (off < 0) {
+		ret = off;
+		goto close_fd;
+	}
+
+	buf = malloc(size + 3);
+	ret = read_full(fd, buf, size);
+	if (ret < 0) {
+		free(buf);
+		goto close_fd;
+	}
+
+	memset(&buf[size], '\0', 3);
+	*out_size = size;
+
+close_fd:
+	close(fd);
+
+	return ret < 0 ? NULL : buf;
+}
+EXPORT_SYMBOL(read_fd);
+
 /**
  * write_file - write a buffer to a file
  * @filename:    The filename to write
-- 
2.39.2