dd: add support for writing to physical drives

A common use of 'dd' is to copy boot images to removable media.
This didn't work on Windows because opening a physical drive
failed with the default flags used by 'dd'.

If the output file is a physical drive remove incompatible flags.

Writing to a physical drive requires elevated privileges and the
drive must be offline.

Adds 80 bytes.

(GitHub issue #435)

1 files changed, 17 insertions, 0 deletions

diff --git a/coreutils/dd.c b/coreutils/dd.c
index a9ef68fbc..e6b88acf5 100644
--- a/coreutils/dd.c
+++ b/coreutils/dd.c
@@ -333,6 +333,17 @@ static void *alloc_buf(size_t size)
         return xmalloc(size);
 }
 
+#if ENABLE_PLATFORM_MINGW32
+// Does 'path' refer to a physical drive in the win32 device namespace?
+static int is_drive_path(const char *path)
+{
+        char *s = auto_string(strdup(path));
+
+        bs_to_slash(s);
+        return strncasecmp(s, "//./PhysicalDrive", 17) == 0;
+}
+#endif
+
 int dd_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int dd_main(int argc UNUSED_PARAM, char **argv)
 {
@@ -558,6 +569,12 @@ int dd_main(int argc UNUSED_PARAM, char **argv)
 # endif
                 }
 #endif
+#if ENABLE_PLATFORM_MINGW32
+                if (is_drive_path(outfile)) {
+                        // Turn off options not supported by Windows device files.
+                        oflag &= ~(O_CREAT | O_TRUNC);
+                }
+#endif
                 xmove_fd(xopen(outfile, oflag), ofd);
 
 #if ENABLE_PLATFORM_MINGW32