win32: restrict visibility of special devices

Handling of the special devices /dev/zero and /dev/urandom was
inconsistent:

- they could be used as arguments to 'cat' but not 'od';
- they could not be used in shell redirection.

Restrict the use of these devices to two places:

- as input files to 'dd' with the 'if=' argument;
- internally within 'shred'.

See GitHub issue #98.

1 files changed, 24 insertions, 7 deletions

diff --git a/win32/mingw.c b/win32/mingw.c
index f9967f1c4..981c50415 100644
--- a/win32/mingw.c
+++ b/win32/mingw.c
@@ -157,6 +157,7 @@ int mingw_open (const char *filename, int oflags, ...)
         va_list args;
         unsigned mode;
         int fd;
+        int special = 0;
         int devnull = 0;
         int devzero = 0;
         int devrand = 0;
@@ -165,16 +166,20 @@ int mingw_open (const char *filename, int oflags, ...)
         mode = va_arg(args, int);
         va_end(args);
 
-        if (oflags & O_NONBLOCK) {
-                oflags &= ~O_NONBLOCK;
+        if (oflags & O_SPECIAL) {
+                oflags &= ~O_SPECIAL;
+                special = 1;
         }
         if (filename && !strncmp(filename, "/dev/", 5)) {
-                if (!strcmp(filename+5, "null"))
+                if (!strcmp(filename+5, "null")) {
                         devnull = 1;
-                else if (!strcmp(filename+5, "zero"))
-                        devzero = 1;
-                else if (!strcmp(filename+5, "urandom"))
-                        devrand = 1;
+                }
+                else if (special) {
+                        if (!strcmp(filename+5, "zero"))
+                                devzero = 1;
+                        else if (!strcmp(filename+5, "urandom"))
+                                devrand = 1;
+                }
 
                 if (devnull || devzero || devrand )
                         filename = "nul";
@@ -194,6 +199,18 @@ int mingw_open (const char *filename, int oflags, ...)
         return fd;
 }
 
+int mingw_xopen(const char *pathname, int flags)
+{
+        int ret;
+
+        /* allow use of special devices */
+        ret = mingw_open(pathname, flags|O_SPECIAL);
+        if (ret < 0) {
+                bb_perror_msg_and_die("can't open '%s'", pathname);
+        }
+        return ret;
+}
+
 #undef fopen
 FILE *mingw_fopen (const char *filename, const char *otype)
 {