copy_file: comment out one condition which is always false.

Add comment explaining POSIX rules for cp - and why
these rules are dangerous. Provide conditionally compiled code
for both POSIX and safe behaviors, select safe for now.
Code shrunk by ~80 bytes.

1 files changed, 50 insertions, 28 deletions

diff --git a/libbb/copy_file.c b/libbb/copy_file.c
index 636fbdc1d..58d657b02 100644
--- a/libbb/copy_file.c
+++ b/libbb/copy_file.c
@@ -11,13 +11,37 @@
 
 #include "libbb.h"
 
-static int retry_overwrite(const char *dest, int flags)
+// POSIX: if exists and -i, ask (w/o -i assume yes).
+// Then open w/o EXCL (yes, not unlink!).
+// If open still fails and -f, try unlink, then try open again.
+// Result: a mess:
+// If dest is a softlink, we overwrite softlink's destination!
+// (or fail, if it points to dir/nonexistent location/etc).
+// This is strange, but POSIX-correct.
+// coreutils cp has --remove-destination to override this...
+
+#define DO_POSIX_CP 0  /* 1 - POSIX behavior, 0 - safe behavior */
+
+
+static int ask_and_unlink(const char *dest, int flags)
 {
+	// If !DO_POSIX_CP, act as if -f is always in effect - we don't want
+	// "'file' exists" msg, we want unlink to be done (silently unless -i
+	// is also in effect).
+	// This prevents safe way from asking more questions than POSIX does.
+#if DO_POSIX_CP
 	if (!(flags & (FILEUTILS_FORCE|FILEUTILS_INTERACTIVE))) {
 		fprintf(stderr, "'%s' exists\n", dest);
 		return -1;
 	}
+#endif
+
+	// TODO: maybe we should do it only if ctty is present?
 	if (flags & FILEUTILS_INTERACTIVE) {
+		// We would not do POSIX insanity. -i asks,
+		// then _unlinks_ the offender. Presto.
+		// (No opening without O_EXCL, no unlinks only if -f)
+		// Or else we will end up having 3 open()s!
 		fprintf(stderr, "%s: overwrite '%s'? ", applet_name, dest);
 		if (!bb_ask_confirmation())
 			return 0; // not allowed to overwrite
@@ -29,6 +53,11 @@ static int retry_overwrite(const char *dest, int flags)
 	return 1; // ok (to try again)
 }
 
+/* Return:
+ * -1 error, copy not made
+ *  0 copy is made or user answered "no" in interactive mode
+ *    (failures to preserve mode/owner/times are not reported in exit code)
+ */
 int copy_file(const char *source, const char *dest, int flags)
 {
 	struct stat source_stat;
@@ -72,13 +101,11 @@ int copy_file(const char *source, const char *dest, int flags)
 				freecon(con);
 				return -1;
 			}
+		} else if (errno == ENOTSUP || errno == ENODATA) {
+			setfscreatecon_or_die(NULL);
 		} else {
-			if (errno == ENOTSUP || errno == ENODATA) {
-				setfscreatecon_or_die(NULL);
-			} else {
-				bb_perror_msg("cannot lgetfilecon %s", source);
-				return -1;
-			}
+			bb_perror_msg("cannot lgetfilecon %s", source);
+			return -1;
 		}
 	}
 #endif
@@ -153,7 +180,7 @@ int copy_file(const char *source, const char *dest, int flags)
 		// (but realpath returns NULL on dangling symlinks...)
 		lf = (flags & FILEUTILS_MAKE_SOFTLINK) ? symlink : link;
 		if (lf(source, dest) < 0) {
-			ovr = retry_overwrite(dest, flags);
+			ovr = ask_and_unlink(dest, flags);
 			if (ovr <= 0)
 				return ovr;
 			if (lf(source, dest) < 0) {
@@ -164,8 +191,8 @@ int copy_file(const char *source, const char *dest, int flags)
 		return 0;
 
 	} else if (S_ISREG(source_stat.st_mode)
-	// Huh? DEREF uses stat, which never returns links IIRC...
-	 || (FLAGS_DEREF && S_ISLNK(source_stat.st_mode))
+	 /* Huh? DEREF uses stat, which never returns links! */
+	 /* || (FLAGS_DEREF && S_ISLNK(source_stat.st_mode)) */
 	) {
 		int src_fd;
 		int dst_fd;
@@ -176,7 +203,7 @@ int copy_file(const char *source, const char *dest, int flags)
 				link_name = is_in_ino_dev_hashtable(&source_stat);
 				if (link_name) {
 					if (link(link_name, dest) < 0) {
-						ovr = retry_overwrite(dest, flags);
+						ovr = ask_and_unlink(dest, flags);
 						if (ovr <= 0)
 							return ovr;
 						if (link(link_name, dest) < 0) {
@@ -196,27 +223,21 @@ int copy_file(const char *source, const char *dest, int flags)
 			return -1;
 		}
 
-		// POSIX: if exists and -i, ask (w/o -i assume yes).
-		// Then open w/o EXCL.
-		// If open still fails and -f, try unlink, then try open again.
-		// Result: a mess:
-		// If dest is a softlink, we overwrite softlink's destination!
-		// (or fail, if it points to dir/nonexistent location/etc).
-		// This is strange, but POSIX-correct.
-		// coreutils cp has --remove-destination to override this...
+#if DO_POSIX_CP  /* POSIX way (a security problem versus symlink attacks!): */
 		dst_fd = open(dest, (flags & FILEUTILS_INTERACTIVE)
-				? O_WRONLY|O_CREAT|O_TRUNC|O_EXCL
+				? O_WRONLY|O_CREAT|O_EXCL
 				: O_WRONLY|O_CREAT|O_TRUNC, source_stat.st_mode);
+#else  /* safe way: */
+		dst_fd = open(dest, O_WRONLY|O_CREAT|O_EXCL, source_stat.st_mode);
+#endif
 		if (dst_fd == -1) {
-			// We would not do POSIX insanity. -i asks,
-			// then _unlinks_ the offender. Presto.
-			// Or else we will end up having 3 open()s!
-			ovr = retry_overwrite(dest, flags);
+			ovr = ask_and_unlink(dest, flags);
 			if (ovr <= 0) {
 				close(src_fd);
 				return ovr;
 			}
-			dst_fd = open(dest, O_WRONLY|O_CREAT|O_TRUNC, source_stat.st_mode);
+			/* It shouldn't exist. If it exists, do not open (symlink attack?) */
+			dst_fd = open(dest, O_WRONLY|O_CREAT|O_EXCL, source_stat.st_mode);
 			if (dst_fd == -1) {
 				bb_perror_msg("cannot open '%s'", dest);
 				close(src_fd);
@@ -227,7 +248,8 @@ int copy_file(const char *source, const char *dest, int flags)
 #if ENABLE_SELINUX
 		if (((flags & FILEUTILS_PRESERVE_SECURITY_CONTEXT)
 		    || (flags & FILEUTILS_SET_SECURITY_CONTEXT))
-		 && is_selinux_enabled() > 0) {
+		 && is_selinux_enabled() > 0
+		) {
 			security_context_t con;  
 			if (getfscreatecon(&con) == -1) {
 				bb_perror_msg("getfscreatecon");
@@ -260,7 +282,7 @@ int copy_file(const char *source, const char *dest, int flags)
 	) {
 		// We are lazy here, a bit lax with races...
 		if (dest_exists) {
-			ovr = retry_overwrite(dest, flags);
+			ovr = ask_and_unlink(dest, flags);
 			if (ovr <= 0)
 				return ovr;
 		}
@@ -273,7 +295,7 @@ int copy_file(const char *source, const char *dest, int flags)
 			char *lpath;
 
 			lpath = xmalloc_readlink_or_warn(source);
-			if (symlink(lpath, dest) < 0) {
+			if (lpath && symlink(lpath, dest) < 0) {
 				bb_perror_msg("cannot create symlink '%s'", dest);
 				free(lpath);
 				return -1;