[PATCH] depmod: Prevent module dependency files corruption due to parallel invocation.

[Michal Suchanek <msuchanek@suse.de>]

Depmod does not use unique filename for temporary files. There is no
guarantee the user does not attempt to run mutiple depmod processes in
parallel. If that happens a temporary file might be created by
depmod(1st), truncated by depmod(2nd), and renamed to final name by
depmod(1st) resulting in corrupted file seen by user.

Due to missing mkstempat() this is more complex than it should be.
Adding PID and random number to the filename should be reasonably
reliable. Adding O_EXCL as mkstemp does fails creating the file rather
than corrupting existing file.

Also prevent dependency files missing. This happens because target files
are removed before renaming the temporary file.

Signed-off-by: Michal Suchanek <msuchanek@suse.de>
---
 tools/depmod.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/tools/depmod.c b/tools/depmod.c
index 989d9077926c..5526ac892cf8 100644
--- a/tools/depmod.c
+++ b/tools/depmod.c
@@ -29,6 +29,7 @@
 #include <string.h>
 #include <unistd.h>
 #include <sys/stat.h>
+#include <sys/time.h>
 #include <sys/utsname.h>
 
 #include <shared/array.h>
@@ -2398,6 +2399,10 @@ static int depmod_output(struct depmod *depmod, FILE *out)
 	};
 	const char *dname = depmod->cfg->dirname;
 	int dfd, err = 0;
+	struct timeval tv;
+
+	gettimeofday(&tv, NULL);
+	srand(tv.tv_sec);
 
 	if (out != NULL)
 		dfd = -1;
@@ -2412,15 +2417,17 @@ static int depmod_output(struct depmod *depmod, FILE *out)
 
 	for (itr = depfiles; itr->name != NULL; itr++) {
 		FILE *fp = out;
-		char tmp[NAME_MAX] = "";
+		char tmp[NAME_MAX + 1] = "";
 		int r, ferr;
 
 		if (fp == NULL) {
-			int flags = O_CREAT | O_TRUNC | O_WRONLY;
+			int flags = O_CREAT | O_TRUNC | O_WRONLY | O_EXCL;
 			int mode = 0644;
 			int fd;
 
-			snprintf(tmp, sizeof(tmp), "%s.tmp", itr->name);
+			snprintf(tmp, sizeof(tmp), "%s.%i.%i", itr->name, getpid(),
+					rand());
+			tmp[NAME_MAX] = 0;
 			fd = openat(dfd, tmp, flags, mode);
 			if (fd < 0) {
 				ERR("openat(%s, %s, %o, %o): %m\n",
@@ -2451,7 +2458,6 @@ static int depmod_output(struct depmod *depmod, FILE *out)
 			break;
 		}
 
-		unlinkat(dfd, itr->name, 0);
 		if (renameat(dfd, tmp, dfd, itr->name) != 0) {
 			err = -errno;
 			CRIT("renameat(%s, %s, %s, %s): %m\n",
-- 
2.19.2