[PATCH v2 2/3] 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 timestamp to the filename should be reasonably reliable.
Adding O_EXCL as mkstemp does fails creating the file rather than
corrupting existing file.

Signed-off-by: Michal Suchanek <msuchanek@suse.de>
---
 tools/depmod.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/tools/depmod.c b/tools/depmod.c
index 18c0d61b2db3..3b6d16e76160 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 +2399,9 @@ 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);
 
 	if (out != NULL)
 		dfd = -1;
@@ -2412,15 +2416,17 @@ 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.%li.%li", itr->name, getpid(),
+					tv.tv_usec, tv.tv_sec);
+			tmp[NAME_MAX] = 0;
 			fd = openat(dfd, tmp, flags, mode);
 			if (fd < 0) {
 				ERR("openat(%s, %s, %o, %o): %m\n",
-- 
2.19.2