confile: improve strprint()

POSIX specifies [1]: "If the value of n is zero on a call to snprintf(), nothing shall be written, the number of bytes that would have been written had n been sufficiently large excluding the terminating null shall be returned, and s may be a null pointer." But in case there are any non-sane libcs out there that do actually dereference the buffer when when 0 is passed as length to snprintf() let's give them a dummy buffer. [1]: The Open Group Base Specifications Issue 7, 2018 edition IEEE Std 1003.1-2017 (Revision of IEEE Std 1003.1-2008) Copyright © 2001-2018 IEEE and The Open Group Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com> Reported-by: Donghwa Jeong <dh48.jeong@samsung.com>

confile: improve strprint()
91bc7fb0 · Christian Brauner · dfd8ceba · 91bc7fb0
Unverified Commit 91bc7fb0 authored May 30, 2018 by Christian Brauner
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confile_utils.h src/lxc/confile_utils.h +18 -15

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--- a/src/lxc/confile_utils.h
+++ b/src/lxc/confile_utils.h
@@ -41,21 +41,24 @@
 #define MACVLAN_MODE_PASSTHRU 8
 #endif
-#define strprint(str, inlen, ...)                                              \
+#define strprint(str, inlen, ...)                                       \
-	do {                                                                   \
+	do {                                                            \
-		len = snprintf(str, inlen, ##__VA_ARGS__);                     \
+		if (str)                                                \
-		if (len < 0) {                                                 \
+			len = snprintf(str, inlen, ##__VA_ARGS__);      \
-			SYSERROR("failed to create string");                   \
+		else                                                    \
-			return -1;                                             \
+			len = snprintf((char *){""}, 0, ##__VA_ARGS__); \
-		};                                                             \
+		if (len < 0) {                                          \
-		fulllen += len;                                                \
+			SYSERROR("failed to create string");            \
-		if (inlen > 0) {                                               \
+			return -1;                                      \
-			if (str)                                               \
+		};                                                      \
-				str += len;                                    \
+		fulllen += len;                                         \
-			inlen -= len;                                          \
+		if (inlen > 0) {                                        \
-			if (inlen < 0)                                         \
+			if (str)                                        \
-				inlen = 0;                                     \
+				str += len;                             \
-		}                                                              \
+			inlen -= len;                                   \
+			if (inlen < 0)                                  \
+				inlen = 0;                              \
+		}                                                       \
 	} while (0);
 extern int parse_idmaps(const char *idmap, char *type, unsigned long *nsid,