debloat realpath's allocation strategy

rather than allocating a PATH_MAX-sized buffer when the caller does
not provide an output buffer, work first with a PATH_MAX-sized temp
buffer with automatic storage, and either copy it to the caller's
buffer or strdup it on success. this not only avoids massive memory
waste, but also avoids pulling in free (and thus the full malloc
implementation) unnecessarily in static programs.

src/misc/realpath.c

@@ -5,6 +5,7 @@
 #include <fcntl.h>
 #include <errno.h>
 #include <unistd.h>
+#include <string.h>
 
 void __procfdname(char *, unsigned);
 
@@ -14,7 +15,7 @@ char *realpath(const char *restrict filename, char *restrict resolved)
 	ssize_t r;
 	struct stat st1, st2;
 	char buf[15+3*sizeof(int)];
-	int alloc = 0;
+	char tmp[PATH_MAX];
 
 	if (!filename) {
 		errno = EINVAL;
@@ -25,27 +26,20 @@ char *realpath(const char *restrict filename, char *restrict resolved)
 	if (fd < 0) return 0;
 	__procfdname(buf, fd);
 
-	if (!resolved) {
-		alloc = 1;
-		resolved = malloc(PATH_MAX);
-		if (!resolved) return 0;
-	}
-
-	r = readlink(buf, resolved, PATH_MAX-1);
+	r = readlink(buf, tmp, sizeof tmp - 1);
 	if (r < 0) goto err;
-	resolved[r] = 0;
+	tmp[r] = 0;
 
 	fstat(fd, &st1);
-	r = stat(resolved, &st2);
+	r = stat(tmp, &st2);
 	if (r<0 || st1.st_dev != st2.st_dev || st1.st_ino != st2.st_ino) {
 		if (!r) errno = ELOOP;
 		goto err;
 	}
 
 	close(fd);
-	return resolved;
+	return resolved ? strcpy(resolved, tmp) : strdup(tmp);
 err:
-	if (alloc) free(resolved);
 	close(fd);
 	return 0;
 }