Fix symbolic mode parsing in parsemode

I found quite a lot of bugs, so I ended up pretty much rewriting as I
followed the spec¹.

Now, it works as follows:

- Determine a mask (who) of bits that can be modified for the subsequent
  operations. If none are specified, select all file mode bits.
- In a loop, determine which operation (+, -, =) to apply.
- If the next character is a permcopy (u, g, o), set the new permissions
  (perm) corresponding to the bits set in the user, group or other parts
  of the existing mode.
- Otherwise, set the new permissions by looping through the next r, w,
  x, s, t characters.
- Now, the set of bits we want to add or remove is (who & perm). Set or
  remove these bits according the the operation (first clearing the
  appropriate bits if the operation is =).
- Continue from the top if the next character is a comma, otherwise,
  process the next operation.

I tested this on some made up inputs, and I believe it is working
correctly now:

  parsemode("g+w", 0644, 0),    before: 0606, now: 0664, fixed
  parsemode("u+rx", 0222, 0),   before: 0077, now: 0722, fixed
  parsemode("+x", 0644, 023),   before: 0754, now: 0754, still works
  parsemode("+w", 0444, 022),   before: 0644, now: 0644, still works
  parsemode("+w", 0444, 0),     before: 0666, now: 0666, still works
  parsemode("+s", 0755, 0),     before: 0755, now: 6755, fixed
  parsemode("u+s", 0755, 0),    before: 0055, now: 4755, fixed
  parsemode("g+s", 0755, 0),    before: 0705, now: 2755, fixed
  parsemode("g=u", 0700, 0),    before: 0000, now: 0770, fixed
  parsemode("go=u-w", 0700, 0), before: 0000, now: 0755, fixed
  parsemode("o+u-g", 0641, 0),  before: 0000, now: 0643, fixed
  parsemode("u=rx,o+w,g-r", 0654, 0) before: error, now: 0516, fixed
  parsemode(",", 0654, 0), before: error, now: error, still works

¹ http://pubs.opengroup.org/onlinepubs/9699919799/utilities/chmod.html

util/mode.c

@@ -16,9 +16,9 @@ mode_t
 parsemode(const char *str, mode_t mode, mode_t mask)
 {
 	char *end;
-	const char *p;
+	const char *p = str;
-	int octal, op = '+';
+	int octal, op;
-	mode_t gmask = 0, m = 0;
+	mode_t who, perm, clear;
 
 	octal = strtol(str, &end, 8);
 	if(*end == '\0') {
@@ -41,61 +41,122 @@ parsemode(const char *str, mode_t mode, mode_t mask)
 		if(octal & 00001) mode |= S_IXOTH;
 		return mode;
 	}
-	for(p = str; *p; p++) {
+next:
+	/* first, determine which bits we will be modifying */
+	for(who = 0; *p; p++) {
 		switch(*p) {
 		/* masks */
 		case 'u':
-			gmask |= S_IRWXU;
+			who |= S_IRWXU|S_ISUID;
-			break;
+			continue;
 		case 'g':
-			gmask |= S_IRWXG;
+			who |= S_IRWXG|S_ISGID;
-			break;
+			continue;
 		case 'o':
-			gmask |= S_IRWXO;
+			who |= S_IRWXO;
-			break;
+			continue;
 		case 'a':
-			gmask |= S_IRWXU|S_IRWXG|S_IRWXO;
+			who |= S_IRWXU|S_ISUID|S_IRWXG|S_ISGID|S_IRWXO;
-			break;
+			continue;
+		}
+		break;
+	}
+	if(who) {
+		clear = who;
+	} else {
+		clear = S_ISUID|S_ISGID|S_ISVTX|S_IRWXU|S_IRWXG|S_IRWXO;
+		who = ~mask;
+	}
+	while(*p) {
+		switch(*p) {
 		/* opers */
 		case '=':
 		case '+':
 		case '-':
 			op = (int)*p;
 			break;
-		/* modes */
-		case 'r':
-			m |= S_IRUSR|S_IRGRP|S_IROTH;
-			break;
-		case 'w':
-			m |= S_IWUSR|S_IWGRP|S_IWOTH;
-			break;
-		case 'x':
-			m |= S_IXUSR|S_IXGRP|S_IXOTH;
-			break;
-		case 's':
-			m |= S_ISUID|S_ISGID;
-			break;
-		case 't':
-			m |= S_ISVTX;
-			break;
 		default:
 			eprintf("%s: invalid mode\n", str);
 			return -1;
 		}
+
+		perm = 0;
+		switch(*++p) {
+		/* copy */
+		case 'u':
+			if(mode & S_IRUSR)
+					perm |= S_IRUSR|S_IRGRP|S_IROTH;
+			if(mode & S_IWUSR)
+					perm |= S_IWUSR|S_IWGRP|S_IWOTH;
+			if(mode & S_IXUSR)
+					perm |= S_IXUSR|S_IXGRP|S_IXOTH;
+			if(mode & S_ISUID)
+					perm |= S_ISUID|S_ISGID;
+			p++;
+			break;
+		case 'g':
+			if(mode & S_IRGRP)
+					perm |= S_IRUSR|S_IRGRP|S_IROTH;
+			if(mode & S_IWGRP)
+					perm |= S_IWUSR|S_IWGRP|S_IWOTH;
+			if(mode & S_IXGRP)
+					perm |= S_IXUSR|S_IXGRP|S_IXOTH;
+			if(mode & S_ISGID)
+					perm |= S_ISUID|S_ISGID;
+			p++;
+			break;
+		case 'o':
+			if(mode & S_IROTH)
+					perm |= S_IRUSR|S_IRGRP|S_IROTH;
+			if(mode & S_IWOTH)
+					perm |= S_IWUSR|S_IWGRP|S_IWOTH;
+			if(mode & S_IXOTH)
+					perm |= S_IXUSR|S_IXGRP|S_IXOTH;
+			p++;
+			break;
+		default:
+			for(; *p; p++) {
+				switch(*p) {
+				/* modes */
+				case 'r':
+					perm |= S_IRUSR|S_IRGRP|S_IROTH;
+					break;
+				case 'w':
+					perm |= S_IWUSR|S_IWGRP|S_IWOTH;
+					break;
+				case 'x':
+					perm |= S_IXUSR|S_IXGRP|S_IXOTH;
+					break;
+				case 's':
+					perm |= S_ISUID|S_ISGID;
+					break;
+				case 't':
+					perm |= S_ISVTX;
+					break;
+				default:
+					goto apply;
+				}
+			}
+		}
+
+	apply:
 		/* apply */
 		switch(op) {
+		case '=':
+			mode &= ~clear;
+			/* fallthrough */
 		case '+':
-			mode |= (m & ((~mask) & 0777));
+			mode |= perm & who;
 			break;
 		case '-':
-			mode &= (~(m & ((~mask) & 0777)));
+			mode &= ~(perm & who);
-			break;
-		case '=':
-			mode = (m & ((~mask) & 0777));
 			break;
 		}
+		/* if we hit a comma, move on to the next clause */
+		if(*p == ',') {
+			p++;
+			goto next;
+		}
 	}
-	if(gmask && op != '=')
-		mode &= ~gmask;
 	return mode;
 }