1 files changed, 136 insertions, 23 deletions

diff --git a/src/misc/realpath.c b/src/misc/realpath.c
index d2708e59..db8b74dc 100644
--- a/src/misc/realpath.c
+++ b/src/misc/realpath.c
@@ -1,43 +1,156 @@
 #include <stdlib.h>
 #include <limits.h>
-#include <sys/stat.h>
-#include <fcntl.h>
 #include <errno.h>
 #include <unistd.h>
 #include <string.h>
-#include "syscall.h"
+
+static size_t slash_len(const char *s)
+{
+	const char *s0 = s;
+	while (*s == '/') s++;
+	return s-s0;
+}
 
 char *realpath(const char *restrict filename, char *restrict resolved)
 {
-	int fd;
-	ssize_t r;
-	struct stat st1, st2;
-	char buf[15+3*sizeof(int)];
-	char tmp[PATH_MAX];
+	char stack[PATH_MAX+1];
+	char output[PATH_MAX];
+	size_t p, q, l, l0, cnt=0, nup=0;
+	int check_dir=0;
 
 	if (!filename) {
 		errno = EINVAL;
 		return 0;
 	}
+	l = strnlen(filename, sizeof stack);
+	if (!l) {
+		errno = ENOENT;
+		return 0;
+	}
+	if (l >= PATH_MAX) goto toolong;
+	p = sizeof stack - l - 1;
+	q = 0;
+	memcpy(stack+p, filename, l+1);
+
+	/* Main loop. Each iteration pops the next part from stack of
+	 * remaining path components and consumes any slashes that follow.
+	 * If not a link, it's moved to output; if a link, contents are
+	 * pushed to the stack. */
+restart:
+	for (; ; p+=slash_len(stack+p)) {
+		/* If stack starts with /, the whole component is / or //
+		 * and the output state must be reset. */
+		if (stack[p] == '/') {
+			check_dir=0;
+			nup=0;
+			q=0;
+			output[q++] = '/';
+			p++;
+			/* Initial // is special. */
+			if (stack[p] == '/' && stack[p+1] != '/')
+				output[q++] = '/';
+			continue;
+		}
+
+		char *z = __strchrnul(stack+p, '/');
+		l0 = l = z-(stack+p);
 
-	fd = sys_open(filename, O_PATH|O_NONBLOCK|O_CLOEXEC);
-	if (fd < 0) return 0;
-	__procfdname(buf, fd);
+		if (!l && !check_dir) break;
 
-	r = readlink(buf, tmp, sizeof tmp - 1);
-	if (r < 0) goto err;
-	tmp[r] = 0;
+		/* Skip any . component but preserve check_dir status. */
+		if (l==1 && stack[p]=='.') {
+			p += l;
+			continue;
+		}
 
-	fstat(fd, &st1);
-	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;
+		/* Copy next component onto output at least temporarily, to
+		 * call readlink, but wait to advance output position until
+		 * determining it's not a link. */
+		if (q && output[q-1] != '/') {
+			if (!p) goto toolong;
+			stack[--p] = '/';
+			l++;
+		}
+		if (q+l >= PATH_MAX) goto toolong;
+		memcpy(output+q, stack+p, l);
+		output[q+l] = 0;
+		p += l;
+
+		int up = 0;
+		if (l0==2 && stack[p-2]=='.' && stack[p-1]=='.') {
+			up = 1;
+			/* Any non-.. path components we could cancel start
+			 * after nup repetitions of the 3-byte string "../";
+			 * if there are none, accumulate .. components to
+			 * later apply to cwd, if needed. */
+			if (q <= 3*nup) {
+				nup++;
+				q += l;
+				continue;
+			}
+			/* When previous components are already known to be
+			 * directories, processing .. can skip readlink. */
+			if (!check_dir) goto skip_readlink;
+		}
+		ssize_t k = readlink(output, stack, p);
+		if (k==p) goto toolong;
+		if (!k) {
+			errno = ENOENT;
+			return 0;
+		}
+		if (k<0) {
+			if (errno != EINVAL) return 0;
+skip_readlink:
+			check_dir = 0;
+			if (up) {
+				while(q && output[q-1]!='/') q--;
+				if (q>1 && (q>2 || output[0]!='/')) q--;
+				continue;
+			}
+			if (l0) q += l;
+			check_dir = stack[p];
+			continue;
+		}
+		if (++cnt == SYMLOOP_MAX) {
+			errno = ELOOP;
+			return 0;
+		}
+
+		/* If link contents end in /, strip any slashes already on
+		 * stack to avoid /->// or //->/// or spurious toolong. */
+		if (stack[k-1]=='/') while (stack[p]=='/') p++;
+		p -= k;
+		memmove(stack+p, stack, k);
+
+		/* Skip the stack advancement in case we have a new
+		 * absolute base path. */
+		goto restart;
 	}
 
-	__syscall(SYS_close, fd);
-	return resolved ? strcpy(resolved, tmp) : strdup(tmp);
-err:
-	__syscall(SYS_close, fd);
+ 	output[q] = 0;
+
+	if (output[0] != '/') {
+		if (!getcwd(stack, sizeof stack)) return 0;
+		l = strlen(stack);
+		/* Cancel any initial .. components. */
+		p = 0;
+		while (nup--) {
+			while(l>1 && stack[l-1]!='/') l--;
+			if (l>1) l--;
+			p += 2;
+			if (p<q) p++;
+		}
+		if (q-p && stack[l-1]!='/') stack[l++] = '/';
+		if (l + (q-p) + 1 >= PATH_MAX) goto toolong;
+		memmove(output + l, output + p, q - p + 1);
+		memcpy(output, stack, l);
+		q = l + q-p;
+	}
+
+	if (resolved) return memcpy(resolved, output, q+1);
+	else return strdup(output);
+
+toolong:
+	errno = ENAMETOOLONG;
 	return 0;
 }