From 283bf91a0e411cd2a1b8b9212752aa7841ca77c7 Mon Sep 17 00:00:00 2001
From: Laslo Hunhold <dev@frign.de>
Date: Wed, 21 Jun 2017 10:27:17 +0200
Subject: [PATCH] Add normabspath() to replace realpath(), making quark work
 with musl

Compiled against musl, quark will not work as musl needs the presence
of procfs to process paths in realpath().
We could wait for it to be implemented[0] or also notice that we don't
want to overengineer the target-resolving. I don't think it's very
suckless if we deploy such a huge infrastructure to resolve paths.

To counteract this and given there are no good solutions available, I
set out to write the function normabspath(), which normalizes an
absolute path.
It is idempotent and works on the buffer passed to it. We don't need a
target, as the resulting resolved path is guaranteed to be of equal
length or shorter. This requires a memcpy in our case before calling it,
but I see it as a nice demonstration of the possibilities and it might
prove to be useful for other projects.

Not requiring a target buffer (that needs to have its length specified),
the one-string-call also simplifies the calling semantics drasticly.

With this function in place, quark works with musl. Statically linked,
stripped and with -Os, it only weighs 102K.

[0]: http://www.openwall.com/lists/musl/2016/11/03/5
---
 quark.c | 46 ++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 44 insertions(+), 2 deletions(-)

diff --git a/quark.c b/quark.c
index dbfbd08..5fcf004 100644
--- a/quark.c
+++ b/quark.c
@@ -456,6 +456,47 @@ sendfile(int fd, char *name, struct request *r, struct stat *st, char *mime,
 	return s;
 }
 
+static int
+normabspath(char *path)
+{
+	size_t len;
+	int done = 0;
+	char *p, *q, *lastp;
+
+	/* force and skip first slash */
+	if (path[0] != '/') {
+		return 1;
+	}
+	p = path + 1;
+
+	/* get length of path */
+	len = strlen(p);
+
+	for (lastp = p; !done; ) {
+		/* bound path component within (p,q) */
+		if (!(q = strchr(p, '/'))) {
+			q = strchr(p, '\0');
+			done = 1;
+		}
+
+		if (p == q || (q - p == 1 && p[0] == '.')) {
+			/* "/" or "./" */
+			memcpy(p, q + 1, len - ((q + 1) - path) + 2);
+			len -= (q + 1) - p;
+		} else if (q - p == 2 && p[0] == '.' && p[1] == '.') {
+			/* "../" */
+			memcpy(lastp, q + 1, len - ((q + 1) - path) + 2);
+			len -= (q + 1) - lastp;
+			p = lastp;
+		} else {
+			lastp = p;
+			p = q + 1;
+		}
+	}
+
+	return 0;
+}
+
 static enum status
 sendresponse(int fd, struct request *r)
 {
@@ -472,8 +513,9 @@ sendresponse(int fd, struct request *r)
 	}
 
 	/* normalize target */
-	if (!realpath(r->target, realtarget)) {
-		return sendstatus(fd, (errno == EACCES) ? S_FORBIDDEN : S_NOT_FOUND);
+	memcpy(realtarget, r->target, sizeof(realtarget));
+	if (normabspath(realtarget)) {
+		return sendstatus(fd, S_BAD_REQUEST);
 	}
 
 	/* reject hidden target */