In __wasilibc_find_abspath, handle relative paths correctly (#632)

Previously, if you called __wasilibc_find_abspath("foo/a.txt"), with
"foo" being a preopened directory, it would return "foo" in abs_prefix
and "foo/a.txt" in relative_path. Fixed this so it correctly returns
"foo" in abs_prefix and "a.txt" in relative_path.

---------

Co-authored-by: Pat Hickey <[email protected]>

Author: catamorphism

libc-bottom-half/sources/preopens.c

@@ -131,7 +131,11 @@ static int internal_register_preopened_fd(filesystem_preopens_own_descriptor_t f
 /// Are the `prefix_len` bytes pointed to by `prefix` a prefix of `path`?
 static bool prefix_matches(const uint8_t *prefix, size_t prefix_len, const char *path) {
     // Allow an empty string as a prefix of any relative path.
-    if (path[0] != '/' /* && prefix_len == 0 */)
+    if (path[0] != '/' && prefix_len == 0)
+        return true;
+
+    // Allow a '/' as a prefix of any relative path.
+    if (path[0] != '/' && prefix_len == 1 && prefix[0] == '/')
         return true;
 
     // Check whether any bytes of the prefix differ.
@@ -212,6 +216,10 @@ int __wasilibc_find_abspath(const char *path,
 
     // The relative path is the substring after the portion that was matched.
     const char *computed = path;
+    if (prefix_matches((const uint8_t*) *abs_prefix, match_len, path)
+        && !((*abs_prefix[0] == '/') && (path[0] != '/')))
+        computed = path + match_len;
+
     if (path[0] == '/')
         computed = path + match_len;
 

test/src/misc/open_relative_path.c

@@ -0,0 +1,50 @@
+//! add-flags.py(RUN): --dir fs::foo
+//! add-flags.py(ARGS): foo
+
+#include <errno.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+#include <sys/stat.h>
+#include "test.h"
+
+#define TEST(c) do { \
+	errno = 0; \
+	if (!(c)) \
+		t_error("%s failed (errno = %d)\n", #c, errno); \
+} while(0)
+
+int main(int argc, char **argv)
+{
+    TEST(argc == 2);
+
+    char *filename1, *filename2, *filename3, *filename4, *filename5;
+    TEST(asprintf(&filename1, "%s/input.txt", argv[1]) != -1);
+
+    int fd = open(filename1, O_WRONLY | O_CREAT);
+    TEST(fd > 2);
+    close(fd);
+
+    TEST(asprintf(&filename2, "%s/dir1", argv[1]) != -1);
+    TEST(mkdir(filename2, 0755) == 0);
+    TEST(asprintf(&filename3, "%s/dir1/dir2", argv[1]) != -1);
+    TEST(mkdir(filename3, 0755) == 0);
+    TEST(asprintf(&filename4, "%s/dir1/dir2/dir3", argv[1]) != -1);
+    TEST(mkdir(filename4, 0755) == 0);
+
+    TEST(asprintf(&filename5, "%s/input.txt", filename4) != -1);
+
+    fd = open(filename5, O_WRONLY | O_CREAT);
+    TEST(fd > 2);
+    close(fd);
+
+    TEST(unlink(filename5)==0);
+    TEST(rmdir(filename4)==0);
+    TEST(rmdir(filename3)==0);
+    TEST(rmdir(filename2)==0);
+    TEST(unlink(filename1)==0);
+
+    return t_status;
+}