Fix a bug in linkat (#690)

This commit fixes a longstanding issue in wasi-libc where the `linkat`
function, when both parameters are `AT_FDCWD` or absolute paths, would
relativize the two input paths in such a way that the previous stomped
over the first, meaning that the wrong actual syscall was issued. This
fixes the problem in the same manner as other two-path-taking functions
which is to use `find_relpath` for one path and `find_relpath_alt` for
the other path.

Author: alexcrichton

libc-bottom-half/sources/posix.c

@@ -408,7 +408,7 @@ __wasilibc_link(const char *oldpath, const char *newpath, int flags)
     char *old_relative_path;
     char *new_relative_path;
     int old_dirfd = find_relpath(oldpath, &old_relative_path);
-    int new_dirfd = find_relpath(newpath, &new_relative_path);
+    int new_dirfd = find_relpath_alt(newpath, &new_relative_path);
 
     // If we can't find a preopen for it, fail as if we can't find the path.
     if (old_dirfd == -1 || new_dirfd == -1) {

test/src/link.c

@@ -25,9 +25,34 @@ int main(void)
         close(fd);
 
         TEST(link(tmp, tmp1)==0);
+        TEST(unlink(tmp1) != -1);
 
+        TEST(linkat(AT_FDCWD, tmp, AT_FDCWD, tmp1, 0)==0);
         TEST(unlink(tmp1) != -1);
+
         TEST(unlink(tmp) != -1);
 
+        char src[] = "/dir/a";
+        char dst[] = "/dir/b";
+
+        TEST(mkdir("dir", 0700) == 0);
+	TEST((fd = open(src, flags | O_WRONLY | O_CREAT | O_EXCL, 0600)) > 2);
+        close(fd);
+
+        TEST(link(src, dst)==0);
+        TEST(unlink(dst) != -1);
+
+        TEST(linkat(AT_FDCWD, src, AT_FDCWD, dst, 0) == 0);
+        TEST(unlink(dst) != -1);
+
+        TEST(chdir("/dir") == 0);
+
+        TEST(linkat(AT_FDCWD, "a", AT_FDCWD, "b", 0) == 0);
+        TEST(unlink("b") != -1);
+
+        TEST(chdir("/") == 0);
+
+        TEST(unlink(src) != -1);
+
 	return t_status;
 }