Change external GC to use directory at configure

This commit changes the external GC API to use `--with-shared-gc=DIR` at
configure time with a directory of the external GC and uses
`RUBY_GC_LIBRARY` environment variable to load the external GC at
runtime.

configure.ac

@@ -4751,7 +4751,7 @@ config_summary "target OS"           "$target_os"
 config_summary "compiler"            "$CC"
 config_summary "with thread"         "$THREAD_MODEL"
 config_summary "with coroutine"      "$coroutine_type"
-config_summary "with shared GC"      "$with_shared_gc"
+config_summary "with shared GC"      "$shared_gc_summary"
 config_summary "enable shared libs"  "$ENABLE_SHARED"
 config_summary "dynamic library ext" "$DLEXT"
 config_summary "CFLAGS"              "$cflags"

gc.c

@@ -748,14 +748,39 @@ typedef struct gc_function_map {
 
 static rb_gc_function_map_t rb_gc_functions;
 
-# define RUBY_GC_LIBRARY_PATH "RUBY_GC_LIBRARY_PATH"
+# define RUBY_GC_LIBRARY "RUBY_GC_LIBRARY"
 
 static void
 ruby_external_gc_init(void)
 {
-    char *gc_so_path = getenv(RUBY_GC_LIBRARY_PATH);
+    // Assert that the directory path ends with a /
+    GC_ASSERT(SHARED_GC_DIR[strlen(SHARED_GC_DIR) - 2] == '/');
+
+    char *gc_so_file = getenv(RUBY_GC_LIBRARY);
+
+    char *gc_so_path = NULL;
     void *handle = NULL;
-    if (gc_so_path && dln_supported_p()) {
+    if (gc_so_file && dln_supported_p()) {
+        /* Check to make sure that gc_so_file matches /[\w-_.]+/ so that it does
+         * not load a shared object outside of the directory. */
+        for (size_t i = 0; i < strlen(gc_so_file); i++) {
+            char c = gc_so_file[i];
+            if (isalnum(c)) continue;
+            switch (c) {
+              case '-':
+              case '_':
+              case '.':
+                break;
+              default:
+                rb_bug("Only alphanumeric, dash, underscore, and period is allowed in "RUBY_GC_LIBRARY"");
+            }
+        }
+
+        gc_so_path = alloca(strlen(SHARED_GC_DIR) + strlen(gc_so_file) + 1);
+        strcpy(gc_so_path, SHARED_GC_DIR);
+        strcpy(gc_so_path + strlen(SHARED_GC_DIR), gc_so_file);
+        gc_so_path[strlen(SHARED_GC_DIR) + strlen(gc_so_file)] = '\0';
+
         char error[1024];
         handle = dln_open(gc_so_path, error, sizeof(error));
         if (!handle) {
@@ -4625,8 +4650,8 @@ void
 Init_GC(void)
 {
 #if USE_SHARED_GC
-    if (getenv(RUBY_GC_LIBRARY_PATH) != NULL && !dln_supported_p()) {
-        rb_warn(RUBY_GC_LIBRARY_PATH " is ignored because this executable file can't load extension libraries");
+    if (getenv(RUBY_GC_LIBRARY) != NULL && !dln_supported_p()) {
+        rb_warn(RUBY_GC_LIBRARY " is ignored because this executable file can't load extension libraries");
     }
 #endif
 

tool/m4/ruby_shared_gc.m4

@@ -1,19 +1,36 @@
 dnl -*- Autoconf -*-
 AC_DEFUN([RUBY_SHARED_GC],[
 AC_ARG_WITH(shared-gc,
-    AS_HELP_STRING([--with-shared-gc],
-    [Enable replacement of Ruby's GC from a shared library.]),
-    [with_shared_gc=$withval], [unset with_shared_gc]
+    AS_HELP_STRING([--with-shared-gc=DIR],
+    [Enable replacement of Ruby's GC from a shared library in the specified directory.]),
+    [shared_gc_dir=$withval], [unset shared_gc_dir]
 )
 
-AC_SUBST([with_shared_gc])
 AC_MSG_CHECKING([if Ruby is build with shared GC support])
-AS_IF([test "$with_shared_gc" = "yes"], [
+AS_IF([test x"$shared_gc_dir" != x], [
     AC_MSG_RESULT([yes])
+
+    # Ensure that shared_gc_dir is always an absolute path so that Ruby
+    # never loads a shared GC from a relative path
+    AS_CASE(["$shared_gc_dir"],
+        [/*], [shared_gc_dir=$shared_gc_dir],
+        [shared_gc_dir=`pwd`/$shared_gc_dir]
+    )
+
+    # Ensure that shared_gc_dir always terminates with a /
+    AS_CASE(["$shared_gc_dir"],
+        [*/], [],
+        [shared_gc_dir="$shared_gc_dir/"]
+    )
+
     AC_DEFINE([USE_SHARED_GC], [1])
+    AC_DEFINE_UNQUOTED([SHARED_GC_DIR], "$shared_gc_dir")
+
+    shared_gc_summary="yes (in $shared_gc_dir)"
 ], [
     AC_MSG_RESULT([no])
-    with_shared_gc="no"
     AC_DEFINE([USE_SHARED_GC], [0])
+
+    shared_gc_summary="no"
 ])
 ])dnl