intercept: Teach libintercept to transmute sonames for Feral Interactive games

Internally libintercept has a number of magic tricks to prevent games from
replacing the system SDL libraries with vendored ones. However, during testing
with DiRT Rally, it was discovered that Feral Interactive have altered the
sonames of their libraries and linked to those, ensuring they never match
the system versions.

This however will lead to a mixed SDL2 stack for those disabling the Steam
runtime, and in turn forces older builds of SDL to be used. To combat this,
we intercept soname requests during early linking based on a simple pattern,
and if they don't match the expected target name, we'll rewrite the request
in place to force the linker to look for the real soname.

In combination with the existing lsi_blacklist_vendor function, this ensures
that a game looking for `libSDL2-2.0.5.so` will correctly load the system's
own `libSDL2-2.0.so.0`.

Signed-off-by: Ikey Doherty <ikey@solus-project.com>

src/intercept/main.c

@@ -230,10 +230,89 @@ char *lsi_search_steam(const char *name)
         return (char *)name;
 }
 
-char *lsi_blacklist_vendor(const char *name)
+/**
+ * Source identifier patterns that indicate a soname replacement is happening.
+ * Note that this is a basic pattern match for the soname and just allows us
+ * to lookup the transmute target
+ */
+static const char *vendor_transmute_source[] = {
+        "libSDL2-2.0.",
+        "libSDL2_image-2.0.",
+};
+
+/**
+ * The transmute target is the intended replacement for any source name, assuming
+ * that the soname doesn't identically match the currently requested soname.
+ * This allows us to do on the fly replacements for re-soname'd libraries.
+ */
+static const char *vendor_transmute_target[] = {
+        "libSDL2-2.0.so.0",
+        "libSDL2_image-2.0.so.0",
+};
+
+/**
+ * emit_replaced_lib is used to pretty-print the debug when we blacklist and
+ * transform a soname to a system name
+ */
+static void emit_replaced_name(const char *lib_name, const char *new_name)
+{
+        fprintf(stderr,
+                "\033[32;1m[LSI:%s]\033[0m: debug: transforming vendor soname: "
+                "\033[31;1m%s\033[0m -> \033[34;1m%s\033[0m\n",
+                matched_process,
+                lib_name,
+                new_name);
+}
+
+/**
+ * lsi_override_soname will deal with LA_SER_ORIG entries, i.e. the original
+ * soname request when the linker tries to load a library.
+ *
+ * Certain Steam games (notably Feral Interactive ports) use their own vendored
+ * SDL libraries with changed sonames, which are implicitly blacklisted by the
+ * lsi_blacklist_vendor function.
+ *
+ * As such, we intercept those renamed libraries, and convert their names back
+ * to the ABI stable system libraries on the fly.
+ */
+static bool lsi_override_soname(unsigned int flag, const char *orig_name, const char **soname)
+{
+        *soname = NULL;
+
+        /* We only need to deal with LA_SER_ORIG */
+        if ((flag & LA_SER_ORIG) != LA_SER_ORIG) {
+                return false;
+        }
+
+        for (size_t i = 0; i < ARRAY_SIZE(vendor_transmute_source); i++) {
+                if (!strstr(orig_name, vendor_transmute_source[i])) {
+                        continue;
+                }
+                /* Ensure we're not just replacing the same thing here as the
+                 * string would be identical, no real replacement would happen,
+                 * and ld will be confused about memory and die.
+                 */
+                if (streq(orig_name, vendor_transmute_target[i])) {
+                        continue;
+                }
+                *soname = vendor_transmute_target[i];
+                emit_replaced_name(orig_name, *soname);
+                return true;
+        }
+
+        return false;
+}
+
+char *lsi_blacklist_vendor(unsigned int flag, const char *name)
 {
         /* Find out if it exists */
         bool file_exists = lsi_file_exists(name);
+        const char *override_soname = NULL;
+
+        /* Find out if we have to rename some libraries on the fly */
+        if (lsi_override_soname(flag, name, &override_soname)) {
+                return (char *)override_soname;
+        }
 
         /* Find out if its a Steam private lib.. These are relative "./" files too! */
         if (name && (strstr(name, "/Steam/") || strncmp(name, "./", 2) == 0)) {
@@ -261,13 +340,13 @@ char *lsi_blacklist_vendor(const char *name)
  * as a private vendored lib
  */
 _nica_public_ char *la_objsearch(const char *name, __lsi_unused__ uintptr_t *cookie,
-                                 __lsi_unused__ unsigned int flag)
+                                 unsigned int flag)
 {
         switch (work_mode) {
         case INTERCEPT_MODE_STEAM:
                 return lsi_search_steam(name);
         case INTERCEPT_MODE_VENDOR_OFFENDER:
-                return lsi_blacklist_vendor(name);
+                return lsi_blacklist_vendor(flag, name);
         case INTERCEPT_MODE_NONE:
         default:
                 return (char *)name;