i#1973: musl: Support late injection (#7254)

Musl supplies no argument to constructors in .init section, thus it's
hard to retrieve environment variables without libc functions. Let's
depend on "environ" variable for now, which is relatively fine in
libdrpreload.so.

This fixes tests requiring late injection on musl, including common.fib,
common.nativeexec and common.nativeexec_{exe,bindnow}*.

Issue: #1973

core/unix/preload.c

@@ -139,13 +139,26 @@ take_over(const char *pname)
 }
 
 INITIALIZER_ATTRIBUTES int
-#if INIT_BEFORE_LIBC
+/* i#1973: Unlike glibc, musl calls constructors without any arguments, thus
+ * it's hard to retrieve envp without depending on any libc symbols.
+ * Retrieve it from stable ABI envrion, which is a simple bare pointer on musl.
+ * XXX: find a more portable way to retrieve environment variables, or detect
+ * libc type and choose the proper method at runtime.
+ */
+#ifdef MUSL
+_init(void)
+{
+    extern char **environ;
+    char **envp = environ;
+#else
+#    if INIT_BEFORE_LIBC
 _init(int argc, char *arg0, ...)
 {
     char **argv = &arg0, **envp = &argv[argc + 1];
-#else
+#    else
 _init(int argc, char **argv, char **envp)
 {
+#    endif
 #endif
     const char *name;
 #if VERBOSE_INIT_FINI
@@ -157,9 +170,11 @@ _init(int argc, char **argv, char **envp)
 
 #if VERBOSE
     {
+#    ifndef MUSL
         int i;
         for (i = 0; i < argc; i++)
             fprintf(stderr, "\targ %d = %s\n", i, argv[i]);
+#    endif
         fprintf(stderr, "env 0 is %s\n", envp[0]);
         fprintf(stderr, "env 1 is %s\n", envp[1]);
         fprintf(stderr, "env 2 is %s\n", envp[2]);