Add basic support for $ORIGIN rpath expansion

This only supports absolute paths, which is enough for ghc, but won't be enough
for other uses. Perhaps it would be better to support it in glibc instead?

Index: hurd-debian/exec/exec.c
===================================================================
--- hurd-debian.orig/exec/exec.c
+++ hurd-debian/exec/exec.c
@@ -890,7 +890,7 @@ do_exec (file_t file,
     secure = (flags & EXEC_SECURE);
     defaults = (flags & EXEC_DEFAULTS);
 
-    /* Now record the big blocks of data we shuffle around unchanged.
+    /* Now record the big blocks of data we shuffle around.
        Whatever arrived inline, we must allocate space for so it can
        survive after this RPC returns.  */
 
@@ -901,11 +901,91 @@ do_exec (file_t file,
       goto stdout;
     boot->argv = argv;
     boot->argvlen = argvlen;
-    envp = servercopy (envp, envplen, envp_copy, &e.error);
-    if (e.error)
-      goto stdout;
+
+    if (filename && filename[0] == '/')
+      {
+	/* Explicit absolute filename, put its dirname in the LD_ORIGIN_PATH
+	   environment variable for $ORIGIN rpath expansion.
+	   XXX: thus does not work with relative paths.  */
+	const char *end = strrchr (filename, '/');
+	size_t pathlen;
+	const char ld_origin_s[] = "\0LD_ORIGIN_PATH=";
+	const char *existing;
+	size_t existing_len = 0;
+	size_t new_envplen;
+	char *new_envp;
+
+	/* Drop trailing slashes.  */
+	while (end > filename && end[-1] == '/')
+	  end--;
+
+	if (end == filename)
+	  /* Root, keep explicit heading/trailing slash.   */
+	  end++;
+
+	pathlen = end - filename;
+
+	if (memcmp (envp, ld_origin_s + 1, sizeof (ld_origin_s) - 2) == 0)
+	  /* Existing variable at the beginning of envp.  */
+	  existing = envp - 1;
+	else
+	  /* Look for the definition.  */
+	  existing = memmem (envp, envplen, ld_origin_s, sizeof (ld_origin_s) - 1);
+
+	if (existing)
+	  {
+	    /* Definition already exists, just replace the content.  */
+	    existing += sizeof (ld_origin_s) - 1;
+	    existing_len = strnlen (existing, envplen - (existing - envp));
+
+	    /* Allocate room for the new content.  */
+	    new_envplen = envplen - existing_len + pathlen;
+	    new_envp = mmap (0, new_envplen,
+			     PROT_READ|PROT_WRITE, MAP_ANON, 0, 0);
+	    if (new_envp == MAP_FAILED)
+	      {
+		e.error = errno;
+		goto stdout;
+	      }
+
+	    /* And copy.  */
+	    memcpy (new_envp, envp, existing - envp);
+	    memcpy (new_envp + (existing - envp), filename, pathlen);
+	    memcpy (new_envp + (existing - envp) + pathlen,
+		    existing + existing_len,
+		    envplen - ((existing - envp) + existing_len));
+	  }
+	else
+	  {
+	    /* No existing definition, prepend one.  */
+	    new_envplen = sizeof (ld_origin_s) - 1 + pathlen + envplen;
+	    new_envp = mmap (0, new_envplen,
+			     PROT_READ|PROT_WRITE, MAP_ANON, 0, 0);
+
+	    memcpy (new_envp, ld_origin_s + 1, sizeof (ld_origin_s) - 2);
+	    memcpy (new_envp + sizeof (ld_origin_s) - 2, filename, pathlen);
+	    new_envp [sizeof (ld_origin_s) - 2 + pathlen] = 0;
+	    memcpy (new_envp + sizeof (ld_origin_s) - 2 + pathlen + 1, envp, envplen);
+	  }
+
+	if (! envp_copy)
+	  /* Deallocate original environment */
+	  munmap (envp, envplen);
+
+	envp = new_envp;
+	envplen = new_envplen;
+      }
+    else
+      {
+	/* No explicit filename, just copy the existing environment */
+	envp = servercopy (envp, envplen, envp_copy, &e.error);
+	if (e.error)
+	  goto stdout;
+      }
+
     boot->envp = envp;
     boot->envplen = envplen;
+
     dtable = servercopy (dtable, dtablesize * sizeof (mach_port_t),
 			 dtable_copy, &e.error);
     if (e.error)