Subject: [bug#41189] [PATCH 0/3] Add Fakechroot engine for 'guix pack -RR'

From: Ludovic Courtès <[email-masked]>
Date: Tue, 12 May 2020 17:32:26 +0200

Hi Carlos,

Carlos O'Donell <[email-masked]> skribis:

> On 5/12/20 6:03 AM, Ludovic Courtès wrote:
>> One thing that won’t work is dlopen because our ‘--library-path’
>> argument is computed statically based on the RUNPATH of the wrapped
>> program.  So for instance if you try to load guile-readline.so from
>> Guile, it eventually fails because libreadline.so isn’t found
>> (libreadline.so is in the RUNPATH of guile-readline.so, but the loader
>> uses non-interposable calls here as well.)  Probably no simple solution
>> to that one.
>
> There is a simple solution. You need to write a dynamic loader audit module
> that handles la_objsearch() and inject your lookup path. See man 7 rtld-audit.
> The dynamic loader audit modules allow you to alter the loader's core behaviour
> with a plugin.

That’s a great idea, and it works like a charm.  Thank you!

Concretely, I can do:

  guix pack -RR -S /bin=bin -S /etc=etc guile guile-readline

and then, on the target machine:

  tar xf pack.tar.gz
  . ./etc/profile
  export GUIX_EXECUTION_ENGINE=fakechroot
  ./bin/guile -c '(use-modules (ice-9 readline))'

Neat!

Ludo’.

diff --git a/Makefile.am b/Makefile.am
index 6cd6e79cab..f65bf5f900 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -338,6 +338,7 @@ AUX_FILES =						\
   gnu/packages/aux-files/linux-libre/4.9-x86_64.conf	\
   gnu/packages/aux-files/linux-libre/4.4-i686.conf	\
   gnu/packages/aux-files/linux-libre/4.4-x86_64.conf	\
+  gnu/packages/aux-files/pack-audit.c			\
   gnu/packages/aux-files/run-in-namespace.c
 
 # Templates, examples.
diff --git a/gnu/packages/aux-files/pack-audit.c b/gnu/packages/aux-files/pack-audit.c
new file mode 100644
index 0000000000..374787e8b9
--- /dev/null
+++ b/gnu/packages/aux-files/pack-audit.c
@@ -0,0 +1,85 @@
+/* GNU Guix --- Functional package management for GNU
+   Copyright (C) 2020 Ludovic Courtès <[email-masked]>
+
+   This file is part of GNU Guix.
+
+   GNU Guix is free software; you can redistribute it and/or modify it
+   under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or (at
+   your option) any later version.
+
+   GNU Guix is distributed in the hope that it will be useful, but
+   WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.  */
+
+/* This file implements part of the GNU ld.so audit interface.  It is used by
+   the "fakechroot" engine of the 'guix pack -RR' wrappers to make sure the
+   loader looks for shared objects under the "fake" root directory.  */
+
+#define _GNU_SOURCE 1
+
+#include <link.h>
+
+#include <error.h>
+#include <stdlib.h>
+#include <string.h>
+#include <assert.h>
+
+/* The pseudo root directory and store that we are relocating to.  */
+static const char *root_directory;
+static char *store;
+
+/* The original store, "/gnu/store" by default.  */
+static const char original_store[] = "@STORE_DIRECTORY@";
+
+/* Like 'malloc', but abort if 'malloc' returns NULL.  */
+static void *
+xmalloc (size_t size)
+{
+  void *result = malloc (size);
+  assert (result != NULL);
+  return result;
+}
+
+unsigned int
+la_version (unsigned int v)
+{
+  if (v != LAV_CURRENT)
+    error (1, 0, "cannot handle interface version %u", v);
+
+  root_directory = getenv ("FAKECHROOT_BASE");
+  if (root_directory == NULL)
+    error (1, 0, "'FAKECHROOT_BASE' is not set");
+
+  store = xmalloc (strlen (root_directory) + sizeof original_store);
+  strcpy (store, root_directory);
+  strcat (store, original_store);
+
+  return v;
+}
+
+/* Return NAME, a shared object file name, relocated under STORE.  This
+   function is called by the loader whenever it looks for a shared object.  */
+char *
+la_objsearch (const char *name, uintptr_t *cookie, unsigned int flag)
+{
+  char *result;
+
+  if (strncmp (name, original_store,
+	       sizeof original_store - 1) == 0)
+    {
+      size_t len = strlen (name) - sizeof original_store
+	+ strlen (store) + 1;
+      result = xmalloc (len);
+      strcpy (result, store);
+      strcat (result, name + sizeof original_store - 1);
+    }
+  else
+    result = strdup (name);
+
+  return result;
+}
diff --git a/gnu/packages/aux-files/run-in-namespace.c b/gnu/packages/aux-files/run-in-namespace.c
index c56c35a510..5ed1c6a1bc 100644
--- a/gnu/packages/aux-files/run-in-namespace.c
+++ b/gnu/packages/aux-files/run-in-namespace.c
@@ -400,7 +400,7 @@ exec_with_loader (const char *store, int argc, char *argv[])
 
   char *loader = concat (store,
 			 PROGRAM_INTERPRETER + sizeof "@STORE_DIRECTORY@");
-  size_t loader_specific_argc = 6;
+  size_t loader_specific_argc = 8;
   size_t loader_argc = argc + loader_specific_argc;
   char *loader_argv[loader_argc + 1];
   loader_argv[0] = argv[0];
@@ -409,7 +409,10 @@ exec_with_loader (const char *store, int argc, char *argv[])
   loader_argv[3] = "--preload";
   loader_argv[4] = concat (store,
 			   FAKECHROOT_LIBRARY + sizeof "@STORE_DIRECTORY@");
-  loader_argv[5] = concat (store,
+  loader_argv[5] = "--audit";
+  loader_argv[6] = concat (store,
+			   LOADER_AUDIT_MODULE + sizeof "@STORE_DIRECTORY@");
+  loader_argv[7] = concat (store,
 			   "@WRAPPED_PROGRAM@" + sizeof "@STORE_DIRECTORY@");
 
   for (size_t i = 0; i < argc; i++)
diff --git a/guix/scripts/pack.scm b/guix/scripts/pack.scm
index 2b37bf5027..13ede581e9 100644
--- a/guix/scripts/pack.scm
+++ b/guix/scripts/pack.scm
@@ -681,6 +681,9 @@ last resort for relocation."
   (define runner
     (local-file (search-auxiliary-file "run-in-namespace.c")))
 
+  (define audit-source
+    (local-file (search-auxiliary-file "pack-audit.c")))
+
   (define (proot)
     (specification->package "proot-static"))
 
@@ -691,6 +694,21 @@ last resort for relocation."
                                    "/lib/fakechroot/libfakechroot.so")
                                 #$output)))
 
+  (define (audit-module)
+    (computed-file "pack-audit.so"
+                   (with-imported-modules '((guix build utils))
+                     #~(begin
+                         (use-modules (guix build utils))
+
+                         (copy-file #$audit-source "audit.c")
+                         (substitute* "audit.c"
+                           (("@STORE_DIRECTORY@")
+                            (%store-directory)))
+
+                         (invoke #$compiler "-std=gnu99"
+                                 "-shared" "-fPIC" "-Os" "-g0"
+                                 "-Wall" "audit.c" "-o" #$output)))))
+
   (define build
     (with-imported-modules (source-module-closure
                             '((guix build utils)
@@ -771,6 +789,8 @@ last resort for relocation."
                                  (string-append "-DFAKECHROOT_LIBRARY=\""
                                                 #$(fakechroot-library) "\"")
 
+                                 (string-append "-DLOADER_AUDIT_MODULE=\""
+                                                #$(audit-module) "\"")
                                  (if gconv
                                      (string-append "-DGCONV_DIRECTORY=\""
                                                     gconv "\"")