Embed the sandbox shell into the statically linked 'nix' binary

With this, Nix will write a copy of the sandbox shell to /bin/sh in
the sandbox rather than bind-mounting it from the host filesystem.
This makes /bin/sh work out of the box with nix-static, i.e. you no
longer get

  /nix/store/qa36xhc5gpf42l3z1a8m1lysi40l9p7s-bootstrap-stage4-stdenv-linux/setup: ./configure: /bin/sh: bad interpreter: No such file or directory

Makefile.config.in

@@ -1,4 +1,3 @@
-HOST_OS = @host_os@
 AR = @AR@
 BDW_GC_LIBS = @BDW_GC_LIBS@
 BOOST_LDFLAGS = @BOOST_LDFLAGS@
@@ -13,13 +12,14 @@ ENABLE_S3 = @ENABLE_S3@
 GTEST_LIBS = @GTEST_LIBS@
 HAVE_LIBCPUID = @HAVE_LIBCPUID@
 HAVE_SECCOMP = @HAVE_SECCOMP@
+HOST_OS = @host_os@
 LDFLAGS = @LDFLAGS@
 LIBARCHIVE_LIBS = @LIBARCHIVE_LIBS@
 LIBBROTLI_LIBS = @LIBBROTLI_LIBS@
 LIBCURL_LIBS = @LIBCURL_LIBS@
+LIBSECCOMP_LIBS = @LIBSECCOMP_LIBS@
 LOWDOWN_LIBS = @LOWDOWN_LIBS@
 OPENSSL_LIBS = @OPENSSL_LIBS@
-LIBSECCOMP_LIBS = @LIBSECCOMP_LIBS@
 PACKAGE_NAME = @PACKAGE_NAME@
 PACKAGE_VERSION = @PACKAGE_VERSION@
 SHELL = @bash@
@@ -31,6 +31,7 @@ datadir = @datadir@
 datarootdir = @datarootdir@
 doc_generate = @doc_generate@
 docdir = @docdir@
+embedded_sandbox_shell = @embedded_sandbox_shell@
 exec_prefix = @exec_prefix@
 includedir = @includedir@
 libdir = @libdir@

configure.ac

@@ -320,6 +320,14 @@ if test ${cross_compiling:-no} = no && ! test -z ${sandbox_shell+x}; then
   fi
 fi
 
+AC_ARG_ENABLE(embedded-sandbox-shell, AS_HELP_STRING([--enable-embedded-sandbox-shell],[include the sandbox shell in the Nix binary [default=no]]),
+  embedded_sandbox_shell=$enableval, embedded_sandbox_shell=no)
+AC_SUBST(embedded_sandbox_shell)
+if test "$embedded_sandbox_shell" = yes; then
+  AC_DEFINE(HAVE_EMBEDDED_SANDBOX_SHELL, 1, [Include the sandbox shell in the Nix binary.])
+fi
+
+
 # Expand all variables in config.status.
 test "$prefix" = NONE && prefix=$ac_default_prefix
 test "$exec_prefix" = NONE && exec_prefix='${prefix}'

flake.nix

@@ -578,7 +578,11 @@
               doInstallCheck=1
             '';
 
-          configureFlags = [ "--sysconfdir=/etc" ];
+          configureFlags =
+            configureFlags ++
+            [ "--sysconfdir=/etc"
+              "--enable-embedded-sandbox-shell"
+            ];
 
           enableParallelBuilding = true;
 

src/libstore/build/local-derivation-goal.cc

@@ -1717,6 +1717,18 @@ void LocalDerivationGoal::runChild()
 
             for (auto & i : dirsInChroot) {
                 if (i.second.source == "/proc") continue; // backwards compatibility
+
+                #if HAVE_EMBEDDED_SANDBOX_SHELL
+                if (i.second.source == "__embedded_sandbox_shell__") {
+                    static unsigned char sh[] = {
+                        #include "embedded-sandbox-shell.gen.hh"
+                    };
+                    auto dst = chrootRootDir + i.first;
+                    createDirs(dirOf(dst));
+                    writeFile(dst, std::string_view((const char *) sh, sizeof(sh)));
+                    chmod_(dst, 0555);
+                } else
+                #endif
                     doBind(i.second.source, chrootRootDir + i.first, i.second.optional);
             }
 

src/libstore/local.mk

@@ -44,9 +44,19 @@ libstore_CXXFLAGS += \
  -DNIX_MAN_DIR=\"$(mandir)\" \
  -DLSOF=\"$(lsof)\"
 
+ifeq ($(embedded_sandbox_shell),yes)
+libstore_CXXFLAGS += -DSANDBOX_SHELL=\"__embedded_sandbox_shell__\"
+
+$(d)/build/local-derivation-goal.cc: $(d)/embedded-sandbox-shell.gen.hh
+
+$(d)/embedded-sandbox-shell.gen.hh: $(sandbox_shell)
+	$(trace-gen) hexdump -v -e '1/1 "0x%x," "\n"' < $< > $@.tmp
+	@mv $@.tmp $@
+else
 ifneq ($(sandbox_shell),)
 libstore_CXXFLAGS += -DSANDBOX_SHELL="\"$(sandbox_shell)\""
 endif
+endif
 
 $(d)/local-store.cc: $(d)/schema.sql.gen.hh $(d)/ca-specific-schema.sql.gen.hh