fatelf-exec: Execute FatElf binaries

Add the ability to execute fatelf binaries from a memfd.
OpenBSD support may be possible with shm_mkstemp.
A future improvement may be to read into a mmap of the memfd.
Use systemd-binfmt to tell the kernel how to execute FatElf binaries.

Author: Henry Wilson

CMakeLists.txt

@@ -1,6 +1,9 @@
 cmake_minimum_required(VERSION 3.0.0)
 project(FatELF)
 
+include(CheckSymbolExists)
+include(GNUInstallDirs)
+
 execute_process(
     COMMAND git rev-list HEAD~..
     WORKING_DIRECTORY "${CMAKE_CURRENT_SOURCE_DIR}"
@@ -37,5 +40,18 @@ add_fatelf_executable(fatelf-verify)
 add_fatelf_executable(fatelf-split)
 add_fatelf_executable(fatelf-validate)
 
+set(CMAKE_REQUIRED_DEFINITIONS -D_GNU_SOURCE)
+check_symbol_exists(memfd_create sys/mman.h MEMFD)
+if(MEMFD)
+    add_fatelf_executable(fatelf-exec)
+
+    find_program(BINFMT systemd-binfmt PATHS /usr/lib/systemd)
+    if(BINFMT)
+        configure_file(fatelf.conf.in fatelf.conf)
+        install(DIRECTORY DESTINATION ${CMAKE_INSTALL_LIBDIR}/binfmt.d)
+        install(FILES ${CMAKE_BINARY_DIR}/fatelf.conf DESTINATION ${CMAKE_INSTALL_LIBDIR}/binfmt.d/)
+    endif()
+endif()
+
 # end of CMakeLists.txt ...
 

fatelf.conf.in

@@ -0,0 +1 @@
+:fatelf-v1:M::\xFA\x70\x0E\x1F\x01\x00::${CMAKE_INSTALL_FULL_BINDIR}/fatelf-exec:

utils/fatelf-exec.c

@@ -0,0 +1,42 @@
+/**
+ * FatELF; support multiple ELF binaries in one file.
+ *
+ * Please see the file LICENSE.txt in the source's root directory.
+ *
+ *  This file written by Henry W. Wilson.
+ */
+
+#define _GNU_SOURCE
+#define FATELF_UTILS 1
+#include "fatelf-utils.h"
+#include <sys/mman.h>
+#include <sys/utsname.h>
+#include <unistd.h>
+
+int main(int argc, char *argv[], char *envp[])
+{
+    xfatelf_init(argc, (char const**)argv);
+    if (argc < 2)
+        xfail("USAGE: %s <in> [args...]", argv[0]);
+
+    const int fd = xopen(argv[1], O_RDONLY | O_CLOEXEC, 0755);
+    FATELF_header *header = xread_fatelf_header(argv[1], fd);
+    struct utsname utsn;
+    uname(&utsn);
+    const int recidx = xfind_fatelf_record(header, utsn.machine);
+    const int outfd = memfd_create("ELF", MFD_CLOEXEC);
+
+    if (outfd < 0)
+        xfail("memfd_create");
+
+    const FATELF_record *rec = &header->records[recidx];
+
+    xcopyfile_range(argv[1], fd, "memfd:ELF", outfd, rec->offset, rec->size);
+    xappend_junk(argv[1], fd, "memfd:ELF", outfd, header);
+
+    fexecve(outfd, (argv + 1), envp);
+    xfail("fexecve");
+    return EXIT_FAILURE;
+} // main
+
+// end of fatelf-exec.c ...