c - seccomp --- how to EXIT_SUCCESS?

Question

Ηow to EXIT_SUCCESS after strict mode seccomp is set. Is it the correct practice, to call syscall(SYS_exit, EXIT_SUCCESS); at the end of main?

#include <stdlib.h>
#include <unistd.h> 
#include <sys/prctl.h>     
#include <linux/seccomp.h> 
#include <sys/syscall.h>

int main(int argc, char **argv) {
  prctl(PR_SET_SECCOMP, SECCOMP_MODE_STRICT);

  //return EXIT_SUCCESS; // does not work
  //_exit(EXIT_SUCCESS); // does not work
  // syscall(__NR_exit, EXIT_SUCCESS); // (EDIT) This works! Is this the ultimate answer and the right way to exit success from seccomp-ed programs?
  syscall(SYS_exit, EXIT_SUCCESS); // (EDIT) works; SYS_exit equals __NR_exit
}

// gcc seccomp.c -o seccomp && ./seccomp; echo "${?}" # I want 0

Answer 1

As explained in eigenstate.org and in SECCOMP (2):

The only system calls that the calling thread is permitted to make are read(2), write(2), _exit(2) (but not exit_group(2)), and sigreturn(2). Other system calls result in the delivery of a SIGKILL signal.

As a result, one would expect _exit() to work, but it's a wrapper function that invokes exit_group(2) which is not allowed in strict mode ([1], [2]), thus the process gets killed.

It's even reported in exit(2) - Linux man page:

In glibc up to version 2.3, the _exit() wrapper function invoked the kernel system call of the same name. Since glibc 2.3, the wrapper function invokes exit_group(2), in order to terminate all of the threads in a process.

Same happens with the return statement, which should end up in killing your process, in the very similar manner with _exit().

Stracing the process will provide further confirmation (to allow this to show up, you have to not set PR_SET_SECCOMP; just comment prctl()) and I got similar output for both non-working cases:

linux12:/home/users/grad1459>gcc seccomp.c -o seccomp
linux12:/home/users/grad1459>strace ./seccomp
execve("./seccomp", ["./seccomp"], [/* 24 vars */]) = 0
brk(0)                                  = 0x8784000
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
mmap2(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb775f000
access("/etc/ld.so.preload", R_OK)      = -1 ENOENT (No such file or directory)
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
fstat64(3, {st_mode=S_IFREG|0644, st_size=97472, ...}) = 0
mmap2(NULL, 97472, PROT_READ, MAP_PRIVATE, 3, 0) = 0xb7747000
close(3)                                = 0
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
open("/lib/i386-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
read(3, "177ELF1113312202261004"..., 512) = 512
fstat64(3, {st_mode=S_IFREG|0755, st_size=1730024, ...}) = 0
mmap2(NULL, 1739484, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0xdd0000
mmap2(0xf73000, 12288, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1a3) = 0xf73000
mmap2(0xf76000, 10972, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0xf76000
close(3)                                = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb7746000
set_thread_area({entry_number:-1 -> 6, base_addr:0xb7746900, limit:1048575, seg_32bit:1, contents:0, read_exec_only:0, limit_in_pages:1, seg_not_present:0, useable:1}) = 0
mprotect(0xf73000, 8192, PROT_READ)     = 0
mprotect(0x8049000, 4096, PROT_READ)    = 0
mprotect(0x16e000, 4096, PROT_READ)     = 0
munmap(0xb7747000, 97472)               = 0
exit_group(0)                           = ?
linux12:/home/users/grad1459>

As you can see, exit_group() is called, explaining everything!

---

Now as you correctly stated, "SYS_exit equals __NR_exit"; for example it's defined in mit.syscall.h:

#define SYS_exit __NR_exit

so the last two calls are equivalent, i.e. you can use the one you like, and the output should be this:

linux12:/home/users/grad1459>gcc seccomp.c -o seccomp && ./seccomp ; echo "${?}" 
0

---

PS

You could of course define a filter yourself and use:

prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, filter);

as explained in the eigenstate link, to allow _exit() (or, strictly speaking, exit_group(2)), but do that only if you really need to and know what you are doing.