Linux Edu Handbook

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**Capceiling** is a new system call added by [[IronPenguin:]] in order to support [[jail]] and other security measures.

For i386 and x86_64 systems, it is defined as:

  #define __NR_capceiling         318
  _syscall1(int, capceiling, unsigned int *, capceiling);

or
  int capceiling(unsigned int *caps);

The capceiling is the absolute limit of the capabilities a process or it's descendants may acquire by any means even if it exectes an suid-root program. Based on that definition, a process can only reduce it's capceiling through this system call. Any capabilities specified in caps that are currently prohibited will be silently masked out.

caps is currently a pointer to an unsigned 32 bit integer bitmask of capabilities. ''include/capceiling.h'' will specify the bitmask of each capability.

On successful return (retval == 0), caps will be set to the new actual capceiling of a process. 

eaxmples:
<code>
  #include <capceiling.h>
  
  int caps = CAP_ALL;
  int ret;

  ret = capceiling(&caps);
  printf("Current capceiling = 0x%08x\n", caps);

  return 0;
</code>

This works since any of CAP_ALL not currently permitted will simply be masked out. 

Capceiling does NOT grant a capability. Having a capability set in a process's capceiling only means it can potentially be granted that capability. For that reason, init is started with capceiling == CAP_ALL. If capceiling is ignored, all processes will inherit that and will behave no differently than a system that doesn't have capceiling at all.

If, instead, caps is assigned:

  int caps = CAP_NET_BIND;

Then as a result, that process and it's children could potentially bind to a privileged port if granted that capability through [[fscaps]], setuid-root, su, or sudo, but can never gain any other capability. Notably, after this, even ''ping'' won't work (as ping requires **CAP_NET_RAW**, that's why ping is usually suid-root).