feature or "feature?" one of my favorite "features" is espfix/espfix64
One of the coolest things the UNIX API can do, but most people don't know: It's possible to send file handles from one process to another with UNIX domain sockets. (Very nice when combined with shm_open().) Reply with your favorite OS API features.
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I quite like weird low-level features as well like ioperm/iopl/modify_ldt/personality. Unfortunately I'm not aware of any major users of these syscalls on Linux (interestingly, many of these got abused on Windows where they're even more undocumented and unsupported)
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memfd_create is pretty cool, use with fexecve for launching ELFs from a buffer
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#!/usr/bin/python3 import ctypes,os memfd_create = {'x86_64': 319} libc = ctypes.CDLL(None) arch = os.uname()[-1] fd_in_mem = libc.syscall(memfd_create[arch],"tempmem",0) file_like = os.fdopen(fd_in_mem, 'wb') file_like.write(elf_bytes) os.execve(fd_in_mem, (new_name,), new_env)
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That when you fork(), you share the same file pointer for open files - a seek() in one process moves the pointer in the other.
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Fork/exec is pretty cool, if a boring answer.
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Using Unix domain sockets again: the ability to query the credentials of the peer (getpeereid, getsockopt with SO_PEERCRED, getpeerucred), so a server can perform fine-grained access control using the euid or egid of the client. This is how you gain privileges without suid.
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kqueue - epoll done right, two years early
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BSD open(). When you forget the 3rd variadic argument to pair with O_CREAT, you get a new file with random permissions pulled from the stack and no compiler error from gcc.
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