.. SPDX-License-Identifier: GPL-2.0

================================== x86-specific ELF Auxiliary Vectors

This document describes the semantics of the x86 auxiliary vectors.

Introduction

ELF Auxiliary vectors enable the kernel to efficiently provide configuration-specific parameters to userspace. In this example, a program allocates an alternate stack based on the kernel-provided size::

#include <sys/auxv.h> #include <elf.h> #include <signal.h> #include <stdlib.h> #include <assert.h> #include <err.h>

#ifndef AT_MINSIGSTKSZ #define AT_MINSIGSTKSZ 51 #endif

.... stack_t ss;

ss.ss_sp = malloc(ss.ss_size); assert(ss.ss_sp);

ss.ss_size = getauxval(AT_MINSIGSTKSZ) + SIGSTKSZ; ss.ss_flags = 0;

if (sigaltstack(&ss, NULL)) err(1, "sigaltstack");

The exposed auxiliary vectors

AT_SYSINFO is used for locating the vsyscall entry point. It is not exported on 64-bit mode.

AT_SYSINFO_EHDR is the start address of the page containing the vDSO.

AT_MINSIGSTKSZ denotes the minimum stack size required by the kernel to deliver a signal to user-space. AT_MINSIGSTKSZ comprehends the space consumed by the kernel to accommodate the user context for the current hardware configuration. It does not comprehend subsequent user-space stack consumption, which must be added by the user. (e.g. Above, user-space adds SIGSTKSZ to AT_MINSIGSTKSZ.)