| Chang S. Bae | 1c33bb0 | 2021-05-18 13:03:17 -0700 | [diff] [blame] | 1 | .. SPDX-License-Identifier: GPL-2.0 |
| 2 | |
| 3 | ================================== |
| 4 | x86-specific ELF Auxiliary Vectors |
| 5 | ================================== |
| 6 | |
| 7 | This document describes the semantics of the x86 auxiliary vectors. |
| 8 | |
| 9 | Introduction |
| 10 | ============ |
| 11 | |
| 12 | ELF Auxiliary vectors enable the kernel to efficiently provide |
| 13 | configuration-specific parameters to userspace. In this example, a program |
| 14 | allocates an alternate stack based on the kernel-provided size:: |
| 15 | |
| 16 | #include <sys/auxv.h> |
| 17 | #include <elf.h> |
| 18 | #include <signal.h> |
| 19 | #include <stdlib.h> |
| 20 | #include <assert.h> |
| 21 | #include <err.h> |
| 22 | |
| 23 | #ifndef AT_MINSIGSTKSZ |
| 24 | #define AT_MINSIGSTKSZ 51 |
| 25 | #endif |
| 26 | |
| 27 | .... |
| 28 | stack_t ss; |
| 29 | |
| 30 | ss.ss_sp = malloc(ss.ss_size); |
| 31 | assert(ss.ss_sp); |
| 32 | |
| 33 | ss.ss_size = getauxval(AT_MINSIGSTKSZ) + SIGSTKSZ; |
| 34 | ss.ss_flags = 0; |
| 35 | |
| 36 | if (sigaltstack(&ss, NULL)) |
| 37 | err(1, "sigaltstack"); |
| 38 | |
| 39 | |
| 40 | The exposed auxiliary vectors |
| 41 | ============================= |
| 42 | |
| 43 | AT_SYSINFO is used for locating the vsyscall entry point. It is not |
| 44 | exported on 64-bit mode. |
| 45 | |
| 46 | AT_SYSINFO_EHDR is the start address of the page containing the vDSO. |
| 47 | |
| 48 | AT_MINSIGSTKSZ denotes the minimum stack size required by the kernel to |
| 49 | deliver a signal to user-space. AT_MINSIGSTKSZ comprehends the space |
| 50 | consumed by the kernel to accommodate the user context for the current |
| 51 | hardware configuration. It does not comprehend subsequent user-space stack |
| 52 | consumption, which must be added by the user. (e.g. Above, user-space adds |
| 53 | SIGSTKSZ to AT_MINSIGSTKSZ.) |