SIGSEGV 信号的捕获与 sigaltstack 系统调用

捕获段错误信号的一个简单程序示例：

#include <stdio.h>
#include <stdlib.h>
#include <signal.h>void segfault(int dummy) {printf("Help!\n");exit(1);
}int main() {int *p = 0;signal(SIGSEGV, segfault);*p = 17;return 0;
}

如果没有在 segfault 中添加 exit 退出程序，sighandler 会由于非法赋值指令重新执行而一直循环。

注释掉 exit 后，一直打印 Help！

栈溢出时 SIGSEGV 信号的处理

上面这个简单的 demo 用于捕获栈溢出时会失败，因为此时已经没有栈空间作为栈帧给 segfault 信号处理函数的调用使用。如果真的需要捕获栈溢出问题，需要首先设定一个可选的栈帧，示例代码如下：

int main() {char myaltstack[SIGSTKSZ];struct sigaction act;stack_t ss;ss.ss_sp = myaltstack;ss.ss_size = sizeof(myaltstack);ss.ss_flags = 0;if (sigaltstack(&ss, NULL))errexit("sigaltstack failed");act.sa_handler = segfault;act.sa_flags = SA_ONSTACK;if (sigaction(SIGSEGV, &act, NULL))errexit("sigaction failed");

以上内容摘自 https://www.win.tue.nl/~aeb/linux/lk/lk-9.html。

man sigaltstack

阅读 sigaltstack 函数的 manual 获取到的使用方法：

申请一块内存空间作为可选的信号处理函数栈使用
使用 sigaltstack 函数通知系统可选的信号处理栈帧的存在及其位置
当使用 sigaction 函数建立一个信号处理函数时，通过指定 SA_ONSTACK 标志通知系统这个信号处理函数应该在可选的栈帧上面执行注册的信号处理函数

manual 中的示例 demo 代码如下：

           stack_t ss;ss.ss_sp = malloc(SIGSTKSZ);if (ss.ss_sp == NULL) {perror("malloc");exit(EXIT_FAILURE);}ss.ss_size = SIGSTKSZ;ss.ss_flags = 0;if (sigaltstack(&ss, NULL) == -1) {perror("sigaltstack");exit(EXIT_FAILURE);}sa.sa_flags = SA_ONSTACK;sa.sa_handler = handler();      /* Address of a signal handler */sigemptyset(&sa.sa_mask);if (sigaction(SIGSEGV, &sa, NULL) == -1) {perror("sigaction");exit(EXIT_FAILURE);}

对 manual 中关键内容的翻译

signaltstack 函数最常见的用法是处理普通程序因为栈溢出而触发的 SIGSEGV 信号的处理过程，在这种情况下 SIGSEGV 信号处理函数不能在进程栈中执行，想要处理这种情况，必须使用一个可选的信号处理栈。

创建一个可选的信号函数执行栈对于那些预期可能会耗尽标准栈的程序非常有用。这种情况是可能发生的，例如因为栈增长的过大触及到了向上增长的堆空间或者栈空间触及到了 setrlimit(RLIMIT_STACK, &rlim) 设定的限制外的内存空间等。

如果标准栈用完，内核将会发送一个 SIGSEGV 信号给程序，在这种情况下捕获这个信号的唯一方式就是使用可选的信号函数执行栈。

在可选栈上执行的信号处理函数调用的函数也会使用这个栈空间来执行，当进程切换到该栈帧上执行时，切换完成后接收到的其它的信号，其信号处理函数也将会在这个栈帧上执行。

这个可选栈与标准的栈不同的地方在于，系统并不会动态扩展可选栈的大小。扩展这个可选栈将会带来不可预料的后果。

需要注意的是一个成功的 execve 系统调用将会移除任何存在的可选信号处理栈。一个使用 fork 创建的子进程将会继承其父进程的可选信号处理栈的一份配置拷贝。

man sigaltstack 的相关信息原文摘录如下：

NAMEsigaltstack - set and/or get signal stack context
SYNOPSIS#include <signal.h>int sigaltstack(const stack_t *ss, stack_t *old_ss);Feature Test Macro Requirements for glibc (see feature_test_macros(7)):sigaltstack():_XOPEN_SOURCE >= 500|| /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L|| /* Glibc versions <= 2.19: */ _BSD_SOURCEDESCRIPTIONsigaltstack()  allows a process to define a new alternate signal stack and/or retrieve the state of an existing alternate signal stack.  An alternate signal stack is used during the execution of a signal handler if the establishment of that handler (see  sigaction(2))  requested it.The normal sequence of events for using an alternate signal stack is the following:1. Allocate an area of memory to be used for the alternate signal stack.2. Use sigaltstack() to inform the system of the existence and location of the alternate signal stack.3. When  establishing  a  signal  handler using sigaction(2), inform the system that the signal handler should be executed on the alternatesignal stack by specifying the SA_ONSTACK flag.The ss argument is used to specify a new alternate signal stack, while the old_ss argument is used to retrieve information about  the  cur‐rently  established  signal stack.  If we are interested in performing just one of these tasks, then the other argument can be specified asNULL.The stack_t type used to type the arguments of this function is defined as follows:typedef struct {void  *ss_sp;     /* Base address of stack */int    ss_flags;  /* Flags */size_t ss_size;   /* Number of bytes in stack */} stack_t;To establish a new alternate signal stack, the fields of this structure are set as follows:ss.ss_flagsThis field contains either 0, or the following flag:SS_AUTODISARM (since Linux 4.7)Clear the alternate signal stack settings on entry to the signal handler.  When the signal handler returns, the previous  al‐ternate signal stack settings are restored.This  flag  was added in order make it safe to switch away from the signal handler with swapcontext(3).  Without this flag, asubsequently handled signal will corrupt the state of the switched-away signal handler.  On kernels where this  flag  is  notsupported, sigaltstack() fails with the error EINVAL when this flag is supplied.ss.ss_spThis  field  specifies the starting address of the stack.  When a signal handler is invoked on the alternate stack, the kernel auto‐matically aligns the address given in ss.ss_sp to a suitable address boundary for the underlying hardware architecture.ss.ss_sizeThis field specifies the size of the stack.  The constant SIGSTKSZ is defined to be large enough to cover the  usual  size  require‐ments for an alternate signal stack, and the constant MINSIGSTKSZ defines the minimum size required to execute a signal handler.To  disable  an existing stack, specify ss.ss_flags as SS_DISABLE.  In this case, the kernel ignores any other flags in ss.ss_flags and theremaining fields in ss.If old_ss is not NULL, then it is used to return information about the alternate signal stack which was in effect  prior  to  the  call  tosigaltstack().  The old_ss.ss_sp and old_ss.ss_size fields return the starting address and size of that stack.  The old_ss.ss_flags may re‐turn either of the following values:SS_ONSTACKThe process is currently executing on the alternate signal stack.  (Note that it is not possible  to  change  the  alternate  signalstack if the process is currently executing on it.)SS_DISABLEThe alternate signal stack is currently disabled.Alternatively,  this value is returned if the process is currently executing on an alternate signal stack that was established usingthe SS_AUTODISARM flag.  In this case, it is safe to switch away from the signal handler with swapcontext(3).  It is  also  possibleto set up a different alternative signal stack using a further call to sigaltstack().SS_AUTODISARMThe alternate signal stack has been marked to be autodisarmed as described above.By specifying ss as NULL, and old_ss as a non-NULL value, one can obtain the current settings for the alternate signal stack without chang‐ing them.RETURN VALUEsigaltstack() returns 0 on success, or -1 on failure with errno set to indicate the error.ERRORSEFAULT Either ss or old_ss is not NULL and points to an area outside of the process's address space.EINVAL ss is not NULL and the ss_flags field contains an invalid flag.ENOMEM The specified size of the new alternate signal stack ss.ss_size was less than MINSTKSZ.EPERM  An attempt was made to change the alternate signal stack while it was active (i.e., the process was already executing on the currentalternate signal stack).NOTESThe  most common usage of an alternate signal stack is to handle the SIGSEGV signal that is generated if the space available for the normalprocess stack is exhausted: in this case, a signal handler for SIGSEGV cannot be invoked on the process stack; if we wish to handle it,  wemust use an alternate signal stack.Establishing an alternate signal stack is useful if a process expects that it may exhaust its standard stack.  This may occur, for example,because the stack grows so large that it encounters the upwardly growing heap, or it reaches  a  limit  established  by  a  call  to  setr‐limit(RLIMIT_STACK, &rlim).  If the standard stack is exhausted, the kernel sends the process a SIGSEGV signal.  In these circumstances theonly way to catch this signal is on an alternate signal stack.On most hardware architectures supported by Linux, stacks grow downward.  sigaltstack() automatically takes account  of  the  direction  ofstack growth.Functions called from a signal handler executing on an alternate signal stack will also use the alternate signal stack.  (This also appliesto any handlers invoked for other signals while the process is executing on the alternate signal stack.)  Unlike the  standard  stack,  thesystem  does  not automatically extend the alternate signal stack.  Exceeding the allocated size of the alternate signal stack will lead tounpredictable results.A successful call to execve(2) removes any existing alternate signal stack.  A child process created via fork(2) inherits  a  copy  of  itsparent's alternate signal stack settings.sigaltstack()  supersedes  the  older  sigstack()  call.  For backward compatibility, glibc also provides sigstack().  All new applicationsshould be written using sigaltstack().History4.2BSD had a sigstack() system call.  It used a slightly different struct, and had the major disadvantage that the caller had to  know  thedirection of stack growth.EXAMPLEThe  following  code segment demonstrates the use of sigaltstack() (and sigaction(2)) to install an alternate signal stack that is employedby a handler for the SIGSEGV signal:stack_t ss;ss.ss_sp = malloc(SIGSTKSZ);if (ss.ss_sp == NULL) {perror("malloc");exit(EXIT_FAILURE);}ss.ss_size = SIGSTKSZ;ss.ss_flags = 0;if (sigaltstack(&ss, NULL) == -1) {perror("sigaltstack");exit(EXIT_FAILURE);}sa.sa_flags = SA_ONSTACK;sa.sa_handler = handler();      /* Address of a signal handler */sigemptyset(&sa.sa_mask);if (sigaction(SIGSEGV, &sa, NULL) == -1) {perror("sigaction");exit(EXIT_FAILURE);}

参考链接：

https://www.win.tue.nl/~aeb/linux/lk/lk-9.html