从Redis的源代码中,在文件src/debug.c中,它使用backtrace()
来记录调用堆栈。在这些操作中,我注意到了getMcontextEip()
,在 Linux 中似乎是这样的:
static void *getMcontextEip(ucontext_t *uc) {
/* Linux */
#if defined(__i386__)
return (void*) uc->uc_mcontext.gregs[14]; /* Linux 32 */
#elif defined(__X86_64__) || defined(__x86_64__)
return (void*) uc->uc_mcontext.gregs[16]; /* Linux 64 */
#elif defined(__ia64__) /* Linux IA64 */
return (void*) uc->uc_mcontext.sc_ip;
#endif
}
所有这些背后的机制是:当有一个信号(即SIGFPE
)时,它将被捕获并尝试将调用堆栈记录到文件中:
void log_stack_trace(ucontext_t *uc) {
void *trace[100];
int fd = open(file);
int trace_size = backtrace(trace, 100); /* get call stack */
/* overwrite sigaction with caller's address */
if (getMcontextEip(uc) != NULL)
trace[1] = getMcontextEip(uc);
backtrace_symbols_fd(trace, trace_size, fd); /* log to file */
}
从评论中,我们知道它旨在用调用者的地址覆盖 sigaction,但是有什么提示吗?我已经模拟了一个SIGFPE
信号并在 GDB 中对其进行了调试,ucontext_t
看起来是这样的:
(gdb) p *uc
$6 = {
uc_flags = 0,
uc_link = 0x0,
uc_stack = {
ss_sp = 0x0,
ss_flags = 2,
ss_size = 0
},
uc_mcontext = {
gregs = {51, 0, 123, 123, 0, 0, -1073745320, -1073745376, -1208258560, 0, -1073745852, 5, 0, 0, 134514547, 115, 2163270, -1073745376, 123},
fpregs = 0xbfffefb0,
oldmask = 0,
cr2 = 0
},
uc_sigmask = {
__val = {0, 0, 44472, 8441088, 0, 0, 4294902655, 4294901760, 4294967295, 0 <repeats 23 times>}
},
__fpregs_mem = {
cw = 0,
sw = 0,
tag = 895,
ipoff = 0,
cssel = 0,
dataoff = 0,
datasel = 0,
_st = {{
significand = {0, 0, 8064, 0},
exponent = 0
}, {
significand = {0, 0, 0, 0},
exponent = 0
}, {
significand = {0, 0, 0, 0},
exponent = 0
}, {
significand = {0, 0, 0, 0},
exponent = 0
}, {
significand = {0, 0, 0, 0},
exponent = 0
}, {
significand = {0, 0, 0, 0},
exponent = 0
}, {
significand = {0, 0, 0, 0},
exponent = 0
}, {
significand = {0, 0, 0, 0},
exponent = 0
}},
status = 0
}
}
在getMcontextEip
,它只是返回uc->uc_mcontext.gregs[14]
i386 平台上的,为什么要这样做?为什么是14而不是其他(有 19 个元素)?