Question

我知道Valgrind以一种允许捕获段错误的方式跟踪内存。但是，为什么没有捕获以下段错误？

int main() {
    char *x = calloc(16, 1);
    char *y = calloc(16, 1);

    x[80] = 'c';
    y[-80] = 'c';

    printf("%c %c\n", *x, *y);
    return 0;
}

是不是应该捕获堆中的越界访问？根据Valgrind的文件：

But it should detect many errors that could crash your program (eg. cause a segmentation fault).

Answer 1

我认为你赋予valgrind超越力量的能力。

它将尝试检测各种类型的错误并向您报告，但它无法检测到所有错误，即使在它尝试检测的某些错误类别中也是如此。

在这种情况下，你正在处理的是一个越界写入数组，如果valgrind设法捕获它，将被报告为“无效写入”错误。通过跟踪哪些地址“有效”来检测它们，因为它们是已知堆块的一部分。

问题是，如果你索引到数组的开头或结尾太远，你实际上最终会得到一个地址，这个地址是相邻块中的有效地址，因此对valgrind来说看起来非常好。为了减少发生这种情况的可能性，valgrind在块的每一侧添加了一个填充区域（称为“红色区域”），但默认情况下只有16个字节。

如果使用--redzone-size=128选项增加红色区域大小，则会发现valgrind确实检测到此程序中的错误。

Answer 2

适合我：

==24344== Memcheck, a memory error detector.
==24344== Copyright (C) 2002-2007, and GNU GPL'd, by Julian Seward et al.
==24344== Using LibVEX rev 1854, a library for dynamic binary translation.
==24344== Copyright (C) 2004-2007, and GNU GPL'd, by OpenWorks LLP.
==24344== Using valgrind-3.3.1-Debian, a dynamic binary instrumentation framework.
==24344== Copyright (C) 2000-2007, and GNU GPL'd, by Julian Seward et al.
==24344== For more details, rerun with: -v
==24344== 
==24344== Invalid write of size 1
==24344==    at 0x8048419: main (testValgrind.c:5)
==24344==  Address 0x418f078 is 0 bytes after a block of size 16 alloc'd
==24344==    at 0x4021E22: calloc (vg_replace_malloc.c:397)
==24344==    by 0x804840F: main (testValgrind.c:3)
==24344== 
==24344== Invalid write of size 1
==24344==    at 0x8048422: main (testValgrind.c:6)
==24344==  Address 0x418f018 is 16 bytes before a block of size 16 alloc'd
==24344==    at 0x4021E22: calloc (vg_replace_malloc.c:397)
==24344==    by 0x80483F8: main (testValgrind.c:2)

==24344== 
==24344== ERROR SUMMARY: 2 errors from 2 contexts (suppressed: 12 from 1)
==24344== malloc/free: in use at exit: 32 bytes in 2 blocks.
==24344== malloc/free: 2 allocs, 0 frees, 32 bytes allocated.
==24344== For counts of detected errors, rerun with: -v
==24344== searching for pointers to 2 not-freed blocks.
==24344== checked 58,940 bytes.
==24344== 
==24344== LEAK SUMMARY:
==24344==    definitely lost: 32 bytes in 2 blocks.
==24344==      possibly lost: 0 bytes in 0 blocks.
==24344==    still reachable: 0 bytes in 0 blocks.
==24344==         suppressed: 0 bytes in 0 blocks.
==24344== Rerun with --leak-check=full to see details of leaked memory.

Valgrind没有抓到Segfaults

2 个答案: