为什么 Valgrind memcheck 不捕获这个 UB？

Question

如标题所说，我真的需要帮助理解，为什么我的系统（linux mint 19、GCC-8.0.1、valgrind-3.13.0、c17）将这段代码视为无效代码：

#include <stdio.h>
#include <string.h>

void printThis( const char *const ptr );

int main( void) {

    char a[10] = "asds";
    char b[10] = "1234567890";

    strcpy ( a, b );
    printThis( a );
}

void printThis( const char *const ptr ){
    printf("Copy completed! : %s\n", ptr );
}

Valgrind 在此处报告问题：

==6973== Memcheck, a memory error detector
==6973== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==6973== Using Valgrind-3.13.0 and LibVEX; rerun with -h for copyright info
==6973== Command: /home/michi/Templates/Cprogram/bin/Debug/Cprogram
==6973== 
==6973== Source and destination overlap in strcpy(0x1ffefffd14, 0x1ffefffd1e)
==6973==    at 0x4C32E97: strcpy (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==6973==    by 0x108724: main (main.c:12)
==6973== 
Copy completed! : 1234567890
==6973== 
==6973== HEAP SUMMARY:
==6973==     in use at exit: 0 bytes in 0 blocks
==6973==   total heap usage: 1 allocs, 1 frees, 1,024 bytes allocated
==6973== 
==6973== All heap blocks were freed -- no leaks are possible
==6973== 
==6973== For counts of detected and suppressed errors, rerun with: -v
==6973== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)

这个有效代码：

#include <stdio.h>

void strcpy2(char *s, char *t);
void printThis( const char *const ptr );

int main( void) {

    char a[10] = "asds";
    char b[10] = "1234567890";

    strcpy2( a, b );
    printThis( a );
}

void strcpy2(char *s, char *t) {
    while ( ( *(s++) = *(t++) ) );
}

void printThis( const char *const ptr ){
    printf("Copy completed! : %s\n", ptr );
}

Valgrind 输出：

==7025== Memcheck, a memory error detector
==7025== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==7025== Using Valgrind-3.13.0 and LibVEX; rerun with -h for copyright info
==7025== Command: /home/michi/Templates/Cprogram/bin/Debug/Cprogram
==7025== 
Copy completed! : 1234567890
==7025== 
==7025== HEAP SUMMARY:
==7025==     in use at exit: 0 bytes in 0 blocks
==7025==   total heap usage: 1 allocs, 1 frees, 1,024 bytes allocated
==7025== 
==7025== All heap blocks were freed -- no leaks are possible
==7025== 
==7025== For counts of detected and suppressed errors, rerun with: -v
==7025== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

使用 O0、O1、O2 和 O3 以及 GCC 标志编译：

-Wpedantic -std=c17 -Wall -Wextra -Werror -Wstrict-prototypes -Wmissing-prototypes -Wmisleading-indentation -Wduplicated-cond -Wold-style-definition -Wconversion -Wshadow -Winit-self -Wfloat-equal -Wwrite-strings -O0 -g

Answer 1

Valgrind 可以捕获只有某些类型的错误。它无法检测堆栈，因此它不会看到您的 strcpy2 的错误。 OTOH strcpy 被一个检查源和目标是否重叠的版本所取代 - 它可以捕获这个 only 因为 a + 10 == b in your 编译程序！

要捕获此类错误，请使用 GCC 的 -fsanitize=address:

% ./a.out 
=================================================================
==3368==ERROR: AddressSanitizer: stack-buffer-overflow on address 0x7fff13832a2a at pc 0x557f05344da8 bp 0x7fff13832990 sp 0x7fff13832980
READ of size 1 at 0x7fff13832a2a thread T0
    #0 0x557f05344da7 in strcpy2 (/a.out+0xda7)
    #1 0x557f05344cca in main (/a.out+0xcca)
    #2 0x7f2d400e5b96 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x21b96)
    #3 0x557f05344a49 in _start (/a.out+0xa49)

Address 0x7fff13832a2a is located in stack of thread T0 at offset 106 in frame
    #0 0x557f05344b39 in main (/a.out+0xb39)

  This frame has 2 object(s):
    [32, 42) 'a'
    [96, 106) 'b' <== Memory access at offset 106 overflows this variable
HINT: this may be a false positive if your program uses some custom stack unwind mechanism or swapcontext
[ ... many more lines follow ... ]

Answer 2

我更新了标题，以便更准确地说明所使用的 Valgrind 工具。

Valgrind 可以检测到此类错误，但如前所述，memcheck.

检测不到

如果我运行使用 Valgrind exp-sgcheck 这个示例代码（exp = 实验性的，sgcheck = 堆栈和全局检查）

valgrind --tool=exp-sgcheck ./so19

然后我得到

==25056== Invalid read of size 1
==25056== at 0x40059D: strcpy2 (so19.c:23)
==25056== by 0x400564: main (so19.c:18)
==25056== Address 0x1ffeffed1a expected vs actual:
==25056== Expected: stack array "b" of size 10 in frame 1 back from here

==25056== Actual: unknown
==25056== Actual: is 0 after Expected
==25056==
==25056== Invalid write of size 1
==25056== at 0x4005A0: strcpy2 (so19.c:23)
==25056== by 0x400564: main (so19.c:18)
==25056== Address 0x1ffeffed2a expected vs actual:
==25056== Expected: stack array "a" of size 10 in frame 1 back from here

==25056== Actual: unknown
==25056== Actual: is 0 after Expected
==25056==
==25056== Invalid read of size 1
==25056== at 0x4005A2: strcpy2 (so19.c:23)
==25056== by 0x400564: main (so19.c:18)
==25056== Address 0x1ffeffed2a expected vs actual:
==25056== Expected: stack array "a" of size 10 in frame 1 back from here

==25056== Actual: unknown
==25056== Actual: is 0 after Expected
==25056==
==25056== Invalid read of size 1
==25056== at 0x4E74C9C: vfprintf (in /lib64/libc-2.12.so)
==25056== by 0x4E7BFF9: printf (in /lib64/libc-2.12.so)
==25056== by 0x4005CD: printThis (so19.c:27)
==25056== by 0x400570: main (so19.c:19)
==25056== Address 0x1ffeffed2a expected vs actual:
==25056== Expected: stack array "a" of size 10 in frame 3 back from here

==25056== Actual: unknown
==25056== Actual: is 0 after Expected
==25056==
==25056== Invalid read of size 1
==25056== at 0x4E9E7C0: _IO_file_xsputn@@GLIBC_2.2.5 (in /lib64/libc-2.12.so)

==25056== by 0x4E74FFF: vfprintf (in /lib64/libc-2.12.so)
==25056== by 0x4E7BFF9: printf (in /lib64/libc-2.12.so)
==25056== by 0x4005CD: printThis (so19.c:27)
==25056== by 0x400570: main (so19.c:19)
==25056== Address 0x1ffeffed2a expected vs actual:
==25056== Expected: stack array "a" of size 10 in frame 4 back from here

==25056== Actual: unknown
==25056== Actual: is 0 after Expected

这是在使用调试信息编译时 (-g)。通过优化构建，未检测到任何错误。

为什么 Valgrind memcheck 不捕获这个 UB？

Why doesn't Valgrind memcheck catch this UB?

c

valgrind

gcc8