GDB不能访问mmap（）的内核分配内存？ [英] GDB can't access mmap()'d kernel allocated memory?

问题描述

我遇到了一个问题，GDB和一些缓冲区分配在内核空间。缓冲区由内核模块分配，内核模块应分配连续的内存块，然后通过mmap（）调用将内存映射到用户空间。然而，GDB似乎不能在任何时候访问这些块。例如，在GDB中击中断点后：

 （gdb）x / 10xb 0x4567e000 
 0x4567e000：但是，查看应用程序在/ proc / smaps中当前映射的内存区域显示：
  
  4567e000-456d3000 rwxs 8913f000 00：0d 883 / dev / cmem 
大小：340 kB 
 Rss ：340 kB 
 Pss：0 kB 
 Shared_Clean：0 kB 
 Shared_Dirty：0 kB 
 Private_Clean：0 kB 
 Private_Dirty：0 kB 
参考：0 kB 
 Swap：0 kB 
  
在运行期间的某个点，该缓冲器地址（或以类似方式分配的另一个）引起SIGSEGV。 
 编程接收信号SIGSEGV，分段故障。 
 [切换到线程0x49aea490（LWP 652）] 
 0x402e4ea8在fwrite（）从/lib/libc.so.6 
（gdb）
（gdb）
 （gdb）其中
＃0 0x402e4ea8在fwrite（）中从/lib/libc.so.6 
＃1 0x000eb394在EncryptedWriter :: Write（this = 0x198600，buffRaw = 0x4567e000< Address 0x4567e000 out of encryption.cpp中的encrypted_writer.cpp：397 
＃2的0x0006b0f4：EncryptionWrapper :: Write（this = 0x3ab2698，buffer = 0x4567e000，size = 719）：54 
  
这种segfault发生，尽管事实上缓冲区已被大量使用直到崩溃，并且/ proc // smaps文件仍然
 
 
 
我完全失去了为什么会发生这种情况，以及为什么映射在/ proc中似乎有效，但从来没有gdb。
解决方案
关于为什么gdb无法访问你想要的内存，我相信Linux不能通过< a href =http://www.kernel.org/doc/man-pages/online/pages/man2/ptrace.2.html> ptrace（）。
 
 
 
根据cmemk.c（我在 linuxutils_2_25.tar.gz ），mmap（）确实设置了内存中的VM_IO标志。
 
 
 
要访问此内存gdb，给你的程序添加一个函数，读取这个内存并让gdb调用这个函数。
 
I'm running into an issue with GDB and some buffers allocated in kernel space.  The buffers are allocated by a kernel module that is supposed to allocate contiguous blocks of memory, and then memory mapped into userspace via a mmap() call.  GDB, however, can't seem to access these blocks at any time.  For example, after hitting a breakpoint in GDB:
(gdb) x /10xb 0x4567e000
0x4567e000:     Cannot access memory at address 0x4567e000
However, looking at the application's currently mapped memory regions in /proc//smaps shows:
4567e000-456d3000 rwxs 8913f000 00:0d 883        /dev/cmem
Size:                340 kB
Rss:                 340 kB
Pss:                   0 kB
Shared_Clean:          0 kB
Shared_Dirty:          0 kB
Private_Clean:         0 kB
Private_Dirty:         0 kB
Referenced:            0 kB
Swap:                  0 kB
The reason I'm even looking into this is because at some point during the run, this buffer address (or another allocated in a similar manner) causes a SIGSEGV.  
Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x49aea490 (LWP 652)]
0x402e4ea8 in fwrite () from /lib/libc.so.6
(gdb)
(gdb)
(gdb) where
#0  0x402e4ea8 in fwrite () from /lib/libc.so.6
#1  0x000eb394 in EncryptedWriter::Write (this=0x198600, buffRaw=0x4567e000 <Address     0x4567e000 out of bounds>, iLenRaw=719) at encrypted_writer.cpp:397
#2  0x0006b0f4 in EncryptionWrapper::Write (this=0x3ab2698, buffer=0x4567e000, size=719) at encryption.cpp:54
This segfault occurs despite the fact that the buffer had been used heavily up until the crash, and the /proc//smaps file still shows this buffer to be mapped as above.

I am completely at a loss as to why this might be happening, and why the mapping seems valid in /proc but never in GDB.
解决方案
About why gdb cannot access the memory you want, I believe Linux does not make I/O memory accessible via ptrace().

According to cmemk.c (which I found in linuxutils_2_25.tar.gz), mmap() does indeed set the VM_IO flag on the memory in question.

To access this memory from gdb, add a function to your program that reads this memory and have gdb call this function.

                        
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