gdb 会暂时给页面写权限吗? [英] Does gdb temporarily give pages write permission?

问题描述

我正在调试 Linux 应用程序中的 seg 错误，该错误是由程序试图更改静态常量数组结构引起的(因此数据位于 ELF 的只读部分中，随后加载到页面中授予只读权限).

I was debugging a seg fault in a Linux app that was caused by a program trying to change a static constant array structure (so the data was in the read-only section of the ELF and subsequently loaded in a page that was then given read-only permission).

在 GDB 中，我在执行错误存储的汇编程序行上放置了一个断点，当它停在那里时，我使用 GDB 手动执行了等效的写入操作.GDB 毫无怨言地做到了这一点，并且读回该值证明它确实已被写入.我查看了/proc/thepid/maps 并且该特定页面仍被标记为不可写".

While in GDB I put a breakpoint on the line of assembler that did the bad store, and when it stopped there I manually performed the equivalent write action using GDB. GDB did this without any complaints, and reading the value back proved it had indeed been written. I looked in /proc/thepid/maps and that particular page was still marked as "not writeable".

所以我的问题是:GDB 是否临时在只读页面上设置写入权限，执行写入，然后重置权限?谢谢.

So my question is: does GDB temporarily set write permissions on a read-only page, perform the write, then reset the permissions? Thanks.

推荐答案

GDB 是否临时设置写权限

does GDB temporarily set write permissions

没有.

在 Linux/*86 上，ptrace()(这是 GDB 用来读取和写入劣质(正在调试的)进程内存的方法)允许对不可读/可写的页面进行读写由劣等人，导致您所描述的混乱.

On Linux/*86, ptrace() (which is what GDB uses to read and write the inferior (being debugged) process memory) allows reads and writes to pages that are not readable/writable by the inferior, leading exactly to the confusion you've described.

这可能被认为是内核中的错误.

This could be considered a bug in the kernel.

需要注意的是，内核已经允许 ptrace 写入通常不可写的 .text 部分，以便调试器能够植入断点(即通过使用断点/陷阱指令覆盖原始指令来完成 -- int3 通过 PTRACE_POKETEXT 请求).

It should be noted that the kernel has to allow ptrace to write to normally non-writable .text section for the debugger to be able to plant breakpoints (which is done by overwriting original instruction with the breakpoint/trap instruction -- int3 via PTRACE_POKETEXT request).

内核不必为 POKE_DATA 做同样的事情，但是 man ptrace 说:

The kernel doesn't have to do the same for POKE_DATA, but man ptrace says:

PTRACE_POKETEXT, PTRACE_POKEDATA
   Copies the word data to location addr in the child's memory.
   As above, the two requests are currently equivalent.

我相信正是这种等价性导致了当前的行为.

I believe it's that equivalentness that causes the current behavior.

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