Messages in this thread | | | From | (Eric W. Biederman) | Date | Thu, 10 Jun 2021 15:57:58 -0500 | Subject | Kernel stack read with PTRACE_EVENT_EXIT and io_uring threads |
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Folks,
Digging through the guts of exit I found something I am not quite certain what to do with. On some architectures such as alpha, m68k, and nios2 the kernel calls into system calls with a subset of the registers saved on the kernel stack, and the kernel calls into signal handling and a few other contexts with all of the registers saved on the kernel stack. The problem is sometimes we read all of the registers from a context where they are not all saved.
When this was initially observed it looked just like a coredump problem and it could be solved by tweaking the coredump code. That change was 77f6ab8b7768 ("don't dump the threads that had been already exiting when zapped.")
However I have looked farther and we have the location where get_signal is called from io_uring, and we have the ptrace_stop in PTRACE_EVENT_EXIT. In PTRACE_EVENT_EXIT we could be called from exit(2) which is a syscall and we definitely won't have everything saved on the kernel stack. I have not doubled checked create_io_thread but I don't think create_io_threads saves all of the registers on the kernel stack.
I think at this point we need to say that the architectures that have a do this need to be fixed to at least call do_exit and the kernel function in create_io_thread with the deeper stack.
Is that reasonable of me to ask? Is there some other way to deal with this issue that I am not seeing? Am I missing some critical detail that makes PTRACE_EVENT_EXIT in do_exit not a problem if someone reads the register with ptrace?
Eric
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