Re: [GIT PULL] Please pull proc and exec work for 5.7-rc1

[Bernd Edlinger]

On 4/30/20 1:59 AM, Jann Horn wrote:
> On Thu, Apr 30, 2020 at 1:22 AM Linus Torvalds
> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>> On Wed, Apr 29, 2020 at 3:38 PM Linus Torvalds
>> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>>>
>>> If you do it properly, with a helper function instead of repeating
>>> that fragile nasty thing, maybe it will look better to me.

I added the BIG FAT WARNNIG comments as a mitigation for that.
Did you like those comments?

>>
>> Side note: if it has a special helper function for the "get lock,
>> repeat if it was invalid", you can do a better job than return
>> -EAGAIN.
>>
>> In particular, you can do this
>>
>>         set_thread_flag(TIF_SIGPENDING);
>>         return -RESTARTNOINTR;
>>
>> which will actually restart the system call. So a ptrace() user (or
>> somebody doing a "write()" to /proc/<pid>/attr/xyz, wouldn't even see
>> the impossible EAGAIN error.
> 
> Wouldn't you end up livelocked in the scenario that currently deadlocks? Like:
> 
>  - tracer attaches to thread A
>  - thread B goes into execve, blocks on waiting for A's death
>  - tracer tries to attach to B and hits the -EAGAIN
> 
> If we make the PTRACE_ATTACH call restart, the tracer will just end up
> looping without ever resolving the deadlock. If we want to get through
> this cleanly with this approach, userspace needs to either
> deprioritize the "I want to attach to pid X" and go back into its
> eventloop, or to just treat -EAGAIN as a fatal error and give up
> trying to attach to that task.
> 

Yes, exactly, the point is the caller is expected to call wait in that
scenario, otherwise the -EAGAIN just repeats forever, that is an API
change, yes, but something unavoidable, and the patch tries hard to
limit it to cases where the live-lock or pseudo-dead-lock is unavoidable
anyway.


Bernd.