Re: [PATCH] RFC: Provide a function to reset IFUNC PLTs

[Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>]

On 09/03/23 12:47, Adhemerval Zanella Netto wrote:
> 
> 
> On 09/03/23 08:32, Jan Kratochvil wrote:
>> On Wed, 08 Mar 2023 21:04:59 +0800, Adhemerval Zanella Netto wrote:
>>> Yes, without a stop-the-world scheme where a helper thread sets PR_GET_DUMPABLE 
>>> and PTRACE_ATTACH the process can not really be sure that any new thread will not 
>>> be created between the time you enumerate the process threads and call the 'freeze'
>>> function.
>>
>> That "Freezer" class does solve the race of new threads. I also do not see
>> a need for ptrace there, it is self-snapshotting/restoring.
>> 	https://github.com/openjdk/crac/pull/41/files#diff-aeec57d804d56002f26a85359fc4ac8b48cfc249d57c656a30a63fc6bf3457adR6029
> 
> I am not sure how the kernel would enumerate new tasks that are created
> while iterating over /proc/self/task.  On closefrom Linux fallback we have
> a similar problem, where the code iterates over /proc/self/fd, and everytime
> it closes a file descriptor it lseeks back to beginning.  It works because
> eventually there will be no more entries on /proc/self/fd, so either you
> will need to certify that kernel adds new tasks at the end of the getdents
> call (used by readdir, or lseek and keep track of all tasks already signaled.

And the 'handler' signal handler has some potential shortcomings as well: 

  * backtrace is not async-signal-safe: glibc implementation on first call
    issues dlopen, which calls malloc; and libgcc_eh.so *might* also calls malloc.
  
  * pthread calls are not async-signal-safe either.

  * it only handles libc.so, other libraries that uses ifunc for function
    selection also fails.

  * the syscall heuristics do not handle partial results (for instance if
    write syscall returns do EINTR).

So this code has the potential of deadlock, specially if you have another
thread issuing malloc.