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Description of problem:
There is a race in the way that linuxltfs uses set_thread_area() after clone().
With suitable delays, the new thread can get canceled before set_thread_area()
takes effect. When this happens, the signal handler in the new thread,
pthread_handle_sigcancel(), will get a SIGSEGV because it uses segment register
%gs but the corresponding segment descriptor has not yet been setup by
Version-Release number of selected component (if applicable):
Steps to Reproduce:
1. run testcases build-i686-linuxltfs/linuxthreads/tst-cancel2 or tst-cancel3,
but provide a slowdown of 100:1 [one hundred to one] or more
Actual Results: Attached partial strace of running tst-cancel2 under a
memory-access checker [provides a slowdown as a side effect] shows that the
fork()ed watchdog sends SIGRT_1 to a child thread before the child thread
Expected Results: Signals whose handler uses register %gs must not be sent to a
thread that does not have set_thread_area() already setup.
Created attachment 94475 [details]
strace -f tst-cancel2 [edited for brevity]
I don't see the problem.
Before DO_SET_THREAD_AREA, %gs is what the initial INIT_THREAD_SELF (, 0)
chose and kernel tls_array in the kernel points to manager_thread.
If SIGCANCEL arrives at this moment, pthread_handle_sigcancel should catch this
(self == manager_thread) and do the right thing.
Now DO_SET_THREAD_AREA calls set_thread_area syscall, changing tls_array
from manager_thread to the actual thread descriptor. Whatever moment the signal
arrives, pthread_handle_sigcancel should either see self == manager_thread or
self == the_new_thread_descriptor, nothing else.
OK; I did not understand the part about pthread_handle_sigcancel handling the
situation in both cases (race won, race lost). This seems to mean that signal
handlers cannot access thread-local storage under linuxltfs (hard to setup
anyway [?], but does mean additional compatibility concerns for apps: "nptl app"
will be unreliable under linuxltfs), and that signal handlers cannot call any
pthread_* functions except pthread_handle_sigcancel.
glibc-2.3.2-87 now does the self == manager_thread -> find self from stack, INIT_THREAD_SELF
in signal handler wrappers too.