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From: Ian McCulloch (ianmcc_at_[hidden])
Date: 2006-03-02 15:56:49

Jim Douglas wrote:

> A recent problem I encountered that caused the total failure of a
> regression test run has highlighted what I (and others) believe to be a
> design flaw in execution_monitor::catch_signals (in file
> execution_monitor.ipp).
> IIUC the purpose of the code in question is to intercept UNIX signals
> and convert them to exceptions which contain a diagnostic message
> indicating the signal type. For most signals this should work reliably.
> However, we suggest that this mechanism is prone to failure when
> attempting to deal with a SIGSEGV, as happened in my case.
> What I observed was a test process that had stalled by blocking against
> a mutex. Detailed analysis showed that the test failed with a memory
> segment violation resulting in a SIGSEGV.
> In brief, the primary execution failure corrupts the program heap. When
> the C++ exception is thrown at execution_monitor:462, the exception
> handling mechanism calls __cxa_allocate_exception which then calls
> std::malloc. But, because of the corrupted heap, this call blocks
> against the malloc mutex.
> This is the specific case with QNX, and other OSs will deal with this in
> different ways. The general point I would like to make is that after a
> memory segment violation, any process's memory will be in an undefined
> state and it is unreasonable to assume that it will be capable of
> continued execution as per the present design. I propose that there is
> no safe way to intercept a SIGSEGV and that it should be allowed to
> terminate the process without intervention.

Completely correct. The SIGSEGV handler is not allowed to return (except in
the case where the signal was generated by a raise()), doing so is
undefined bahaviour. Whatever extra stuff needs to be done to report the
error, needs to be done in the signal handler itself, using
async-signal-safe functions only. Definitely no malloc().


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