Invalid program counters and unwinding

Florian Weimer
Thu Jun 28 14:18:00 GMT 2018

On 06/28/2018 04:16 AM, Jeff Law wrote:
>> Previous discussions:
>>    (patch with a spread lock, still not async-signal-safe)

> You might also want to look at RH BZ 1293594 which I think has pointers
> back to an issue from 2008 :(

Interesting.  That does suspiciously look like a concurrent dlclose. 
It's just that the crash handler crashes, after the application crash. 
I think this one is really NOTABUG, both technically and from user 
impact: we do not cause the crash, we just react poorly to the 
application triggering undefined behavior.

In the bug, you mentioned this code fragment for x86-64:

42        unsigned char *pc = context->ra;
43        struct sigcontext *sc;
44        long new_cfa;
46        /* movq __NR_rt_sigreturn, %rax ; syscall  */
47        if (*(unsigned char *)(pc+0) == 0x48
48            && *(unsigned long *)(pc+1) == 0x050f0000000fc0c7)

I'm not sure I agree that it is “dumb”, but I think it proves 
conclusively that you cannot feed random addresses to the unwinder. 8-)


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