Re: reliable reproducer, was Re: core dump analysis

[Finn Thain <fthain@linux-m68k.org>]

On Sun, 23 Apr 2023, Michael Schmitz wrote:

> Am 23.04.2023 um 13:41 schrieb Michael Schmitz:
> 
> Though the question remains - is this expected behaviour for programs 
> that do deep recursion on the stack while taking signals (and the reason 
> for the option to run signal handlers on an alternate stack)?
> 

I don't understand how "deep recursion" can be used to explain this. We've 
seen crashes with only 1.8 MB of stack usage.

The best reason I can think of for having a signal stack would be that it 
may be better for signal delivery to fail than for the target process to 
fail. But I've no idea whether the kernel makes that kind of defensive 
programming possible (?)

> And why does this almost always appear to happen after bus error exceptions
> (frame format b)? The extra exception stack information isn't even accounted
> for in the above frame end address!
> 
> Result with sa_sigaction handler:
> 
> parent usp  : 0xef969e28
> handler tos : 0xef969e6c
> handler stack overwrote usp!
> frame end   : 0xef969e7c
> frame start : 0xef969b58
> handler usp : 0xef969b40
> signal usp  : 0xef969e04
> signal pc   : 0x80000696
> signal fmtv : 0x114
> 
> parent usp  : 0xef955008
> handler tos : 0xef955064
> handler stack overwrote usp!
> frame end   : 0xef955074
> frame start : 0xef954d50
> handler usp : 0xef954d38
> signal usp  : 0xef954ffc
> signal pc   : 0x80000680
> signal fmtv : 0xb008
> 
> parent usp  : 0xef945eb8
> handler tos : 0xef945f0c
> handler stack overwrote usp!
> frame end   : 0xef945f1c
> frame start : 0xef945bf8
> handler usp : 0xef945be0
> signal usp  : 0xef945ea8
> signal pc   : 0xc009f37a
> signal fmtv : 0x80
> 
> parent usp  : 0xef933eb8
> handler tos : 0xef933f0c
> handler stack overwrote usp!
> frame end   : 0xef933f1c
> frame start : 0xef933bf8
> handler usp : 0xef933be0
> signal usp  : 0xef933ea8
> signal pc   : 0xc009f37a
> signal fmtv : 0x80
> 
> parent usp  : 0xef921edc
> handler tos : 0xef9aaca4
> handler stack overwrote usp!
> frame end   : 0xef9aacb4
> frame start : 0xef9aa990
> handler usp : 0xef9aa978
> signal usp  : 0xef9aac40
> signal pc   : 0x80000782
> signal fmtv : 0x114
> 
> Illegal instruction (core dumped)
> 

I don't understand these results. If usp was really overwritten, the 
program would have crashed early, no?

> Exception right before crash was an interrupt in this case (only seen 
> that once in this context, though I've seen lots of those in the course 
> of the test runs). Frame start calculated from siginfo pointer value in 
> this case.
> 

I didn't realize that you could get a crash from a signal delivered 
following an interrupt. I'll try to modify the kernel such that signals 
are not delivered after page faults.