[webkit-dev] WTFCrash()

[Michael Catanzaro]

On Wed, Mar 7, 2018 at 12:36 AM, Mark Lam <mark.lam at apple.com> wrote:
> On Darwin, we currently only use WTFCrash() on Debug builds (see 
> definition of the CRASH() macro).  Feel free to make linux do the 
> same.  FWIW, I use this crash trace a lot when debugging crashes when 
> I don’t already have a debugger attached yet.  Of course, with a 
> debugger attached, it is of less value.

Unfortunately that somehow breaks the top frames in the backtrace on 
Linux, so we can't do that.

> The reason for release builds using WTFBreakpointTrap() is so that we 
> can get a crash with minimal perturbation to the register state, to 
> help with post-mortem crash analysis.  Debug builds are only used 
> during development and internal testing.  So, we take the opportunity 
> to get more crash info there (hence, the dumping of the crash trace). 
>  The reason that ASan builds use __builtin_trap() is because ASan 
> does not like the memory access of 0xbadbbeef (if I remember 
> correctly).
> 
> In addition, we also have infrastructure in place that looks for 
> these crash patterns.  So, changing to use SIGABRT (even if it 
> generates a crash log on Darwin) would mean additional cost and churn 
> to update that infrastructure, with not much gain to show for it.  
> Hence, I object to the change.

OK, interesting. I won't change the behavior for Darwin, then.

Our crash telemetry does not care about the crash signal that's used; 
we'll get a nice backtrace regardless. Linux developers kinda expect 
SIGABRT to be used for intentional crashes like WTFCrash(), though, so 
it would be (very) slightly more useful for us to use abort().

> Feel free to change the linux implementation of CRASH() to use 
> abort() if that works better for linux folks.  BTW, CRASH() is what 
> you want to define/redirect.  WTFCrash() is only one implementation 
> of CRASH().  No client should be calling WTFCrash() directly.

CRASH() always calls WTFCrash(), except in Darwin release builds. So 
one option is to change only WTFCrash(). I experimented with changing 
CRASH() to call abort() directly and decided it doesn't really matter 
either way. The advantage of changing CRASH() is that there is one 
fewer uninteresting frame at the top of the backtrace, since WTFCrash 
will not be present. The cost is it will be very slightly harder to 
notice that CRASH() was called, since WTFCrash() won't be present in 
the backtrace.

Michael