[webkit-dev] WTFCrash()

[Mark Lam]

> On Mar 6, 2018, at 9:09 PM, Michael Catanzaro <mcatanzaro at igalia.com> wrote:
> 
> Hi,
> 
> After [1], WTFCrash() is used only in debug builds on Darwin. For Darwin release builds, inline assembly is used to trigger a SIGTRAP. I experimented with this today and found it works quite badly on Linux, somehow confusing gdb and clobbering the top frames of the stacktrace, so I think we should leave that unchanged and keep it Darwin-only. So this mail applies only to debug builds on Darwin, and to non-Darwin ports.
> 
> Now, currently, WTFCrash() does three things:
> 
> (1) Calls a user-configurable crash hook
> (2) Print a low-quality backtrace to stderr
> (3) Crash somehow:
>  - If ASAN is used, with __builtin_trap() (that's SIGILL on Linux x86_64)
>  - Then with *(int *)(uintptr_t)0xbbadbeef = 0, which might fail to crash if 0xbadbeef is a valid address, and is SIGSEGV otherwise
>  - Then with __builtin_trap() if COMPILER(GCC_OR_CLANG)
>  - Then with ((void(*)())0)() otherwise (presumably SIGSEGV or SIGBUS, dunno)
> 
> This is all rather more complicated than it needs to be.
> 
> First off, the crash hook is (almost) unused and should simply be removed, see [2].
> 
> Next, the low-quality backtrace. Does anyone think this is useful? It's mainly annoying to me, because it's not anywhere near as good as a proper backtrace that shows stack members, it's mangled so function names are unnecessarily-difficult to read, and it takes all of five seconds to get a much nicer one with modern Linux developer tools. If other developers like it, perhaps we could keep it for debug builds only, and skip right to the crashing in release builds? I suppose we could keep printing it always if there is desire to do this, because it has never caused any problems with Linux crash telemetry and doesn't seem to be harming anything, but otherwise my instinct is to simplify.

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.

> Now, as for how exactly to crash. Current logic, with asan disabled, prefers SIGSEGV, then SIGILL if that fails, then SIGSEGV again. I don't like that WTFCrash() triggers a SIGSEGV. This is not very clean; at least on Linux, it's conventional for assertion failures and other intentional crashes to cause SIGABRT instead of trying to write to 0xbadbeef. SIGILL is also quite unusual. I  think I'd be happy if we replace all of WTFCrash() with a single call to abort(). Any objections to this? Is there a special reason for the current convoluted logic?

For Darwin, I think we want assertion failures to generate crash logs for post-mortem analysis.  I’m not sure if SIGABRT will give us the crash logs we want.

The choice of a SIGSEGV on 0xbadbeef (for debug builds) and WTFBreakpointTrap() e.g. int3 on x86_64 (for release builds), is so that we can discern the crash log for an assertion failure vs any other crash.

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.

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.

Mark

> Michael
> 
> [1] https://bugs.webkit.org/show_bug.cgi?id=153996
> [2] https://bugs.webkit.org/show_bug.cgi?id=183369
> 
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