Re: Creative ideas on how to debug heap corruption

I assume you're familiar with the following from https://www.aosabook.org/en/ghc.html and that this facility is still there. Just in case you are not: So, the debug RTS has an optional mode that we call *sanity checking*. Sanity checking enables all kinds of expensive assertions, and can make the program run many times more slowly. In particular, sanity checking runs a full scan of the heap to check for dangling pointers (amongst other things), before *and* after every GC. The first job when investigating a runtime crash is to run the program with sanity checking turned on; sometimes this will catch the invariant violation well before the program actually crashes. On Mon, Aug 31, 2020 at 11:08 AM Csaba Hruska wrote:

Dump the whole heap into file during GC traversal or taking the whole allocated area. hmm, maybe this is the same as core dump.

On Mon, Aug 31, 2020 at 11:00 AM Ben Lippmeier wrote:

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On 31 Aug 2020, at 5:54 pm, Moritz Angermann < moritz.angermann@gmail.com> wrote:

If anyone has some create ideas, I'd love to hear them. I've been wondering if just logging allocations (offset, range, type) would help figuring out what we expected to be there; and then maybe try to break on the allocation, (and subsequent writes).

I'm sure some have been down this road before.

Force a GC before every allocation, and make the GC check the validity of the objects before it moves anything. I think this used to be possible by compiling the runtime system in debug mode.

The usual pain of heap corruption is that once the heap is corrupted it may be several GC cycles before you get the actual crash, and in the meantime the objects have all been moved around. The GC walks over all the objects by nature, so get it to validate the heap every time it does, then force it to run as often as you possibly can.

A user space approach is to use a library like vacuum or packman that also walks over the heap objects directly.

http://hackage.haskell.org/package/vacuum-2.2.0.0/docs/GHC-Vacuum.html https://hackage.haskell.org/package/packman

Ben.

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