On which platforms is PHC enabled? Windows, macOS, Linux, and/or Android?
On 2023-12-21 10:19 PM, Paul Bone wrote:
TL;DR: we're rolling out PHC, currently at 1% soon at 10%. If you
already know about PHC that's the new information.
I've been filing a few extra crash reports with memory errors such as
buffer overruns and use after frees. I found a comment on one of them
this morning "Where do these extra stacks come from?". That's a great
question and tells me I haven't communicated this widely enough yet.
I'd like to do that properly later, but for now I'd like this e-mail
to serve as an informal introduction.
The Probabilistic Heap Checker (PHC) is a component in Firefox that
will, probabilistically, redirect a `malloc()` request into a special
area where it can perform extra checking for memory errors. It can
detect buffer overruns and use-after-free errors. But the really cool
thing is that it records the stack at the time of allocation and
free. So that for a use-after-free, the crash report constrains not
only the stack where the error occurred, but stacks that describe the
object's lifetime.
It also records the address and size of the memory allocation.
This information is behind "Protected Data Access", engineers with the
appropriate crash-stats permission have access, it's also not
symbolicised so far. So most people are going to notice it when bugs
are filed against their components. They'll see the extra stacks in
the bug report and might wonder where it came from if they don't have
protected data access.
Right now this runs for everybody in Firefox Nightly (and has done for
years), now we're rolling it out in Firefox Release, it's enabled for
roughly 1% of our population in Firefox 120 and 121 and we already
have 13 crash reports annotated with PHC stacks from december (not all
of them are genuine). We're planning to roll out to 10% of release in
January which means we could expect ~130 crash reports (some
percentage of which will be genuine bugs).
We're looking forward to having this new capability to find and
diagnose memory errors. I'll be filing bugs for these as appropriate
and attaching these extra stacks. I'll also continue to be making
improvements to PHC in the new year. Until then, if you're taking a
break this time of year have a safe and happy holiday season.
Cheers.
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