684619 - Assertion failure: addr % Cell::CellSize == 0, at jsgc.h:665

Status: RESOLVED FIXED

(Core :: JavaScript Engine, defect)

Description

[Christian Holler (:decoder)]

Attachment: Test case for shell (see README file inside).

The attached test asserts on mozilla-central revision a351ae35f2c4 (with shell build fix from mozilla-inbound rev fff3dc9478ce). See README for options and running instructions. This test only works on 32 bit debug builds.

Stepping through the assert will crash:

Program received signal SIGSEGV, Segmentation fault.
0x0805a65b in js::gc::ArenaHeader::allocated (this=0xdadad000) at ../../jsgc.h:391
391             JS_ASSERT(allocKind <= FINALIZE_LIMIT);

Comment 1

[Brian Hackett [Laid off!]]

A GC gets triggered in the middle of the FindReferences shell-only debugging function, when a call to JS_SetElement allocates some shapes.  FindReferences uses a GC tracer to walk the heap, and after the GC it winds up with a garbage value in its worklist (don't know why, but one possibility is a shifting set of conservative roots caused previously reachable things to become unreachable and get collected).

Can someone with the right bits remove the s-s?

Comment 2

[Christian Holler (:decoder)]

I didn't see that FindReferences is involved here. My question would be if FindReferences is generally harmful (and should be deleted in fuzz driver before testing) or if this is a real bug and FindReferences is generally safe.

Comment 3

[Brian Hackett [Laid off!]]

(In reply to Christian Holler (:decoder) from comment #2)
> I didn't see that FindReferences is involved here. My question would be if
> FindReferences is generally harmful (and should be deleted in fuzz driver
> before testing) or if this is a real bug and FindReferences is generally
> safe.

This is a real bug, FindReferences should not be crashing here or anywhere.  However, until this gets fixed you might want to blacklist FindReferences from the fuzzer, as even if a GC crashing testcase uses FindReferences there isn't a simple way to tell it is crashing *because* of the FindReferences.

Comment 4

[Jim Blandy :jimb]

Attachment: Make HeapReverser itself root all nodes referred to by roots.

I can't reproduce this bug myself, but if bhackett's analysis is correct, I'd expect this to fix the problem. Explanation in the comments.

Comment 5

[Jim Blandy :jimb]

(That is, I started with a351ae35f2c4, applied the patch in fff3dc9478ce, build as a 32-bit executable, and then ran the test case according to the README, and didn't get an assertion. Not too surprising, if it indeed is a conservative scanning issue.)

Comment 6

[Jim Blandy :jimb]

https://hg.mozilla.org/integration/mozilla-inbound/rev/d88e2c0f2732

Comment 7

[Marco Bonardo [:mak]]

https://hg.mozilla.org/mozilla-central/rev/d88e2c0f2732

Comment 9

[LegNeato]

Do we care about this for Fx9 (with TI) or could we ship as-is?

Comment 10

[David Mandelin [:dmandelin]]

(In reply to Christian Legnitto [:LegNeato] from comment #9)
> Do we care about this for Fx9 (with TI) or could we ship as-is?

The bug and fix are both in a shell-only debugging function so it doesn't affect Fx9.