747870 - Unaligned access in XPCCallContext::XPCCallContext

Status: RESOLVED FIXED

(Core :: XPConnect, defect)

Description

[Mike Hommey [:glandium]]

I got a report about Iceweasel crashing at startup on sparc. The relevant part of the backtrace is here:
#0  XPCCallContext::XPCCallContext (this=0xffff566c, callerLanguage=XPCContext::LANG_JS, cx=0xf7924340, callBeginRequest=0, obj=0xf0c9d900, 
    flattenedJSObject=0xf0c9d900, wrapper=0x0, tearOff=0x0)
    at js/xpconnect/src/XPCCallContext.cpp:83
#1  0xf6deef30 in XPCLazyCallContext::GetXPCCallContext (this=0xffff5648)
    at js/xpconnect/src/xpcprivate.h:1335

The problem comes from using the placement new operator with an array of char, which is not correctly aligned for the XPCCallContext class.

Thanks to Jurij Smakov for tracking down the root cause.

Comment 1

[Mike Hommey [:glandium]]

Attachment: Properly align XPCLazyCallContext::mData

Comment 2

[:Ms2ger (he/him; ⌚ UTC+1/+2)]

Comment on attachment 617439 [details] [diff] [review]
Properly align XPCLazyCallContext::mData

Review of attachment 617439 [details] [diff] [review]:
-----------------------------------------------------------------

::: js/xpconnect/src/xpcprivate.h
@@ +1315,1 @@
>                                             mCallBeginRequest == CALL_BEGINREQUEST,

Fix indentation somehow, please.

Comment 3

[Bobby Holley (:bholley)]

Comment on attachment 617439 [details] [diff] [review]
Properly align XPCLazyCallContext::mData

This won't cause any extra code to be run on XPCLazyCallContext instantiation, right? These things need to be super fast, at least until the new DOM bindings are done.

r=bholley with the indentation fixed.

Comment 4

[Mike Hommey [:glandium]]

No, it's just a POD type with the right alignment.

Comment 5

[Mike Hommey [:glandium]]

(In reply to Ms2ger from comment #2)
> Comment on attachment 617439 [details] [diff] [review]
> Properly align XPCLazyCallContext::mData
> 
> Review of attachment 617439 [details] [diff] [review]:
> -----------------------------------------------------------------
> 
> ::: js/xpconnect/src/xpcprivate.h
> @@ +1315,1 @@
> >                                             mCallBeginRequest == CALL_BEGINREQUEST,
> 
> Fix indentation somehow, please.

As indenting would make that line you quote go overboard, what style would you prefer?
mCcxToDestroy = mCcx = new (mData.addr())
    XPCCallContext(mCallerLanguage, (...)

mCcxToDestroy = mCcx =
    new (mData.addr())
        XPCCallContext(mCallerLanguage, (...)

char *data = mData.addr();
mCcxToDestroy = mCcx =
    new (data) XPCCallContext(mCallerLanguage, (...)

other ?

(The latter case allowing the current indentation being kept)

Comment 6

[Mike Hommey [:glandium]]

(In reply to Mike Hommey [:glandium] from comment #5)
> char *data = mData.addr();

XPCCallContext *data, even.

Comment 7

[Bobby Holley (:bholley)]

(In reply to Mike Hommey [:glandium] from comment #5)
> (The latter case allowing the current indentation being kept)

SGTM.

Comment 8

[Mike Hommey [:glandium]]

https://hg.mozilla.org/integration/mozilla-inbound/rev/77e68aa8f24c

Comment 9

[(no longer active)]

https://hg.mozilla.org/mozilla-central/rev/77e68aa8f24c