Bug 286477 – REGRESSION(288455@main): Crash in WebCore::ImageBackingStore::clearRect when opening emoji chooser on Slack

• Component: WebCore Misc.
• Status: RESOLVED FIXED

[Michael Catanzaro]

Created attachment 474001 [details]
Full backtrace

When opening the emoji chooser on Red Hat's internal Slack website, the web process immediately crashes when decoding GIF images:

#0  __pthread_kill_implementation (threadid=<optimized out>, signo=signo@entry=6, no_tid=no_tid@entry=0) at pthread_kill.c:44
#1  0x00007f060fc9ae23 in __pthread_kill_internal (threadid=<optimized out>, signo=6) at pthread_kill.c:78
#2  0x00007f060fc4208e in __GI_raise (sig=sig@entry=6) at ../sysdeps/posix/raise.c:26
#3  0x00007f060fc29882 in __GI_abort () at abort.c:79
#4  0x00007f060b0e8c1d in std::__glibcxx_assert_fail (file=<optimized out>, line=line@entry=433, function=<optimized out>, condition=<optimized out>)
    at ../../../../../libstdc++-v3/src/c++11/assert_fail.cc:41
#5  0x00007f0610feec8a in WebCore::ImageBackingStore::clearRect (this=0x7f0283381f00, rect=...)
    at /buildstream/gnome/sdk/webkitgtk-6.0.bst/Source/WebCore/platform/graphics/ImageBackingStore.h:105
#6  0x00007f0610fee680 in WebCore::GIFImageDecoder::initFrameBuffer (this=0x7f02832ed900, frameIndex=<optimized out>)
    at /buildstream/gnome/sdk/webkitgtk-6.0.bst/Source/WebCore/platform/image-decoders/gif/GIFImageDecoder.cpp:411
#7  0x00007f0610fee15e in WebCore::GIFImageDecoder::haveDecodedRow
    (this=0x7f02832ed900, frameIndex=2, rowBuffer=WTF::Vector of length 106, capacity 106 = {...}, width=<optimized out>, rowNumber=<optimized out>, repeatCount=1, writeTransparentPixels=<optimized out>)
    at /buildstream/gnome/sdk/webkitgtk-6.0.bst/Source/WebCore/platform/image-decoders/gif/GIFImageDecoder.cpp:237
#8  0x00007f0610feef3e in GIFLZWContext::outputRow (this=0x7f0283387a60)
    at /buildstream/gnome/sdk/webkitgtk-6.0.bst/Source/WebCore/platform/image-decoders/gif/GIFImageReader.cpp:156
#9  0x00007f0610fef3ac in GIFLZWContext::doLZW (this=0x7f0283387a60, block=Python Exception <class 'gdb.error'>: value has been optimized out
)
    at /buildstream/gnome/sdk/webkitgtk-6.0.bst/Source/WebCore/platform/image-decoders/gif/GIFImageReader.cpp:307
#10 0x00007f0610fef48b in GIFFrameContext::decode
    (this=0x7f02832ff9c0, data=std::span of length 19802 = {...}, client=<optimized out>, frameDecoded=@0x7ffc4f9f0ccf: false)
    at /buildstream/gnome/sdk/webkitgtk-6.0.bst/Source/WebCore/platform/image-decoders/gif/GIFImageReader.cpp:341
#11 0x00007f0610fef868 in GIFImageReader::decode (this=0x7f02832e3180, query=<optimized out>, haltAtFrame=3)
    at /buildstream/gnome/sdk/webkitgtk-6.0.bst/Source/WebCore/platform/image-decoders/gif/GIFImageReader.cpp:378
#12 0x00007f0610fedca4 in WebCore::GIFImageDecoder::decode
    (this=0x7f02832ed900, haltAtFrame=3, query=WebCore::GIFImageDecoder::GIFFullQuery, allDataReceived=true)
    at /buildstream/gnome/sdk/webkitgtk-6.0.bst/Source/WebCore/platform/image-decoders/gif/GIFImageDecoder.cpp:35

I will attach the full backtrace. Unfortunately it is not detailed enough for me to understand where exactly the assertion is coming from, though I guess we're probably reading out of the bounds of the IntRect, but the assertion is .

This problem never occurs on webkit.slack.com, only on Red Hat's internal Slack. I think Red Hat's emoji chooser is different (bigger).

Unfortunately the problem is not occurring in my development environment, but I'm hoping that's because we don't enable standard library assertions by default. I'll find out. If I can reproduce it in my development environment, then I can bisect it.

[Michael Catanzaro]

I was able to reproduce after adding -DCMAKE_CXX_FLAGS="-Wp,-D_GLIBCXX_ASSERTIONS". We really need to do that on the bots. Anyway, got an error message:

/usr/bin/../lib/gcc/x86_64-redhat-linux/14/../../../../include/c++/14/span:433: span<element_type, dynamic_extent> std::span<unsigned int>::subspan(size_type, size_type) const [_Type = unsigned int, _Extent = 18446744073709551615]: Assertion '__offset <= size()' failed.

It's caused by 288455@main "Adopt more spans in ImageBackingStore.h"

[Michael Catanzaro]

I changed all the ASSERTS to RELEASE_ASSERTs, but none of them are hit.

Next I added some debug:

@@ -100,6 +100,7 @@ public:
         auto pixels = pixelsStartingAt(rect.x(), rect.y());
         for (int i = 0; i < rect.height(); ++i) {
             zeroSpan(pixels.first(rect.width()));
+            WTFLogAlways("About to skip %d pixels in span of size %zu", m_size.width(), pixels.size());
             skip(pixels, m_size.width());
         }
     }

Output is:

About to skip 128 pixels in span of size 15475
About to skip 128 pixels in span of size 15347
About to skip 128 pixels in span of size 15219
About to skip 128 pixels in span of size 15091
About to skip 128 pixels in span of size 14963
About to skip 128 pixels in span of size 14835
About to skip 128 pixels in span of size 14707
About to skip 128 pixels in span of size 14579
About to skip 128 pixels in span of size 14451
About to skip 128 pixels in span of size 14323
About to skip 128 pixels in span of size 14195
About to skip 128 pixels in span of size 14067
About to skip 128 pixels in span of size 13939
About to skip 128 pixels in span of size 13811
About to skip 128 pixels in span of size 13683
About to skip 128 pixels in span of size 13555
About to skip 128 pixels in span of size 13427
About to skip 128 pixels in span of size 13299
About to skip 128 pixels in span of size 13171
About to skip 128 pixels in span of size 13043
About to skip 128 pixels in span of size 12915
About to skip 128 pixels in span of size 12787
About to skip 128 pixels in span of size 12659
About to skip 128 pixels in span of size 12531
About to skip 128 pixels in span of size 12403
About to skip 128 pixels in span of size 12275
About to skip 128 pixels in span of size 12147
About to skip 128 pixels in span of size 12019
About to skip 128 pixels in span of size 11891
About to skip 128 pixels in span of size 11763
About to skip 128 pixels in span of size 11635
About to skip 128 pixels in span of size 11507
About to skip 128 pixels in span of size 11379
About to skip 128 pixels in span of size 11251
About to skip 128 pixels in span of size 11123
About to skip 128 pixels in span of size 10995
About to skip 128 pixels in span of size 10867
About to skip 128 pixels in span of size 10739
About to skip 128 pixels in span of size 10611
About to skip 128 pixels in span of size 10483
About to skip 128 pixels in span of size 10355
About to skip 128 pixels in span of size 10227
About to skip 128 pixels in span of size 10099
About to skip 128 pixels in span of size 9971
About to skip 128 pixels in span of size 9843
About to skip 128 pixels in span of size 9715
About to skip 128 pixels in span of size 9587
About to skip 128 pixels in span of size 9459
About to skip 128 pixels in span of size 9331
About to skip 128 pixels in span of size 9203
About to skip 128 pixels in span of size 9075
About to skip 128 pixels in span of size 8947
About to skip 128 pixels in span of size 8819
About to skip 128 pixels in span of size 8691
About to skip 128 pixels in span of size 8563
About to skip 128 pixels in span of size 8435
About to skip 128 pixels in span of size 8307
About to skip 128 pixels in span of size 8179
About to skip 128 pixels in span of size 8051
About to skip 128 pixels in span of size 7923
About to skip 128 pixels in span of size 7795
About to skip 128 pixels in span of size 7667
About to skip 128 pixels in span of size 7539
About to skip 128 pixels in span of size 7411
About to skip 128 pixels in span of size 7283
About to skip 128 pixels in span of size 7155
About to skip 128 pixels in span of size 7027
About to skip 128 pixels in span of size 6899
About to skip 128 pixels in span of size 6771
About to skip 128 pixels in span of size 6643
About to skip 128 pixels in span of size 6515
About to skip 128 pixels in span of size 6387
About to skip 128 pixels in span of size 6259
About to skip 128 pixels in span of size 6131
About to skip 128 pixels in span of size 6003
About to skip 128 pixels in span of size 5875
About to skip 128 pixels in span of size 5747
About to skip 128 pixels in span of size 5619
About to skip 128 pixels in span of size 5491
About to skip 128 pixels in span of size 5363
About to skip 128 pixels in span of size 5235
About to skip 128 pixels in span of size 5107
About to skip 128 pixels in span of size 4979
About to skip 128 pixels in span of size 4851
About to skip 128 pixels in span of size 4723
About to skip 128 pixels in span of size 4595
About to skip 128 pixels in span of size 4467
About to skip 128 pixels in span of size 4339
About to skip 128 pixels in span of size 4211
About to skip 128 pixels in span of size 4083
About to skip 128 pixels in span of size 3955
About to skip 128 pixels in span of size 3827
About to skip 128 pixels in span of size 3699
About to skip 128 pixels in span of size 3571
About to skip 128 pixels in span of size 3443
About to skip 128 pixels in span of size 3315
About to skip 128 pixels in span of size 3187
About to skip 128 pixels in span of size 3059
About to skip 128 pixels in span of size 2931
About to skip 128 pixels in span of size 2803
About to skip 128 pixels in span of size 2675
About to skip 128 pixels in span of size 2547
About to skip 128 pixels in span of size 2419
About to skip 128 pixels in span of size 2291
About to skip 128 pixels in span of size 2163
About to skip 128 pixels in span of size 2035
About to skip 128 pixels in span of size 1907
About to skip 128 pixels in span of size 1779
About to skip 128 pixels in span of size 1651
About to skip 128 pixels in span of size 1523
About to skip 128 pixels in span of size 1395
About to skip 128 pixels in span of size 1267
About to skip 128 pixels in span of size 1139
About to skip 128 pixels in span of size 1011
About to skip 128 pixels in span of size 883
About to skip 128 pixels in span of size 755
About to skip 128 pixels in span of size 627
About to skip 128 pixels in span of size 499
About to skip 128 pixels in span of size 371
About to skip 128 pixels in span of size 243
About to skip 128 pixels in span of size 115
/usr/bin/../lib/gcc/x86_64-redhat-linux/14/../../../../include/c++/14/span:433: span<element_type, dynamic_extent> std::span<unsigned int>::subspan(size_type, size_type) const [_Type = unsigned int, _Extent = 18446744073709551615]: Assertion '__offset <= size()' failed.

Can't skip 128 bytes in a span of size 115.

The clearRect function assumes that rect.height() % rect.width() == 0, but it's not. I'm not sure, but this might be a preexisting bug that was only exposed by the switch to std::span?

[Michael Catanzaro]

> The clearRect function assumes that rect.height() % rect.width() == 0, but it's not. I'm not sure, but this might be a preexisting bug that was only exposed by the switch to std::span?

Oops, that precondition should be: pixelsStartingAt(rect.x(), rect.y()).size() % rect.width() == 0

[Radar WebKit Bug Importer]

<rdar://problem/143960229>

[Michael Catanzaro]

This is also causing crashes when scrolling on youtube.com player views.

[Michael Catanzaro]

Test image: https://emoji.slack-edge.com/T030G10V24F/clapping-7001/65bfed96fee186b2.gif

If somebody wants to figure out how to construct an image that similarly triggers the crash, then we could add a layout test for this. I'm not sure about including this image directly into WebKit due to unclear licensing.

[Michael Catanzaro]

Pull request: https://github.com/WebKit/WebKit/pull/40730

[EWS]

Committed 290611@main (32dd4c71d022): <https://commits.webkit.org/290611@main>

Reviewed commits have been landed. Closing PR #40730 and removing active labels.