0002857: Segfault in lwc__intern

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0002857

LibWapcaplet

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public

2023-08-07 03:27

2026-10-05 22:31

Reporter

Michael Orlitzky

Assigned To

Priority

normal

Severity

crash

Reproducibility

always

Status

acknowledged

Resolution

open

Summary

0002857: Segfault in lwc__intern

Description

I'm trying to use libsvgtiny to load SVGs for GTK, replacing the usual librsvg loader. I can only guess that this is some kind of thread-safety issue, because it happens "randomly" while also being easy to reproduce. The steps to reproduce it however are a little insane. The short version is that it happens when I try to load a lot of icons at the same time, each of which involves parsing an SVG document. That uses libdom, which in turn uses libwapcaplet. Here's the segfault; it looks like the "str" pointer has become invalid?

Thread 73 "pool-gtk4-icon-" received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x7fffb57fa6c0 (LWP 30309)]
0x00007fffe6ba83ef in lwc__intern (s=0x7fffe6c1770a "circle", slen=6,
    ret=0x7fffd00c6458, hasher=0x7fffe6ba81a7 <lwc__calculate_hash>,
    compare=0x7ffff72aeee0 <strncmp>, copy=0x7ffff72a2ea0 <memmove>)
    at src/libwapcaplet.c:107
107 if ((str->hash == h) && (str->len == slen)) {
(gdb) p str
$1 = (lwc_string *) 0xa5a5a5a5a5a5a5a5
(gdb) p str->len
Cannot access memory at address 0xa5a5a5a5a5a5a5b5

Steps To Reproduce

If you're in the mood for an adventure...

First, build this, and give it your real prefix/libdir:

  http://gitweb.michael.orlitzky.com/?p=libsvgtiny-pixbuf.git

Next, delete the pixbuf loader and thumbnailer that come with librsvg. The thumbnailers can be found in /usr/share/thumbnailers, and the loader directory can be found with,

  $ pkg-config --variable gdk_pixbuf_moduledir gdk-pixbuf-2.0

Then, install libsvgtiny-pixbuf (sudo make install), and update the loader cache:

  $ sudo gdk-pixbuf-query-loaders --update-cache

Now GTK should be using libsvgtiny to produce SVGs. One at a time, it more or less works. But if you launch the "gtk-icon-browser" and click around for a while, it will almost certainly crash with the segfault above. (If you don't have gtk-icon-browser, it comes with GTK; maybe built it from source.)

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Notes
~0002414 Michael Orlitzky (reporter) 2023-08-08 00:38	Wrapping svgtiny_parse() in a mutex (to prevent concurrent executions) seems to have fixed the problem, further suggesting that it's a thread-safety issue. If you're going to try to reproduce this issue, keep in mind that libsvgtiny-pixbuf.git now has a comit specifically intended to work around it; you'll have to revert that commit first.

~0002509 John-Mark Bell (administrator) 2026-10-05 22:30 Last edited: 2026-10-05 22:31 View 2 revisions	Yes: none of the libraries are explicitly designed for thread safety (as NetSurf itself is single threaded). That said, all the higher-level ones should be fine as they operate using context objects (which cannot be shared between threads, but separate threads could reasonably make use of independent contexts at the same time). This currently falls apart (as you've noticed) because libwapcaplet underpins several of the libraries and contains a bunch of global datastructures with no thread safety mechanisms in place. Some thought is needed as to the "best" answer here (as we can't guarantee that all the supported platform runtimes provide sane locking mechanisms).

Notes

Date Modified	Username	Field	Change
Issue History
2023-08-07 03:27	Michael Orlitzky	New Issue
2023-08-08 00:38	Michael Orlitzky	Note Added: 0002414
2026-10-05 22:30	John-Mark Bell	Status	new => acknowledged
2026-10-05 22:30	John-Mark Bell	Note Added: 0002509
2026-10-05 22:31	John-Mark Bell	Note Edited: 0002509	View Revisions

Issue History