07372: X11 Lightgun routine does not check for valid X11 display, resulting in crash (segmentation fault)

ID	Category [?]	Severity [?]	Reproducibility	Date Submitted	Last Update
07372	Crash/Freeze	Critical (emulator)	Always	Jul 6, 2019, 22:43	Oct 19, 2019, 18:52

Tester	GeorgeMcMullen	View Status	Public	Platform	MAME (Self-compiled)
Assigned To	GeorgeMcMullen	Resolution	Fixed	OS	Other
Status [?]	Resolved			Driver
Version	0.211	Fixed in Version	0.215	Build	32-bit
		Fixed in Git Commit		Github Pull Request #

Summary	07372: X11 Lightgun routine does not check for valid X11 display, resulting in crash (segmentation fault)
Description	Greetings, First bug report, thanks in advance for your patience. @MrGW reported an issue (https://mametesters.org/view.php?id=7350) with regards to MAME v0.210 crashing immediately on startup, which seemed to be caused by changes in v0.210 which enable X11 and XInput on Linux based systems by default. The workaround is to either compile with X11/XInput explicitly disabled, or start MAME with the option '-lightgunprovider none'. I believe I've found the cause of the issue. I'm filing a new bug because I'm unable to comment on or reopen the original bug. Also, this is a lower level bug report for the code itself. Platforms Affected: - Raspberry Pi (3, 3+, 2, 0) - Raspbian Stretch or RetroPie 4.4 (based on Raspbian Stretch) The issue is caused by the Lightgun routine attempting to reference an X11 display when there is none. Thus, this issue is likely to occur on other Linux based platforms where X11 is not being used (e.g. directfb). The actual crash happens at this line: https://github.com/mamedev/mame/blob/173962bf18acf20ada60d1b592bf798be28a32a5/src/osd/modules/input/input_x11.cpp#L316 That routine should check that m_display is not null before proceeding to XPending(), which evidently also doesn't check that it's been passed a null value. But this may also be averted by checking that the X11 initialization routine has actually successfully completed as in the following line: https://github.com/mamedev/mame/blob/173962bf18acf20ada60d1b592bf798be28a32a5/src/osd/modules/input/input_x11.cpp#L454 A simple check on the return value of initialize will avert the lightgun routine from completing its own initialization, and subsequently causing the crash.
Steps To Reproduce	git pull latest MAME source. Compile MAME using the standard 'makefile' using the 'make' command Launch MAME. The UI will just start to show and then it dies with a segmentation fault. Launch MAME with the following command line options: mame -verbose You will get output such as the following: Lightgun: Begin initialization Unable to connect to X server Lightgun: End initialization Joystick: Start initialization Joystick: End initialization Searching font Liberation Sans in -. path/s font Liberation Sans\|Regular is not TrueType or BDF, using MAME default Segmentation fault Launching MAME with the following command line options is a workaround, which will allow you to use MAME. mame -lightgunprovider none
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No.16684 GeorgeMcMullen Tester Jul 20, 2019, 16:51	I checked out the history of the input_x11.cpp OSD module and the lines that I mentioned are actually from the original refactoring of the file. As such, knowledge about the inner workings and potential regressions may be hard to come by. Barring anybody else's contribution, I'll do some additional tracing of the code to see if there are any possible hitches and submit a pull request.
No.17084 GeorgeMcMullen Tester Oct 13, 2019, 17:15	Quick update. I've been working on a pull request and have gotten some feedback. https://github.com/mamedev/mame/pull/5725
No.17097 GeorgeMcMullen Tester Oct 18, 2019, 19:03	My pull request has been merged into master. Thank you! https://github.com/mamedev/mame/commit/4a2d4b3911488fc73d3443ec569093860e37be38