Bug 102474

There've been several fixes in all those trees upstream.

Can you try with upstream code instead (for both mutter, gnome-shell, Xwayland) so we can rule out a problem with the Ubuntu backport?

Does it occur only in kvm? What about real hardware?

> There've been several fixes in all those trees upstream.

> Can you try with upstream code instead (for both mutter, gnome-shell, Xwayland) > so we can rule out a problem with the Ubuntu backport?

I can try having a go, is there specific branches to try? or just GNOME trunk (3.25) ones?

> Does it occur only in kvm? What about real hardware?

sorry if the description was not clear, that's using GNOME on a laptop not in kvm, it's start kvm in the session to do tested on a daily iso which takes xwayland down

I can reproduce a similar crash using qemu-kvm with SDL backend and alt-tabbing between the qemu window and some ohter native window in gnome-shell:

Thread 1 "Xwayland" received signal SIGSEGV, Segmentation fault.
0x000000000042b77e in zwp_pointer_constraints_v1_lock_pointer (lifetime=2, region=0x0, pointer=<optimized out>, surface=<optimized out>, 
    zwp_pointer_constraints_v1=0x1ef37d0) at pointer-constraints-unstable-v1-client-protocol.h:347
347		id = wl_proxy_marshal_constructor((struct wl_proxy *) zwp_pointer_constraints_v1,
(gdb) bt
#0  0x000000000042b77e in zwp_pointer_constraints_v1_lock_pointer (lifetime=2, region=0x0, pointer=<optimized out>, surface=<optimized out>, 
    zwp_pointer_constraints_v1=0x1ef37d0) at pointer-constraints-unstable-v1-client-protocol.h:347
#1  xwl_pointer_warp_emulator_lock (warp_emulator=0x2c26db0) at xwayland-input.c:2584
#2  0x000000000042d998 in xwl_seat_maybe_lock_on_hidden_cursor (xwl_seat=xwl_seat@entry=0x29142f0) at xwayland-input.c:2756
#3  0x000000000042ef15 in xwl_seat_maybe_lock_on_hidden_cursor (xwl_seat=0x29142f0) at xwayland-input.c:2765
#4  xwl_seat_cursor_visibility_changed (xwl_seat=xwl_seat@entry=0x29142f0) at xwayland-input.c:2768
#5  0x000000000042fa68 in xwl_set_cursor (device=<optimized out>, screen=<optimized out>, cursor=<optimized out>, x=<optimized out>, y=<optimized out>)
    at xwayland-cursor.c:245
#6  0x00000000004c4b8a in miPointerUpdateSprite (pDev=0x28e6fa0) at mipointer.c:468
#7  0x00000000004c4dda in miPointerDisplayCursor (pDev=0x28e6fa0, pScreen=0x1ee6740, pCursor=0x2b988a0) at mipointer.c:206
#8  0x00000000004b2250 in CursorDisplayCursor (pDev=0x28e6fa0, pScreen=0x1ee6740, pCursor=0x2b988a0) at cursor.c:150
#9  0x000000000052e44f in AnimCurDisplayCursor (pDev=0x28e6fa0, pScreen=0x1ee6740, pCursor=0x2b988a0) at animcur.c:220
#10 0x000000000044cc4b in ChangeToCursor (pDev=0x28e6fa0, cursor=0x2b988a0) at events.c:936
#11 0x0000000000453d3e in ActivatePointerGrab (mouse=0x28e6fa0, grab=0x2d1d7f0, time=..., autoGrab=<optimized out>) at events.c:1542
#12 0x000000000044ecd1 in GrabDevice (client=client@entry=0x2ae5720, dev=dev@entry=0x28e6fa0, pointer_mode=1, keyboard_mode=1, grabWindow=<optimized out>, 
    ownerEvents=<optimized out>, ctime=0, mask=0x7ffce3a623f0, grabtype=1, curs=0, confineToWin=4194314, status=0x7ffce3a623ef "") at events.c:5120
#13 0x000000000045180a in ProcGrabPointer (client=0x2ae5720) at events.c:4908
#14 0x0000000000443ffa in Dispatch () at dispatch.c:478
#15 0x0000000000447f08 in dix_main (argc=10, argv=0x7ffce3a625e8, envp=<optimized out>) at main.c:276
#16 0x00007efc0168250a in __libc_start_main () from /lib64/libc.so.6
#17 0x000000000042979a in _start ()

(gdb) f 1
#1  xwl_pointer_warp_emulator_lock (warp_emulator=0x2c26db0) at xwayland-input.c:2584
2584	        zwp_pointer_constraints_v1_lock_pointer(pointer_constraints,
(gdb) list
2579	    struct xwl_window *lock_window = xwl_seat->focus_window;
2580	
2581	    warp_emulator->locked_window = lock_window;
2582	
2583	    warp_emulator->locked_pointer =
2584	        zwp_pointer_constraints_v1_lock_pointer(pointer_constraints,
2585	                                                lock_window->surface,
2586	                                                xwl_seat->wl_pointer,
2587	                                                NULL,
2588	                                                ZWP_POINTER_CONSTRAINTS_V1_LIFETIME_PERSISTENT);
(gdb) p *xwl_seat
value has been optimized out
(gdb) p *lock_window
Cannot access memory at address 0x0

Possible fix here:

https://patchwork.freedesktop.org/patch/174476/

the patch seems to fix the segfault indeed

Patch has bee pushed in git master, commit cdd0352b