On Sun, Dec 23, 2018 at 02:55:52PM +0100, Noralf TrÃnnes wrote:
Den 23.12.2018 01.55, skrev Peter Wu:
After drm_fb_helper_fbdev_setup calls drm_fb_helper_init,
"dev->fb_helper" will be initialized (and thus drm_fb_helper_fini will
have some effect). After that, drm_fb_helper_initial_config is called
which may call the "fb_probe" driver callback.
This driver callback may call drm_fb_helper_defio_init (as is done by
drm_fb_helper_generic_probe) or set a framebuffer (as is done by bochs)
as documented. These are normally cleaned up on exit by
drm_fb_helper_fbdev_teardown which also calls drm_fb_helper_fini.
If an error occurs after "fb_probe", but before setup is complete, then
calling just drm_fb_helper_fini will leak resources. This was triggered
by df2052cc922 ("bochs: convert to drm_fb_helper_fbdev_setup/teardown"):
[ 50.008030] bochsdrmfb: enable CONFIG_FB_LITTLE_ENDIAN to support this framebuffer
[ 50.009436] bochs-drm 0000:00:02.0: [drm:drm_fb_helper_fbdev_setup] *ERROR* fbdev: Failed to set configuration (ret=-38)
[ 50.011456] [drm] Initialized bochs-drm 1.0.0 20130925 for 0000:00:02.0 on minor 2
[ 50.013604] WARNING: CPU: 1 PID: 1 at drivers/gpu/drm/drm_mode_config.c:477 drm_mode_config_cleanup+0x280/0x2a0
[ 50.016175] CPU: 1 PID: 1 Comm: swapper/0 Tainted: G T 4.20.0-rc7 #1
[ 50.017732] EIP: drm_mode_config_cleanup+0x280/0x2a0
[ 50.023155] Call Trace:
[ 50.023155] ? bochs_kms_fini+0x1e/0x30
[ 50.023155] ? bochs_unload+0x18/0x40
This can be reproduced with QEMU and CONFIG_FB_LITTLE_ENDIAN=n.
Fixes: 8741216396b2 ("drm/fb-helper: Add drm_fb_helper_fbdev_setup/teardown()")
Reported-by: kernel test robot <rong.a.chen@xxxxxxxxx>
Cc: Noralf TrÃnnes <noralf@xxxxxxxxxxx>
Signed-off-by: Peter Wu <peter@xxxxxxxxxxxxx>
drivers/gpu/drm/drm_fb_helper.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
index 9d64f874f965..432e0f3b9267 100644
@@ -2860,7 +2860,7 @@ int drm_fb_helper_fbdev_setup(struct drm_device *dev,
This change will break the error path for drm_fbdev_generic_setup()
because drm_fb_helper_generic_probe() cleans up on error but doesn't
clear drm_fb_helper->fb resulting in a double drm_framebuffer_remove().
This should probably considered a bug of drm_fb_helper_generic_probe.
Ownership of fb_helper should remain with the caller. The caller can
detect an error and act accordingly.
My assumption has been that the drm_fb_helper_funcs->fb_probe callback
cleans up its resources on error. Clearly this is not the case for bochs, so
my take on this is that bochsfb_create() needs to clean up on error.
That assumption still holds for bochs. The problem is this sequence:
- drm_fb_helper_fbdev_setup is called.
- fb_probe succeeds (this is crucial).
- register_framebuffer fails.
- error path of setup is triggered.
As fb_helper is fully setup by drivers, the drm_fb_helper core should
fully deallocate it again on the error path or else a leak occurs.
Gerd has a patchset that switches bochs over to the generic fbdev
emulation, but ofc that doesn't help with 4.20:
And that does not help with other users of the drm_fb_helper who use
functions like drm_fb_helper_defio_init. They will likely run in the
I don't have a way to test tinydrm or other drivers, but if you force
register_framebuffer to fail, you should be able to reproduce the
problem with drm_fb_helper_generic_probe.