Messages in this thread | | | Date | Wed, 4 May 2022 13:08:43 +0200 | Subject | Re: [PATCH v2] fbdev: Use helper to get fb_info in all file operations | From | Thomas Zimmermann <> |
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Hi
Am 04.05.22 um 12:55 schrieb Daniel Vetter: > On Wed, May 04, 2022 at 11:28:07AM +0200, Javier Martinez Canillas wrote: >> Hello Daniel, >> >> On 5/4/22 11:02, Daniel Vetter wrote: >>> On Tue, May 03, 2022 at 10:19:34PM +0200, Javier Martinez Canillas wrote: >>>> A reference to the framebuffer device struct fb_info is stored in the file >>>> private data, but this reference could no longer be valid and must not be >>>> accessed directly. Instead, the file_fb_info() accessor function must be >>>> used since it does sanity checking to make sure that the fb_info is valid. >>>> >>>> This can happen for example if the registered framebuffer device is for a >>>> driver that just uses a framebuffer provided by the system firmware. In >>>> that case, the fbdev core would unregister the framebuffer device when a >>>> real video driver is probed and ask to remove conflicting framebuffers. >>>> >>>> Most fbdev file operations already use the helper to get the fb_info but >>>> get_fb_unmapped_area() and fb_deferred_io_fsync() don't. Fix those two. >>>> >>>> Since fb_deferred_io_fsync() is not in fbmem.o, the helper has to be >>>> exported. Rename it and add a fb_ prefix to denote that is public now. >>>> >>>> Reported-by: Junxiao Chang <junxiao.chang@intel.com> >>>> Signed-off-by: Javier Martinez Canillas <javierm@redhat.com> >>> >>> Note that fb_file_info is hilariously racy since there's nothing >>> preventing a concurrenct framebuffer_unregister. Or at least I'm not >>> seeing anything. See cf4a3ae4ef33 ("fbdev: lock_fb_info cannot fail") for >>> context, maybe reference that commit here in your patch. >>> >>> Either way this doesn't really make anything worse, so >>> Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch> >>> >> >> Yes, I noticed is racy but at least checking this makes less likely to >> occur. And thanks, I'll reference that commit in the description of v3. >> >> BTW, I also noticed that the same race that happens with open(),read(), >> close(), etc happens with the VM operations: >> >> int fb_deferred_io_mmap(struct fb_info *info, struct vm_area_struct *vma) >> { >> ... >> vma->vm_private_data = info; >> ... >> } >> >> static vm_fault_t fb_deferred_io_fault(struct vm_fault *vmf) >> { >> ... >> struct fb_info *info = vmf->vma->vm_private_data; >> ... >> } >> >> static vm_fault_t fb_deferred_io_mkwrite(struct vm_fault *vmf) >> { >> ... >> struct fb_info *info = vmf->vma->vm_private_data; >> ... >> } >> >> So something similar to fb_file_fb_info() is needed to check if >> the vm_private_data is still valid. I guess that could be done >> by using the vmf->vma->vm_file and attempting the same trick that >> fb_file_fb_info() does ? > > Yeah should work, except if the ptes are set up already there's kinda not > much that this will prevent. We'd need to tear down mappings and SIGBUS or > alternatively have something else in place there so userspace doesn't blow > up in funny ways (which is what we're doing on the drm side, or at least > trying to). > > I'm also not sure how much we should care, since ideally for drm drivers > this is all taken care of by drm_dev_enter in the right places. It does > mean though that fbdev mmap either needs to have it's own memory or be > fully redirected to the drm gem mmap. > > And then we can afford to just not care to fix fbdev itself.
While the problem has been there ever since, the bug didn't happen until we fixed hot-unplugging for fbdev. Not doing anything is probably not the right thing.
Best regards Thomas
> -Daniel
-- Thomas Zimmermann Graphics Driver Developer SUSE Software Solutions Germany GmbH Maxfeldstr. 5, 90409 Nürnberg, Germany (HRB 36809, AG Nürnberg) Geschäftsführer: Ivo Totev [unhandled content-type:application/pgp-signature] | |