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This commit reworks the permission handling of the two ioctls. In particular it enforced the CAP_SYS_ADMIN check only, if: - we're issuing the ioctl from process other than the one which opened the node, and - we are, or were master in the past This ensures that we: - do not regress the systemd-logind style of DRM_MASTER arbitrator - allow applications which do not use systemd-logind to drop their master capabilities (and regain them at later point) ... w/o running as root. See the comment above drm_master_check_perm() for more details. v1: - Tweak wording, fixup all checks, add igt test v2: - Add a few more comments, grammar nitpicks. Cc: Adam Jackson <ajax@redhat.com> Cc: Daniel Vetter <daniel.vetter@ffwll.ch> Cc: Pekka Paalanen <ppaalanen@gmail.com> Testcase: igt/core_setmaster/master-drop-set-user Signed-off-by: Emil Velikov <emil.velikov@collabora.com> Reviewed-by: Adam Jackson <ajax@redhat.com> Link: https://patchwork.freedesktop.org/patch/msgid/20200319172930.230583-1-emil.l.velikov@gmail.com