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[ Upstream commit0c3057a5a0] The function qdisc_tree_reduce_backlog() uses TC_H_ROOT as a termination condition when traversing up the qdisc tree to update parent backlog counters. However, if a class is created with classid TC_H_ROOT, the traversal terminates prematurely at this class instead of reaching the actual root qdisc, causing parent statistics to be incorrectly maintained. In case of DRR, this could lead to a crash as reported by Mingi Cho. Prevent the creation of any Qdisc class with classid TC_H_ROOT (0xFFFFFFFF) across all qdisc types, as suggested by Jamal. Reported-by: Mingi Cho <mincho@theori.io> Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com> Reviewed-by: Simon Horman <horms@kernel.org> Fixes:066a3b5b23("[NET_SCHED] sch_api: fix qdisc_tree_decrease_qlen() loop") Link: https://patch.msgid.link/20250306232355.93864-2-xiyou.wangcong@gmail.com Signed-off-by: Jakub Kicinski <kuba@kernel.org> Signed-off-by: Sasha Levin <sashal@kernel.org>