sched_ext: Reject oversized automatic CID shards

Automatic topology packing starts a new shard before adding a core when
the current shard would exceed its target size. However, a first core is
always accepted so that cores remain indivisible. An indivisible core with
more than SCX_CID_SHARD_MAX_CPUS CPUs cannot be represented without
exceeding the hard shard limit.

Per-shard state uses fixed-size inline cmask storage sized for
SCX_CID_SHARD_MAX_CPUS. Passing an oversized shard to alloc_pshard() can
therefore make cmask initialization overwrite the following fields.

Reject an indivisible core larger than SCX_CID_SHARD_MAX_CPUS before adding
it to a shard. Existing non-empty shard splitting already keeps the total
within the hard cap, while an indivisible core may still exceed
cid_shard_size when it fits within the hard limit.

Report the oversized core and the hard limit with scx_error() so that the
rejection is diagnosable.

tj: The bug is only reproducible with artificial VM topologies. It can't
    happen on real hardware.

Fixes: e2c841b91bf9 ("sched_ext: Add CID sharding")
Reviewed-by: Andrea Righi <arighi@nvidia.com>
Tested-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Hui Su <sh_def@163.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

0 files changed, 0 insertions, 0 deletions