sched/topology: Fix sched_domain_span()

[ Upstream commit e379dce ]

Commit 8e8e23d ("sched/topology: Compute sd_weight considering
cpuset partitions") ends up relying on the fact that structure
initialization should not touch the flexible array.

However, the official GCC specification for "Arrays of Length Zero"
[*] says:

  Although the size of a zero-length array is zero, an array member of
  this kind may increase the size of the enclosing type as a result of
  tail padding.

Additionally, structure initialization will zero tail padding. With
the end result that since offsetof(*type, member) < sizeof(*type),
array initialization will clobber the flex array.

Luckily, the way flexible array sizes are calculated is:

  sizeof(*type) + count * sizeof(*type->member)

This means we have the complete size of the flex array *outside* of
sizeof(*type), so use that instead of relying on the broken flex array
definition.

[*] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html

Fixes: 8e8e23d ("sched/topology: Compute sd_weight considering cpuset partitions")
Reported-by: Nathan Chancellor <nathan@kernel.org>
Debugged-by: K Prateek Nayak <kprateek.nayak@amd.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Jon Hunter <jonathanh@nvidia.com>
Tested-by: Chen Yu <yu.c.chen@intel.com>
Tested-by: K Prateek Nayak <kprateek.nayak@amd.com>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Link: https://patch.msgid.link/20260323093627.GY3738010@noisy.programming.kicks-ass.net
Signed-off-by: Sasha Levin <sashal@kernel.org>

Author: Peter Zijlstra

include/linux/sched/topology.h

@@ -141,18 +141,30 @@ struct sched_domain {
 
 	unsigned int span_weight;
 	/*
-	 * Span of all CPUs in this domain.
+	 * See sched_domain_span(), on why flex arrays are broken.
 	 *
-	 * NOTE: this field is variable length. (Allocated dynamically
-	 * by attaching extra space to the end of the structure,
-	 * depending on how many CPUs the kernel has booted up with)
-	 */
 	unsigned long span[];
+	 */
 };
 
 static inline struct cpumask *sched_domain_span(struct sched_domain *sd)
 {
-	return to_cpumask(sd->span);
+	/*
+	 * Turns out that C flexible arrays are fundamentally broken since it
+	 * is allowed for offsetof(*sd, span) < sizeof(*sd), this means that
+	 * structure initialzation *sd = { ... }; which writes every byte
+	 * inside sizeof(*type), will over-write the start of the flexible
+	 * array.
+	 *
+	 * Luckily, the way we allocate sched_domain is by:
+	 *
+	 *   sizeof(*sd) + cpumask_size()
+	 *
+	 * this means that we have sufficient space for the whole flex array
+	 * *outside* of sizeof(*sd). So use that, and avoid using sd->span.
+	 */
+	unsigned long *bitmap = (void *)sd + sizeof(*sd);
+	return to_cpumask(bitmap);
 }
 
 extern void partition_sched_domains(int ndoms_new, cpumask_var_t doms_new[],