[tip: sched/core] sched/topology: Fix sched_domain_span()

[tip-bot2 for Peter Zijlstra]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     e379dce8af11d8d6040b4348316a499bfd174bfb
Gitweb:        https://git.kernel.org/tip/e379dce8af11d8d6040b4348316a499bfd174bfb
Author:        Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate:    Mon, 23 Mar 2026 10:36:27 +01:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Tue, 24 Mar 2026 10:07:04 +01:00

sched/topology: Fix sched_domain_span()

Commit 8e8e23dea43e ("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: 8e8e23dea43e ("sched/topology: Compute sd_weight considering cpuset partitions")
Reported-by: Nathan Chancellor <nathan@xxxxxxxxxx>
Debugged-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Tested-by: Jon Hunter <jonathanh@xxxxxxxxxx>
Tested-by: Chen Yu <yu.c.chen@xxxxxxxxx>
Tested-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
Tested-by: Nathan Chancellor <nathan@xxxxxxxxxx>
Link: https://patch.msgid.link/20260323093627.GY3738010@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
---
 include/linux/sched/topology.h | 24 ++++++++++++++++++------
 1 file changed, 18 insertions(+), 6 deletions(-)

diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
index 51c2958..36553e1 100644
--- a/include/linux/sched/topology.h
+++ b/include/linux/sched/topology.h
@@ -142,18 +142,30 @@ 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[],