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@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

diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
index 51c29581f15ebf25d9fddea5cb7706d88de66e0e..36553e14866d0dffdbb57afbfb0e8a7fc80a8e40 100644 (file)
--- 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[],