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sched/topology: Fix sched_domain_span()

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Commit 8e8e23dea4 ("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: 8e8e23dea4 ("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
This commit is contained in:
Peter Zijlstra
2026-03-23 10:36:27 +01:00
parent fe7171d0d5
commit e379dce8af
1 changed files with 18 additions and 6 deletions
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24
include/linux/sched/topology.h
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@@ -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[],