Re: [PATCH v2] percpu: add basic double free check

Andrew Morton
2026-01-17T03:15:49Z

On Thu, 15 Jan 2026 18:32:16 -0800 Dennis Zhou <dennis@kernel.org> wrote:

> This adds a basic double free check by validating the first bit of the
> allocation in alloc_map and bound_map are set. If the alloc_map bit is
> not set, then this means the area is currently unallocated. If the
> bound_map bit is not set, then we are not freeing from the beginning of
> the allocation.
>
> This is a respin of [1] adding the requested changes from me and
> Christoph.
>
> ...
>
> @@ -1276,18 +1277,24 @@ static int pcpu_alloc_area(struct pcpu_chunk *chunk, int alloc_bits,
> static int pcpu_free_area(struct pcpu_chunk *chunk, int off)
> {
> struct pcpu_block_md *chunk_md = &chunk->chunk_md;
> + int region_bits = pcpu_chunk_map_bits(chunk);
> int bit_off, bits, end, oslot, freed;
>
> lockdep_assert_held(&pcpu_lock);
> - pcpu_stats_area_dealloc(chunk);
>
> oslot = pcpu_chunk_slot(chunk);
>
> bit_off = off / PCPU_MIN_ALLOC_SIZE;
> + if (unlikely(bit_off < 0 || bit_off >= region_bits))
> + return 0;

This (which looks sensible) wasn't changelogged?

> @@ -2242,6 +2252,13 @@ void free_percpu(void __percpu *ptr)
>
> spin_lock_irqsave(&pcpu_lock, flags);
> size = pcpu_free_area(chunk, off);
> + if (size == 0) {
> + spin_unlock_irqrestore(&pcpu_lock, flags);
> +
> + if (__ratelimit(&_rs))
> + WARN(1, "percpu double free or bad ptr\n");

Is ratelimiting really needed? A WARN_ON_ONCE is enough to tell people
that this kernel is wrecked?

> + return;
> + }

The patch does appear to do that which it set out to do. But do we
want to do it? Is there a history of callers double-freeing percpu
memory? Was there some bug which would have been more rapidly and
easily solved had this change been in place?