Re: [PATCH v5] bpf: Fix smp_processor_id() call trace for preemptible kernels

[Andrii Nakryiko]

On Tue, Jun 30, 2026 at 5:27 PM Edward Adam Davis <eadavis@xxxxxx> wrote:
>
> bpf_mem_cache_free_rcu() maybe called in preemptible context, this
> will trigger the below warning message:
>
> BUG: using smp_processor_id() in preemptible [00000000] code: syz.0.17/5820
> caller is bpf_mem_cache_free_rcu+0x48/0xc0 kernel/bpf/memalloc.c:954
> Call Trace:
>  check_preemption_disabled+0xd3/0xe0 lib/smp_processor_id.c:47
>  bpf_mem_cache_free_rcu+0x48/0xc0 kernel/bpf/memalloc.c:954
>  rhtab_delete_elem+0x185a/0x1b30 kernel/bpf/hashtab.c:2969
>  __rhtab_map_lookup_and_delete_batch+0x935/0xcb0 kernel/bpf/hashtab.c:3349
>  bpf_map_do_batch+0x445/0x630 kernel/bpf/syscall.c:-1
>  __sys_bpf+0x906/0xd90 kernel/bpf/syscall.c:-1
>
> this_cpu_ptr() requires the caller to prevent task migration.
> These helpers currently do not enforce that requirement and may
> be invoked from preemptible contexts, leading to accesses to another
> CPU's per-CPU cache after migration. Use get_cpu_ptr()/put_cpu_ptr()
> to pin the task while accessing the per-CPU allocator state.
>
> Fixes: 5af6807bdb10 ("bpf: Introduce bpf_mem_free_rcu() similar to kfree_rcu().")
> Fixes: 7c8199e24fa0 ("bpf: Introduce any context BPF specific memory allocator.")
> Reported-by: syzbot+fd7e415d891073b83e1f@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://syzkaller.appspot.com/bug?extid=fd7e415d891073b83e1f
> Signed-off-by: Edward Adam Davis <eadavis@xxxxxx>
> ---

from what I can see, bpf_mem_free() is called through bpf kfuncs only,
and all BPF programs run with migration disabled. So this seems like a
false positive. For per-cpu checking, it should probably check that
either preemption is disabled or migration is disabled. tl;dr, there
doesn't seem to be any

pw-bot: cr

> v1 -> v2: using guard against preemption
> v2 -> v3: replace get/put_cpu() to bpf_disable/enable_instrumentation()
> v3 -> v4: disable preempt to make this_cpu_ptr() work
> v4 -> v5: in mem free disable preemption
>

maybe throttle your patch resubmission spree a bit?..