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

[Edward Adam Davis <eadavis@qq.com>]

On Wed, 1 Jul 2026 12:57:52 -0700, Andrii Nakryiko 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@syzkaller.appspotmail.com
> > Closes: https://syzkaller.appspot.com/bug?extid=fd7e415d891073b83e1f
> > Signed-off-by: Edward Adam Davis <eadavis@qq.com>
> > ---
> 
> 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
Today, I researched the call sites of bpf_mem_free() in detail; it is
called directly by bpf kfuns only. 

Different BPF program types execute in distinct kernel contexts: for
instance, XDP and TC typically run in non-sleepable networking contexts,
whereas `raw_tp`, `tracing`, `LSM`, `syscall` and especially sleepable
BPF programs may run in contexts where preemption and migration are
enabled. Consequently, any shared BPF code (such as a helper or kfunc)
that accesses per-CPU data must not rely on the caller having disabled
migration; instead, it must internally ensure that CPU migration does
not occur during the access.

Taking into account the CI reviewer's feedback [1] and my further
investigation into BPF programs as described above, the adjustments
related to bpf_mem_free() should be retained.

[1]
https://lore.kernel.org/all/20260630094823.897BD1F00A3D@smtp.kernel.org
> 
> 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?..
My earlier actions were driven by a desire to keep pace with the CI
reviewer's response speed.
Moving forward, I will slow down the cadence of new patch submissions,
as you suggested.