On 23/9/26 08:08, Donggeun Yoo wrote:
> pcpu_init_value() initializes the per-cpu area of a newly created
> [lru_]percpu_hash element. The area is recycled, so when the value
> comes from a BPF program (onallcpus == false) it writes the running
> CPU's slot and zeroes the rest.
>
> bpf_percpu_hash_update() passes onallcpus == true, which delegates to
> pcpu_copy_value(). Since BPF_F_CPU was added that writes only the CPU
> named in map_flags when the flag is set, so on the create path the other
> slots keep the recycled element's values:
>
> update(k1, 0xdeadc0de, BPF_F_ALL_CPUS) every CPU holds 0xdeadc0de
> delete(k1) element back on the freelist
> update(k2, 0xc0ffee, BPF_F_CPU | 0) creates, writes CPU 0 only
> lookup(k2) CPU 0 0xc0ffee, rest 0xdeadc0de
>
> Zero-fill the other CPUs on that arm too.
>
> Fixes: c6936161fd55 ("bpf: Add BPF_F_CPU and BPF_F_ALL_CPUS flags support for
> percpu_hash and lru_percpu_hash maps")
> Signed-off-by: Donggeun Yoo <[email protected]>
> ---
> The merged arm no longer calls bpf_obj_cancel_fields() on the named
> CPU. That call is inert here: it acts only on BPF_TIMER,
> BPF_WORKQUEUE and BPF_TASK_WORK, which map_check_btf() rejects for
> [lru_]percpu_hash.
>
> v2: name the BPF_F_CPU entry condition in the block comment.
>
> kernel/bpf/hashtab.c | 10 ++++++----
> 1 file changed, 6 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
> index 4f495dcbf670c..f7a9c432d9b76 100644
> --- a/kernel/bpf/hashtab.c
> +++ b/kernel/bpf/hashtab.c
> @@ -1054,14 +1054,16 @@ static void pcpu_init_value(struct bpf_htab *htab,
> void __percpu *pptr,
> /* When not setting the initial value on all cpus, zero-fill element
> * values for other cpus. Otherwise, bpf program has no way to ensure
> * known initial values for cpus other than current one
> - * (onallcpus=false always when coming from bpf prog).
> + * (onallcpus=false always when coming from bpf prog,
> + * map_flags & BPF_F_CPU when coming from syscall but setting
> + * only one cpu).
> */
> - if (!onallcpus) {
> - int current_cpu = raw_smp_processor_id();
> + if (!onallcpus || (map_flags & BPF_F_CPU)) {
> + int init_cpu = onallcpus ? map_flags >> 32 :
> raw_smp_processor_id();
'onallcpus ? map_flags >> 32' does not look reasonable.
Based on the 'if' above, either of these looks straightforward to me.
int init_cpu = !onallcpus ? raw_smp_processor_id() : map_flags >> 32;
or
int init_cpu = (map_flags & BPF_F_CPU) ? map_flags >> 32 :
raw_smp_processor_id();
Thanks,
Leon
> int cpu;
>
> for_each_possible_cpu(cpu) {
> - if (cpu == current_cpu)
> + if (cpu == init_cpu)
> copy_map_value(&htab->map, per_cpu_ptr(pptr,
> cpu), value);
> else /* Since elem is preallocated, we cannot touch
> special fields */
> zero_map_value(&htab->map, per_cpu_ptr(pptr,
> cpu));
