Re: [PATCH net-next 2/3] bpf: introduce BPF_MAP_TYPE_STACK_TRACE

Peter Zijlstra
2016-02-25T14:23:39Z

On Wed, Feb 17, 2016 at 07:58:58PM -0800, Alexei Starovoitov wrote:
> +static u64 bpf_get_stackid(u64 r1, u64 r2, u64 flags, u64 r4, u64 r5)
> +{
> + struct pt_regs *regs = (struct pt_regs *) (long) r1;
> + struct bpf_map *map = (struct bpf_map *) (long) r2;
> + struct bpf_stack_map *smap = container_of(map, struct bpf_stack_map, map);
> + struct perf_callchain_entry *trace;
> + struct stack_map_bucket *bucket, *new_bucket, *old_bucket;
> + u32 max_depth = map->value_size / 8;
> + /* stack_map_alloc() checks that max_depth <= PERF_MAX_STACK_DEPTH */
> + u32 init_nr = PERF_MAX_STACK_DEPTH - max_depth;
> + u32 skip = flags & BPF_F_SKIP_FIELD_MASK;
> + u32 hash, id, trace_nr, trace_len;
> + bool user = flags & BPF_F_USER_STACK;
> + bool kernel = !user;
> + u64 *ips;
> +
> + if (unlikely(flags & ~(BPF_F_SKIP_FIELD_MASK | BPF_F_USER_STACK |
> + BPF_F_FAST_STACK_CMP | BPF_F_REUSE_STACKID)))
> + return -EINVAL;
> +
> + trace = get_perf_callchain(regs, init_nr, kernel, user, false, false);
> +
> + if (unlikely(!trace))
> + /* couldn't fetch the stack trace */
> + return -EFAULT;
> +
> + /* get_perf_callchain() guarantees that trace->nr >= init_nr
> + * and trace-nr <= PERF_MAX_STACK_DEPTH, so trace_nr <= max_depth
> + */
> + trace_nr = trace->nr - init_nr;
> +
> + if (trace_nr <= skip)
> + /* skipping more than usable stack trace */
> + return -EFAULT;
> +
> + trace_nr -= skip;
> + trace_len = trace_nr * sizeof(u64);
> + ips = trace->ip + skip + init_nr;
> + hash = jhash2((u32 *)ips, trace_len / sizeof(u32), 0);
> + id = hash & (smap->n_buckets - 1);

Its not at all clear where the corresponding rcu_read_lock() is at.

> + bucket = rcu_dereference(smap->buckets[id]);
> +
> + if (bucket && bucket->hash == hash) {
> + if (flags & BPF_F_FAST_STACK_CMP)
> + return id;
> + if (bucket->nr == trace_nr &&
> + memcmp(bucket->ip, ips, trace_len) == 0)
> + return id;
> + }
> +
> + /* this call stack is not in the map, try to add it */
> + if (bucket && !(flags & BPF_F_REUSE_STACKID))
> + return -EEXIST;
> +
> + new_bucket = kmalloc(sizeof(struct stack_map_bucket) + map->value_size,
> + GFP_ATOMIC | __GFP_NOWARN);
> + if (unlikely(!new_bucket))
> + return -ENOMEM;
> +
> + memcpy(new_bucket->ip, ips, trace_len);
> + memset(new_bucket->ip + trace_len / 8, 0, map->value_size - trace_len);
> + new_bucket->hash = hash;
> + new_bucket->nr = trace_nr;
> +
> + old_bucket = xchg(&smap->buckets[id], new_bucket);
> + if (old_bucket)
> + kfree_rcu(old_bucket, rcu);
> + return id;
> +}