On Thu, Jul 16, 2020 at 03:59:32PM -0700, Song Liu wrote:
> +
> +BPF_CALL_3(bpf_get_stackid_pe, struct bpf_perf_event_data_kern *, ctx,
> + struct bpf_map *, map, u64, flags)
> +{
> + struct perf_event *event = ctx->event;
> + struct perf_callchain_entry *trace;
> + bool has_kernel, has_user;
> + bool kernel, user;
> +
> + /* perf_sample_data doesn't have callchain, use bpf_get_stackid */
> + if (!(event->attr.sample_type & __PERF_SAMPLE_CALLCHAIN_EARLY))
what if event was not created with PERF_SAMPLE_CALLCHAIN ?
Calling the helper will still cause crashes, no?
> + return bpf_get_stackid((unsigned long)(ctx->regs),
> + (unsigned long) map, flags, 0, 0);
> +
> + if (unlikely(flags & ~(BPF_F_SKIP_FIELD_MASK | BPF_F_USER_STACK |
> + BPF_F_FAST_STACK_CMP | BPF_F_REUSE_STACKID)))
> + return -EINVAL;
> +
> + user = flags & BPF_F_USER_STACK;
> + kernel = !user;
> +
> + has_kernel = !event->attr.exclude_callchain_kernel;
> + has_user = !event->attr.exclude_callchain_user;
> +
> + if ((kernel && !has_kernel) || (user && !has_user))
> + return -EINVAL;
this will break existing users in a way that will be very hard for them to
debug.
If they happen to set exclude_callchain_* flags during perf_event_open
the helpers will be failing at run-time.
One can argue that when precise_ip=1 the bpf_get_stack is broken, but
this is a change in behavior.
It also seems to be broken when PERF_SAMPLE_CALLCHAIN was not set at event
creation time, but precise_ip=1 was.
> +
> + trace = ctx->data->callchain;
> + if (unlikely(!trace))
> + return -EFAULT;
> +
> + if (has_kernel && has_user) {
shouldn't it be || ?
> + __u64 nr_kernel = count_kernel_ip(trace);
> + int ret;
> +
> + if (kernel) {
> + __u64 nr = trace->nr;
> +
> + trace->nr = nr_kernel;
> + ret = __bpf_get_stackid(map, trace, flags);
> +
> + /* restore nr */
> + trace->nr = nr;
> + } else { /* user */
> + u64 skip = flags & BPF_F_SKIP_FIELD_MASK;
> +
> + skip += nr_kernel;
> + if (skip > BPF_F_SKIP_FIELD_MASK)
> + return -EFAULT;
> +
> + flags = (flags & ~BPF_F_SKIP_FIELD_MASK) | skip;
> + ret = __bpf_get_stackid(map, trace, flags);
> + }
> + return ret;
> + }
> + return __bpf_get_stackid(map, trace, flags);
...
> + if (has_kernel && has_user) {
> + __u64 nr_kernel = count_kernel_ip(trace);
> + int ret;
> +
> + if (kernel) {
> + __u64 nr = trace->nr;
> +
> + trace->nr = nr_kernel;
> + ret = __bpf_get_stack(ctx->regs, NULL, trace, buf,
> + size, flags);
> +
> + /* restore nr */
> + trace->nr = nr;
> + } else { /* user */
> + u64 skip = flags & BPF_F_SKIP_FIELD_MASK;
> +
> + skip += nr_kernel;
> + if (skip > BPF_F_SKIP_FIELD_MASK)
> + goto clear;
> +
> + flags = (flags & ~BPF_F_SKIP_FIELD_MASK) | skip;
> + ret = __bpf_get_stack(ctx->regs, NULL, trace, buf,
> + size, flags);
> + }
Looks like copy-paste. I think there should be a way to make it
into common helper.
I think the main isssue is wrong interaction with event attr flags.
I think the verifier should detect that bpf_get_stack/bpf_get_stackid
were used and prevent attaching to perf_event with attr.precise_ip=1
and PERF_SAMPLE_CALLCHAIN is not specified.
I was thinking whether attaching bpf to event can force setting of
PERF_SAMPLE_CALLCHAIN, but that would be a surprising behavior,
so not a good idea.
So the only thing left is to reject attach when bpf_get_stack is used
in two cases:
if attr.precise_ip=1 and PERF_SAMPLE_CALLCHAIN is not set.
(since it will lead to crashes)
if attr.precise_ip=1 and PERF_SAMPLE_CALLCHAIN is set,
but exclude_callchain_[user|kernel]=1 is set too.
(since it will lead to surprising behavior of bpf_get_stack)
Other ideas?
