Re: [PATCH v4 2/4] tracing/user_events: Introduce multi-format events

[Steven Rostedt]

On Thu, 22 Feb 2024 00:18:05 +0000
Beau Belgrave <beaub@xxxxxxxxxxxxxxxxxxx> wrote:

> Currently user_events supports 1 event with the same name and must have
> the exact same format when referenced by multiple programs. This opens
> an opportunity for malicous or poorly thought through programs to

  malicious? ;-)

-- Steve

> create events that others use with different formats. Another scenario
> is user programs wishing to use the same event name but add more fields
> later when the software updates. Various versions of a program may be
> running side-by-side, which is prevented by the current single format
> requirement.
> 
> Add a new register flag (USER_EVENT_REG_MULTI_FORMAT) which indicates
> the user program wishes to use the same user_event name, but may have
> several different formats of the event. When this flag is used, create
> the underlying tracepoint backing the user_event with a unique name
> per-version of the format. It's important that existing ABI users do
> not get this logic automatically, even if one of the multi format
> events matches the format. This ensures existing programs that create
> events and assume the tracepoint name will match exactly continue to
> work as expected. Add logic to only check multi-format events with
> other multi-format events and single-format events to only check
> single-format events during find.
> 
> Change system name of the multi-format event tracepoint to ensure that
> multi-format events are isolated completely from single-format events.
> This prevents single-format names from conflicting with multi-format
> events if they end with the same suffix as the multi-format events.
> 
> Add a register_name (reg_name) to the user_event struct which allows for
> split naming of events. We now have the name that was used to register
> within user_events as well as the unique name for the tracepoint. Upon
> registering events ensure matches based on first the reg_name, followed
> by the fields and format of the event. This allows for multiple events
> with the same registered name to have different formats. The underlying
> tracepoint will have a unique name in the format of {reg_name}.{unique_id}.
> 
> For example, if both "test u32 value" and "test u64 value" are used with
> the USER_EVENT_REG_MULTI_FORMAT the system would have 2 unique
> tracepoints. The dynamic_events file would then show the following:
>   u:test u64 count
>   u:test u32 count
> 
> The actual tracepoint names look like this:
>   test.0
>   test.1
> 
> Both would be under the new user_events_multi system name to prevent the
> older ABI from being used to squat on multi-formatted events and block
> their use.
> 
> Deleting events via "!u:test u64 count" would only delete the first
> tracepoint that matched that format. When the delete ABI is used all
> events with the same name will be attempted to be deleted. If
> per-version deletion is required, user programs should either not use
> persistent events or delete them via dynamic_events.
> 
> Signed-off-by: Beau Belgrave <beaub@xxxxxxxxxxxxxxxxxxx>