PAPI/perf_event allows creating events that have invalid names

[Vince Weaver]

Yan Liu reports that PAPI lets her create the following event without error: DTLB_LOAD_MISSES:MISS_CAUSES_A_WALK,DTLB_STORE_MISSES:MISS_CAUSES_A_WALK:u=1

PAPI seems to be treating the second event stuck on the end as a umask, but doesn't seem to mind that it's not a valid umask.

[Philip J Mucci]

Hmm. Seems like this should be handled by libpfm, no?

[Vince Weaver]

The pe_libpfm4 code is difficult to wade through, especially since user/kernel u=1/k=1 mask support was added.

The issue seems to be we parse out the event names and umasks separately.

To match umasks, we do a strncmp() of each mask in the proposed event against each possible umask. The problem is we strncmp with the size set to the possible umask, so if our umask matches but is longer it will be detected as a match even if what we are passing in is much longer.

So I guess the fix is to do a size check. This is actually a tricky bug, it means if there are multiple umasks that start with the same characters we might accidentally match to the wrong one.

[Vince Weaver]

I take it back, we actually do match exactly and check for size. Yet somehow a umask with garbage on the end still gets accepted as good. Still investigating.

[Vince Weaver]

Found the bug, we weren't failing on invalid event.

Fixed by 4bc699f5f737b0730881ed3d9b5122230d438429

[Vince Weaver]

as a follow up, PAPI wasn't handling this right, but indeed libpfm4 also has a bug and is accepting events with invalid umasks. I'll report this upstream too.