Heiko Carstens reported[1] a bug when running the ftrace selftests. After running them, he noticed that the enabled_functions file had all functions enabled in it. That means something was registered to ftrace that shouldn't be.
The issue was with the accounting of the new fprobe logic which was built on top of the function graph infrastructure. Patch 3 of this series is the fix for that bug, but while debugging that, 3 other accounting bugs were discovered. The bug fix for the reported bug was that fprobes would update its counter every time a new fprobe was added, even if a new fprobe was attached to a function that was already attached to another fprobe. But when removing the fprobe, it would not decrement the counter of an fprobe with a duplicate. This left the fprobe ops attached to function graph and when it removed the last probe from the hash, it would create an empty filter hash which would tell function graph that it wanted to trace all functions. The solution was to always decrement the counter when a fprobe was removed. The first of the other bugs was that when enabling a second subops to the function graph infrastructure, it would add all functions to be called back and not just the functions for the two subops for tracing. This was due to the creation of the filter hash for the manager ops that controls the subops. The first iteration where the manage ops hash was NULL was mistaken as an "empty hash" which means to trace all functions. The second bug was when adding a function to the hash where the hash already had that function, it would allocate and create a new entry for it. This leaves duplicates and causes unnecessary overhead and memory wastage. The third bug was when the last fprobe was removed, it would just unregister from function graph but it did not remove the function from its own ops hash. When adding a new fprobe, it would not only enable the function for that new fprobe, but it would also enable the function of the last fprobe that was removed. Finally, a test was added to check and fail if any of the bugs were introduced, with the exception of the duplicate hash entries, as that was more of a memory waste and not something visible by user space. [1] https://lore.kernel.org/all/20250217114918.10397-a-...@linux.ibm.com/ Steven Rostedt (5): ftrace: Fix accounting of adding subops to a manager ops ftrace: Do not add duplicate entries in subops manager ops fprobe: Fix accounting of when to unregister from function graph fprobe: Always unregister fgraph function from ops selftests/ftrace: Update fprobe test to check enabled_functions file ---- kernel/trace/fprobe.c | 8 ++-- kernel/trace/ftrace.c | 33 +++++++++---- .../ftrace/test.d/dynevent/add_remove_fprobe.tc | 54 ++++++++++++++++++++++ 3 files changed, 81 insertions(+), 14 deletions(-)
