On Tue, Jun 29, 2021 at 6:55 AM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote: > > From: "Steven Rostedt (VMware)" <rostedt@xxxxxxxxxxx> > > All internal use cases for tracepoint_probe_register() is set to not ever > be called with the same function and data. If it is, it is considered a > bug, as that means the accounting of handling tracepoints is corrupted. > If the function and data for a tracepoint is already registered when > tracepoint_probe_register() is called, it will call WARN_ON_ONCE() and > return with EEXISTS. > > The BPF system call can end up calling tracepoint_probe_register() with > the same data, which now means that this can trigger the warning because > of a user space process. As WARN_ON_ONCE() should not be called because > user space called a system call with bad data, there needs to be a way to > register a tracepoint without triggering a warning. > > Enter tracepoint_probe_register_may_exist(), which can be called, but will > not cause a WARN_ON() if the probe already exists. It will still error out > with EEXIST, which will then be sent to the user space that performed the > BPF system call. > > This keeps the previous testing for issues with other users of the > tracepoint code, while letting BPF call it with duplicated data and not > warn about it. There doesn't seem to be anything conceptually wrong with attaching the same BPF program twice to the same tracepoint. Is it a hard requirement to have a unique tp+callback combination, or was it done mostly to detect an API misuse? How hard is it to support such use cases? I was surprised to discover this is not supported (though I never had a use for this, had to construct a test to see the warning). > > Link: https://lore.kernel.org/lkml/20210626135845.4080-1-penguin-kernel@xxxxxxxxxxxxxxxxxxx/ > Link: https://syzkaller.appspot.com/bug?id=41f4318cf01762389f4d1c1c459da4f542fe5153 [1]` > > Cc: stable@xxxxxxxxxxxxxxx > Fixes: c4f6699dfcb85 ("bpf: introduce BPF_RAW_TRACEPOINT") > Reported-by: syzbot <syzbot+721aa903751db87aa244@xxxxxxxxxxxxxxxxxxxxxxxxx> > Reported-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx> > Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx> > --- > > #syz test: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master > > include/linux/tracepoint.h | 10 ++++++++++ > kernel/trace/bpf_trace.c | 3 ++- > kernel/tracepoint.c | 33 ++++++++++++++++++++++++++++++--- > 3 files changed, 42 insertions(+), 4 deletions(-) > [...]
