Messages in this thread | | | Date | Mon, 17 Jan 2011 21:34:59 +0100 | From | Oleg Nesterov <> | Subject | Re: Q: perf_event && task->ptrace_bps[] |
| |
On 11/08, Oleg Nesterov wrote: > > I am trying to understand the usage of hw-breakpoints in arch_ptrace(). > ptrace_set_debugreg() and related code looks obviously racy. Nothing > protects us against flush_ptrace_hw_breakpoint() called by the dying > tracee. Afaics we can leak perf_event or use the already freed memory > or both. > > Am I missed something? > > Looking into the git history, I don't even know which patch should be > blamed (if I am right), there were too many changes. I noticed that > 2ebd4ffb6d0cb877787b1e42be8485820158857e "perf events: Split out task > search into helper" moved the PF_EXITING check from find_get_context(). > This check coould help if sys_ptrace() races with SIGKILL, but it was > racy anyway.
Ping.
Any idea how to fix this cleanly? May be we can reuse perf_event_mutex, but this looks soooo ugly. And do_exit()->flush_ptrace_hw_breakpoint() has the strange "FIXME:" comment which doesn't help me to understand what can we do.
Probably the best fix is to change this code so that the tracer owns ->ptrace_bps[], not the tracee. But this is not trivial, and needs a lot of changes in ptrace code.
I am reading perf_event.c, but all I found so far is a couple of trivial methods to crash the kernel via sys_perf_event_open(), will report tomorrow...
Oleg.
| |