Em Fri, Jan 17, 2014 at 10:00:20AM +0100, Stephane Eranian escreveu: > Hi, > > I have been debugging a NULL pointer issue with perf stat unit/scale code > and in the process I ran into what appeared like a double-free issue reported > by glibc. It took me a while to realize that it was because of memory > corruption > caused by a recent change in how evsel are freed. > > My test case is simple. I used RAPL but I think any event with a suggested > cpumask in /sys/devices/XXX/cpumask will do: > > # perf stat -a -e power/energy-cores/ ls > > The issue boils down to the fact that evsels have their file descriptors > closed > twice nowadays. Once in __run_per_stat() via perf_evsel__close_fd() and > twice in perf_evlist__close(). > > Now, calling close() twice is okay. However the fd is then set to -1. > That's still okay with close(). The problem is elsewhere. > > It comes from the ncpus argument passed to perf_evsel__close(). It is > DIFFERENT between the evsel and the evlist when cpumask are used.
Oops, at some point I knew that set of globals and mixup of evlists in builtin-stat would bite :-\ I guess it was introduced in: commit 7ae92e744e3fb389afb1e24920ecda331d360c61 Author: Yan, Zheng <[email protected]> Date: Mon Sep 10 15:53:50 2012 +0800 perf stat: Check PMU cpumask file I need to untangle that direct usage of the target, and global evlist to properly fix this, but in the meantime I'll take a look at your patch, thanks for doing this work. > Take my case, 8 CPUs machine but a 1 CPU cpumask. The evsel allocates > the xyarray for 1 CPU 1 thread. The fd are first close with 1 CPU, 1 thread. > But then evlist_close() comes in and STILL thinks the events were using > 8 CPUs, 1 thread and thus a xyarray of that size. And this causes writes > to entries that are beyond the xyarray when the fds are set to -1, thereby > causing memory corruption which I was lucky to catch via glibc. > > First, why are we closing the descriptors twice? > > Second, I have a fix that seems to work for me. It uses the evsel->cpus > if evsel->cpus exists, otherwise it defaults to evtlist->cpus. Looks like > a reasonable thing to do to me, but is it? I would rather avoid the double > close altogether. > > > Opinion? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
