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SubjectRe: [PATCH] perf top: expand the range of multithreaded phase
Hello,

Sorry for the late reply.

On Fri, Mar 17, 2023 at 3:05 AM Hangliang Lai <laihangliang1@huawei.com> wrote:
>
> In __cmd_top, perf_set_multithreaded is used to enable pthread_rwlock, thus
> donw_read and down_write can work to handle concurrency problems. Then top
> use perf_set_singlethreaded and switch to single threaded phase, assuming
> that no thread concurrency will happen later.
>
> However, a UAF problem could occur in perf top in single threaded phase,
> The concurrent procedure is like this:
>
> display_thread process_thread
> -------------- --------------
>
> thread__comm_len
> -> thread__comm_str
> -> __thread__comm_str(thread)
> thread__delete
> -> comm__free
> -> comm_str__put
> -> zfree(&cs->str)
> -> thread->comm_len = strlen(comm);
>
> Since in single thread phase, perf_singlethreaded is true, down_read and
> down_write can not work to avoid concurrency problems.
>
> This patch put perf_set_singlethreaded to the function tail to expand the
> multithreaded phase range, make display_thread and process_thread run
> safe.

I think it should be unconditional as perf top is always multi-threaded.

>
> Signed-off-by: Hangliang Lai <laihangliang1@huawei.com>
> Reported-by: Wenyu Liu <liuwenyu7@huawei.com>
> Reviewed-by: Yunfeng Ye <yeyunfeng@huawei.com>
> ---
> tools/perf/builtin-top.c | 7 ++++---
> 1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/tools/perf/builtin-top.c b/tools/perf/builtin-top.c
> index 7c6413447..74239940b 100644
> --- a/tools/perf/builtin-top.c
> +++ b/tools/perf/builtin-top.c
> @@ -1280,9 +1280,6 @@ static int __cmd_top(struct perf_top *top)
> top->evlist->core.threads, false,
> top->nr_threads_synthesize);
>
> - if (top->nr_threads_synthesize > 1)
> - perf_set_singlethreaded();

Instead, we can simply do

perf_set_multithreaded();

If top->nr_threads_synthesize > 1, no effect. If not, it turns
the switch on here.

> -
> if (perf_hpp_list.socket) {
> ret = perf_env__read_cpu_topology_map(&perf_env);
> if (ret < 0) {
> @@ -1359,6 +1356,10 @@ out_join:
> out_join_thread:
> pthread_cond_signal(&top->qe.cond);
> pthread_join(thread_process, NULL);
> +
> + if (top->nr_threads_synthesize > 1)
> + perf_set_singlethreaded();

And remove the condition here.

Thanks,
Namhyung


> +
> return ret;
> }
>
> --
> 2.33.0
>

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