Re: [PATCH] perf record: Fix coredump with --overwrite and --max-size

[Namhyung Kim]

On Tue, Dec 27, 2022 at 5:10 AM Yang Jihong <yangjihong1@xxxxxxxxxx> wrote:
>
> When --overwrite and --max-size options of perf record are used together,
> a segmentation fault occurs. The following is an example:
>
>  # perf record -e sched:sched* --overwrite --max-size 1M -a -- sleep 1
>   [ perf record: Woken up 1 times to write data ]
>   perf: Segmentation fault
>   Obtained 1 stack frames.
>   [0xc4c67f]
>   Segmentation fault (core dumped)
>
> backtrace of the core file is as follows:
>
>   #0  0x0000000000417990 in process_locked_synthesized_event (tool=0x0, event=0x15, sample=0x1de0, machine=0xf8) at builtin-record.c:630
>   #1  0x000000000057ee53 in perf_event__synthesize_threads (nr_threads_synthesize=21, mmap_data=<optimized out>, needs_mmap=<optimized out>, machine=0x17ad9b0, process=<optimized out>, tool=0x0) at util/synthetic-events.c:1950
>   #2  __machine__synthesize_threads (nr_threads_synthesize=0, data_mmap=<optimized out>, needs_mmap=<optimized out>, process=<optimized out>, threads=0x8, target=0x8, tool=0x0, machine=0x17ad9b0) at util/synthetic-events.c:1936
>   #3  machine__synthesize_threads (machine=0x17ad9b0, target=0x8, threads=0x8, needs_mmap=<optimized out>, data_mmap=<optimized out>, nr_threads_synthesize=0) at util/synthetic-events.c:1947
>   #4  0x000000000040165d in record__synthesize (tail=<optimized out>, rec=0xbe2520 <record>) at builtin-record.c:2010
>   #5  0x0000000000403989 in __cmd_record (argc=<optimized out>, argv=<optimized out>, rec=0xbe2520 <record>) at builtin-record.c:2810
>   #6  0x00000000004196ba in record__init_thread_user_masks (rec=0xbe2520 <record>, cpus=0x17a65f0) at builtin-record.c:3837
>   #7  record__init_thread_masks (rec=0xbe2520 <record>) at builtin-record.c:3938
>   #8  cmd_record (argc=1, argv=0x7ffdd692dc60) at builtin-record.c:4241
>   #9  0x00000000004b701d in pager_command_config (var=0x0, value=0x15 <error: Cannot access memory at address 0x15>, data=0x1de0) at perf.c:117
>   #10 0x00000000004b732b in get_leaf_frame_caller_aarch64 (sample=0xfffffffb, thread=0x0, usr_idx=<optimized out>) at util/arm64-frame-pointer-unwind-support.c:56
>   #11 0x0000000000406331 in execv_dashed_external (argv=0x7ffdd692d9e8) at perf.c:410
>   #12 run_argv (argcp=<synthetic pointer>, argv=<synthetic pointer>) at perf.c:431
>   #13 main (argc=<optimized out>, argv=0x7ffdd692d9e8) at perf.c:562

I'm not sure this callstack is correct.

>
> The reason is that record__bytes_written accesses the freed memory rec->thread_data,
> The process is as follows:
>   __cmd_record
>     -> record__free_thread_data
>       -> zfree(&rec->thread_data)         // free rec->thread_data
>     -> record__synthesize
>       -> perf_event__synthesize_id_index
>         -> process_synthesized_event
>           -> record__write
>             -> record__bytes_written     // access rec->thread_data
>
> In the overwrite scenario, to synthesize non-sample events,
> we do not need to check perf size limit.

Hmm.. I think we should prevent this kind of access after
record__free_thread_data().  We may set nr_threads to 0
and save the bytes_written for threads separately.

Thanks,
Namhyung


>
> Fixes: 6d57581659f7 ("perf record: Add support for limit perf output file size")
> Signed-off-by: Yang Jihong <yangjihong1@xxxxxxxxxx>
> ---
>  tools/perf/builtin-record.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c
> index 29dcd454b8e2..c5f169150d63 100644
> --- a/tools/perf/builtin-record.c
> +++ b/tools/perf/builtin-record.c
> @@ -260,7 +260,7 @@ static int record__write(struct record *rec, struct mmap *map __maybe_unused,
>         else
>                 rec->bytes_written += size;
>
> -       if (record__output_max_size_exceeded(rec) && !done) {
> +       if (!rec->opts.tail_synthesize && record__output_max_size_exceeded(rec) && !done) {
>                 fprintf(stderr, "[ perf record: perf size limit reached (%" PRIu64 " KB),"
>                                 " stopping session ]\n",
>                                 record__bytes_written(rec) >> 10);
> --
> 2.17.1
>