Re: [PATCH v2] perf record: Fix coredump with --overwrite and --max-size

[Yang Jihong]

Hello,

On 2023/1/7 5:12, Namhyung Kim wrote:
> Hello,
>
> On Wed, Jan 4, 2023 at 8:09 PM Yang Jihong <yangjihong1@xxxxxxxxxx> wrote:
> > Hello,
> >
> > On 2023/1/4 0:50, Namhyung Kim wrote:
> > > On Mon, Jan 2, 2023 at 8:20 AM Arnaldo Carvalho de Melo <acme@xxxxxxxxxx> wrote:
> > > > Em Thu, Dec 29, 2022 at 12:47:28PM +0000, Yang Jihong escreveu:
> > > > > 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
> > > > >
> > > > > 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
> > > > >
> > > > > we only need to check the value of done first.
> > > > > Also add variable check in record__bytes_written for code hardening,
> > > > > and save bytes_written separately to reduce one calculation.
> > > > >
> > > > > Fixes: 6d57581659f7 ("perf record: Add support for limit perf output file size")
> > > > > Signed-off-by: Yang Jihong <yangjihong1@xxxxxxxxxx>
> > > > > ---
> > > > >
> > > > > Changes since v1:
> > > > >    - Add variable check in record__bytes_written for code hardening.
> > > > >    - Save bytes_written separately to reduce one calculation.
> > > > >    - Remove rec->opts.tail_synthesize check.
> > > >
> > > > Namhyung, are you ok with this now?
> > > >
> > > > - Arnaldo
> > > >
> > > > >    tools/perf/builtin-record.c | 26 +++++++++++++++++---------
> > > > >    1 file changed, 17 insertions(+), 9 deletions(-)
> > > > >
> > > > > diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c
> > > > > index 29dcd454b8e2..acba9e43e519 100644
> > > > > --- a/tools/perf/builtin-record.c
> > > > > +++ b/tools/perf/builtin-record.c
> > > > > @@ -230,16 +230,29 @@ static u64 record__bytes_written(struct record *rec)
> > > > >         u64 bytes_written = rec->bytes_written;
> > > > >         struct record_thread *thread_data = rec->thread_data;
> > > > >
> > > > > +     if (thread_data == NULL)
> > > > > +             return bytes_written;
> > > > > +
> > >
> > > Then it won't count bytes written by threads, right?
> > > I think it needs to be saved somewhere.
> > I'm not sure here. Can you explain it more clearly, thanks :)
> > I can modify it accordingly.
> >
> > I think if thread_data == NULL, it is not thread data.
> > In this case, we just return rec->bytes_written.
>
> It can be thread data but freed before tail synthesis, right?
> In that case, I think it needs to add bytes_written by threads
> to calculate the correct data size.
Em... In the __cmd_record function, record__stop_threads is called 
before record__free_thread_data, so if the thread has been freed, there 
will be no thread data.
I think it's okay to ignore the situation you mentioned above.

Thanks,
Yang