linux-kernel - Re: [PATCH] perf inject jit: Ignore memfd mmap events if jitdump present

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Date:   Thu, 4 Aug 2022 08:22:14 -0700
From:   Ian Rogers <irogers@...gle.com>
To:     Brian Robbins <brianrob@...ux.microsoft.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] perf inject jit: Ignore memfd mmap events if jitdump present

On Tue, Aug 2, 2022 at 11:25 AM Brian Robbins
<brianrob@...ux.microsoft.com> wrote:
>
> Some processes store jitted code in memfd mappings to avoid having rwx
> mappings.  These processes map the code with a writeable mapping and a
> read-execute mapping.  They write the code using the writeable mapping
> and then unmap the writeable mapping.  All subsequent execution is
> through the read-execute mapping.
>
> perf inject --jit ignores //anon* mappings for each process where a
> jitdump is present because it expects to inject mmap events for each
> jitted code range, and said jitted code ranges will overlap with the
> //anon* mappings.
>
> Ignore /memfd: mappings so that jitted code contained in /memfd:
> mappings is treated the same way as jitted code contained in //anon*
> mappings.
>
> Signed-off-by: Brian Robbins <brianrob@...ux.microsoft.com>

Acked-by: Ian Rogers <irogers@...gle.com>

> ---
>  tools/perf/util/jitdump.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/tools/perf/util/jitdump.c b/tools/perf/util/jitdump.c
> index a23255773c60..335a3c61940b 100644
> --- a/tools/perf/util/jitdump.c
> +++ b/tools/perf/util/jitdump.c
> @@ -845,8 +845,12 @@ jit_process(struct perf_session *session,
>         if (jit_detect(filename, pid, nsi)) {
>                 nsinfo__put(nsi);
>
> -               // Strip //anon* mmaps if we processed a jitdump for this pid
> -               if (jit_has_pid(machine, pid) && (strncmp(filename, "//anon", 6) == 0))
> +               /*
> +                * Strip //anon* and /memfd:* mmaps if we processed a jitdump for this pid
> +                */
> +               if (jit_has_pid(machine, pid) &&
> +                       ((strncmp(filename, "//anon", 6) == 0) ||
> +                        (strncmp(filename, "/memfd:", 7) == 0))

Related to this there is the prctl PR_SET_VMA_ANON_NAME which will
name mapping to start with "[anon:"
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/filesystems/proc.rst#n434
I wonder also we should be checking the pages are executable.

Thanks,
Ian

>                         return 1;
>
>                 return 0;
> --
> 2.25.1
>