Re: [PATCH 2/3] perf unwind-libdw: Handle JIT-generated DSOs properly

[Ian Rogers]

On Mon, Dec 11, 2023 at 11:05 PM Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
>
> Usually DSOs are mapped from the beginning of the file, so the base
> address of the DSO can be calculated by map->start - map->pgoff.
>
> However, JIT DSOs which are generated by `perf inject -j`, are mapped
> only the code segment.  This makes unwind-libdw code confusing and
> rejects processing unwinds in the JIT DSOs.  It should use the map
> start address as base for them to fix the confusion.
>
> Fixes: 1fe627da3033 ("perf unwind: Take pgoff into account when reporting elf to libdwfl")
> Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>
> ---
>  tools/perf/util/unwind-libdw.c | 21 +++++++++++++++++----
>  1 file changed, 17 insertions(+), 4 deletions(-)
>
> diff --git a/tools/perf/util/unwind-libdw.c b/tools/perf/util/unwind-libdw.c
> index 8554db3fc0d7..6013335a8dae 100644
> --- a/tools/perf/util/unwind-libdw.c
> +++ b/tools/perf/util/unwind-libdw.c
> @@ -46,6 +46,7 @@ static int __report_module(struct addr_location *al, u64 ip,
>  {
>         Dwfl_Module *mod;
>         struct dso *dso = NULL;
> +       Dwarf_Addr base;
>         /*
>          * Some callers will use al->sym, so we can't just use the
>          * cheaper thread__find_map() here.
> @@ -58,13 +59,25 @@ static int __report_module(struct addr_location *al, u64 ip,
>         if (!dso)
>                 return 0;
>
> +       /*
> +        * The generated JIT DSO files only map the code segment without
> +        * ELF headers.  Since JIT codes used to be packed in a memory
> +        * segment, calculating the base address using pgoff falls into
> +        * a different code in another DSO.  So just use the map->start
> +        * directly to pick the correct one.
> +        */
> +       if (!strncmp(dso->long_name, "/tmp/jitted-", 12))

Perhaps it would be better to test:
dso->symtab_type == DSO_BINARY_TYPE__JAVA_JIT

Thanks,
Ian

> +               base = map__start(al->map);
> +       else
> +               base = map__start(al->map) - map__pgoff(al->map);
> +
>         mod = dwfl_addrmodule(ui->dwfl, ip);
>         if (mod) {
>                 Dwarf_Addr s;
>
>                 dwfl_module_info(mod, NULL, &s, NULL, NULL, NULL, NULL, NULL);
> -               if (s != map__start(al->map) - map__pgoff(al->map))
> -                       mod = 0;
> +               if (s != base)
> +                       mod = NULL;
>         }
>
>         if (!mod) {
> @@ -72,14 +85,14 @@ static int __report_module(struct addr_location *al, u64 ip,
>
>                 __symbol__join_symfs(filename, sizeof(filename), dso->long_name);
>                 mod = dwfl_report_elf(ui->dwfl, dso->short_name, filename, -1,
> -                                     map__start(al->map) - map__pgoff(al->map), false);
> +                                     base, false);
>         }
>         if (!mod) {
>                 char filename[PATH_MAX];
>
>                 if (dso__build_id_filename(dso, filename, sizeof(filename), false))
>                         mod = dwfl_report_elf(ui->dwfl, dso->short_name, filename, -1,
> -                                             map__start(al->map) - map__pgoff(al->map), false);
> +                                             base, false);
>         }
>
>         if (mod) {
> --
> 2.43.0.472.g3155946c3a-goog
>