linux-kernel - Re: [PATCHv3] perf tool: fix dereferencing NULL al->maps

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Message-ID: <CAP-5=fUjbsv93DKDxaNXzfe3b9H_0MQyjauNxsxM1XTN7T8QAw@mail.gmail.com>
Date: Sat, 20 Jul 2024 22:48:22 -0700
From: Ian Rogers <irogers@...gle.com>
To: Casey Chen <cachen@...estorage.com>
Cc: linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org, 
	namhyung@...nel.org, yzhong@...estorage.com
Subject: Re: [PATCHv3] perf tool: fix dereferencing NULL al->maps

On Sat, Jul 20, 2024 at 8:20 PM Casey Chen <cachen@...estorage.com> wrote:
>
> With 0dd5041c9a0e ("perf addr_location: Add init/exit/copy functions"),
> when cpumode is 3 (macro PERF_RECORD_MISC_HYPERVISOR),
> thread__find_map() could return with al->maps being NULL.
>
> The path below could add a callchain_cursor_node with NULL ms.maps.
>
> add_callchain_ip()
>   thread__find_symbol(.., &al)
>     thread__find_map(.., &al)   // al->maps becomes NULL
>   ms.maps = maps__get(al.maps)
>   callchain_cursor_append(..., &ms, ...)
>     node->ms.maps = maps__get(ms->maps)
>
> Then the path below would dereference NULL maps and get segfault.
>
> fill_callchain_info()
>   maps__machine(node->ms.maps);
>
> Fix it by checking if maps is NULL in maps__machine().

The intent with the accessors is that maps is like a "this" pointer.
In C++ a this pointer of NULL would be undefined behavior. As such I'd
prefer if we could add the NULL test in the caller.

Thanks,
Ian

> ---
>  tools/perf/util/maps.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/tools/perf/util/maps.c b/tools/perf/util/maps.c
> index 432399cbe5dd..d243cb794a99 100644
> --- a/tools/perf/util/maps.c
> +++ b/tools/perf/util/maps.c
> @@ -169,7 +169,7 @@ static void maps__set_maps_by_name_sorted(struct maps *maps, bool value)
>
>  struct machine *maps__machine(const struct maps *maps)
>  {
> -       return RC_CHK_ACCESS(maps)->machine;
> +       return maps ? RC_CHK_ACCESS(maps)->machine : NULL;
>  }
>
>  unsigned int maps__nr_maps(const struct maps *maps)
> --
> 2.45.2
>
>