Re: [PATCH 1/6] perf offcpu: Fix a build failure on old kernels

[Arnaldo Carvalho de Melo <acme@kernel.org>]

Em Fri, Jun 24, 2022 at 04:13:08PM -0700, Namhyung Kim escreveu:
> Old kernels have task_struct which contains "state" field and newer
> kernels have "__state".  While the get_task_state() in the BPF code
> handles that in some way, it assumed the current kernel has the new
> definition and it caused a build error on old kernels.
> 
> We should not assume anything and access them carefully.  Do not use
> the task struct directly and access them using new and old definitions
> in a row.

I added a:

Fixes: edc41a1099c2d08c ("perf record: Enable off-cpu analysis with BPF")

Ok?

- Arnaldo
 
> Reported-by: Ian Rogers <irogers@google.com>
> Signed-off-by: Namhyung Kim <namhyung@kernel.org>
> ---
>  tools/perf/util/bpf_skel/off_cpu.bpf.c | 20 ++++++++++++++------
>  1 file changed, 14 insertions(+), 6 deletions(-)
> 
> diff --git a/tools/perf/util/bpf_skel/off_cpu.bpf.c b/tools/perf/util/bpf_skel/off_cpu.bpf.c
> index 792ae2847080..cc6d7fd55118 100644
> --- a/tools/perf/util/bpf_skel/off_cpu.bpf.c
> +++ b/tools/perf/util/bpf_skel/off_cpu.bpf.c
> @@ -71,6 +71,11 @@ struct {
>  	__uint(max_entries, 1);
>  } cgroup_filter SEC(".maps");
>  
> +/* new kernel task_struct definition */
> +struct task_struct___new {
> +	long __state;
> +} __attribute__((preserve_access_index));
> +
>  /* old kernel task_struct definition */
>  struct task_struct___old {
>  	long state;
> @@ -93,14 +98,17 @@ const volatile bool uses_cgroup_v1 = false;
>   */
>  static inline int get_task_state(struct task_struct *t)
>  {
> -	if (bpf_core_field_exists(t->__state))
> -		return BPF_CORE_READ(t, __state);
> +	/* recast pointer to capture new type for compiler */
> +	struct task_struct___new *t_new = (void *)t;
>  
> -	/* recast pointer to capture task_struct___old type for compiler */
> -	struct task_struct___old *t_old = (void *)t;
> +	if (bpf_core_field_exists(t_new->__state)) {
> +		return BPF_CORE_READ(t_new, __state);
> +	} else {
> +		/* recast pointer to capture old type for compiler */
> +		struct task_struct___old *t_old = (void *)t;
>  
> -	/* now use old "state" name of the field */
> -	return BPF_CORE_READ(t_old, state);
> +		return BPF_CORE_READ(t_old, state);
> +	}
>  }
>  
>  static inline __u64 get_cgroup_id(struct task_struct *t)
> -- 
> 2.37.0.rc0.161.g10f37bed90-goog

-- 

- Arnaldo