Re: [PATCH bpf-next 4/4] bpftool: use a local bpf_perf_event_value to fix accessing its fields

[Jiri Olsa <olsajiri@gmail.com>]

On Fri, May 12, 2023 at 11:33:54AM +0100, Quentin Monnet wrote:
> From: Alexander Lobakin <alobakin@pm.me>
> 
> Fix the following error when building bpftool:
> 
>   CLANG   profiler.bpf.o
>   CLANG   pid_iter.bpf.o
> skeleton/profiler.bpf.c:18:21: error: invalid application of 'sizeof' to an incomplete type 'struct bpf_perf_event_value'
>         __uint(value_size, sizeof(struct bpf_perf_event_value));
>                            ^     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> tools/bpf/bpftool/bootstrap/libbpf/include/bpf/bpf_helpers.h:13:39: note: expanded from macro '__uint'
> tools/bpf/bpftool/bootstrap/libbpf/include/bpf/bpf_helper_defs.h:7:8: note: forward declaration of 'struct bpf_perf_event_value'
> struct bpf_perf_event_value;
>        ^
> 
> struct bpf_perf_event_value is being used in the kernel only when
> CONFIG_BPF_EVENTS is enabled, so it misses a BTF entry then.

hi,
when I switch off CONFIG_BPF_EVENTS the bpftool build fails for me
with missing BTF error:

  GEN     vmlinux.h
libbpf: failed to find '.BTF' ELF section in /home/jolsa/kernel/linux-qemu/vmlinux
Error: failed to load BTF from /home/jolsa/kernel/linux-qemu/vmlinux: No data available
make: *** [Makefile:208: vmlinux.h] Error 195
make: *** Deleting file 'vmlinux.h'

so I wonder you need to care about bpf_perf_event_value
in that case

jirka

> Define struct bpf_perf_event_value___local with the
> `preserve_access_index` attribute inside the pid_iter BPF prog to
> allow compiling on any configs. It is a full mirror of a UAPI
> structure, so is compatible both with and w/o CO-RE.
> bpf_perf_event_read_value() requires a pointer of the original type,
> so a cast is needed.
> 
> Fixes: 47c09d6a9f67 ("bpftool: Introduce "prog profile" command")
> Suggested-by: Andrii Nakryiko <andrii@kernel.org>
> Signed-off-by: Alexander Lobakin <alobakin@pm.me>
> Signed-off-by: Quentin Monnet <quentin@isovalent.com>
> ---
>  tools/bpf/bpftool/skeleton/profiler.bpf.c | 27 ++++++++++++++---------
>  1 file changed, 17 insertions(+), 10 deletions(-)
> 
> diff --git a/tools/bpf/bpftool/skeleton/profiler.bpf.c b/tools/bpf/bpftool/skeleton/profiler.bpf.c
> index ce5b65e07ab1..2f80edc682f1 100644
> --- a/tools/bpf/bpftool/skeleton/profiler.bpf.c
> +++ b/tools/bpf/bpftool/skeleton/profiler.bpf.c
> @@ -4,6 +4,12 @@
>  #include <bpf/bpf_helpers.h>
>  #include <bpf/bpf_tracing.h>
>  
> +struct bpf_perf_event_value___local {
> +	__u64 counter;
> +	__u64 enabled;
> +	__u64 running;
> +} __attribute__((preserve_access_index));
> +
>  /* map of perf event fds, num_cpu * num_metric entries */
>  struct {
>  	__uint(type, BPF_MAP_TYPE_PERF_EVENT_ARRAY);
> @@ -15,14 +21,14 @@ struct {
>  struct {
>  	__uint(type, BPF_MAP_TYPE_PERCPU_ARRAY);
>  	__uint(key_size, sizeof(u32));
> -	__uint(value_size, sizeof(struct bpf_perf_event_value));
> +	__uint(value_size, sizeof(struct bpf_perf_event_value___local));
>  } fentry_readings SEC(".maps");
>  
>  /* accumulated readings */
>  struct {
>  	__uint(type, BPF_MAP_TYPE_PERCPU_ARRAY);
>  	__uint(key_size, sizeof(u32));
> -	__uint(value_size, sizeof(struct bpf_perf_event_value));
> +	__uint(value_size, sizeof(struct bpf_perf_event_value___local));
>  } accum_readings SEC(".maps");
>  
>  /* sample counts, one per cpu */
> @@ -39,7 +45,7 @@ const volatile __u32 num_metric = 1;
>  SEC("fentry/XXX")
>  int BPF_PROG(fentry_XXX)
>  {
> -	struct bpf_perf_event_value *ptrs[MAX_NUM_MATRICS];
> +	struct bpf_perf_event_value___local *ptrs[MAX_NUM_MATRICS];
>  	u32 key = bpf_get_smp_processor_id();
>  	u32 i;
>  
> @@ -53,10 +59,10 @@ int BPF_PROG(fentry_XXX)
>  	}
>  
>  	for (i = 0; i < num_metric && i < MAX_NUM_MATRICS; i++) {
> -		struct bpf_perf_event_value reading;
> +		struct bpf_perf_event_value___local reading;
>  		int err;
>  
> -		err = bpf_perf_event_read_value(&events, key, &reading,
> +		err = bpf_perf_event_read_value(&events, key, (void *)&reading,
>  						sizeof(reading));
>  		if (err)
>  			return 0;
> @@ -68,14 +74,14 @@ int BPF_PROG(fentry_XXX)
>  }
>  
>  static inline void
> -fexit_update_maps(u32 id, struct bpf_perf_event_value *after)
> +fexit_update_maps(u32 id, struct bpf_perf_event_value___local *after)
>  {
> -	struct bpf_perf_event_value *before, diff;
> +	struct bpf_perf_event_value___local *before, diff;
>  
>  	before = bpf_map_lookup_elem(&fentry_readings, &id);
>  	/* only account samples with a valid fentry_reading */
>  	if (before && before->counter) {
> -		struct bpf_perf_event_value *accum;
> +		struct bpf_perf_event_value___local *accum;
>  
>  		diff.counter = after->counter - before->counter;
>  		diff.enabled = after->enabled - before->enabled;
> @@ -93,7 +99,7 @@ fexit_update_maps(u32 id, struct bpf_perf_event_value *after)
>  SEC("fexit/XXX")
>  int BPF_PROG(fexit_XXX)
>  {
> -	struct bpf_perf_event_value readings[MAX_NUM_MATRICS];
> +	struct bpf_perf_event_value___local readings[MAX_NUM_MATRICS];
>  	u32 cpu = bpf_get_smp_processor_id();
>  	u32 i, zero = 0;
>  	int err;
> @@ -102,7 +108,8 @@ int BPF_PROG(fexit_XXX)
>  	/* read all events before updating the maps, to reduce error */
>  	for (i = 0; i < num_metric && i < MAX_NUM_MATRICS; i++) {
>  		err = bpf_perf_event_read_value(&events, cpu + i * num_cpu,
> -						readings + i, sizeof(*readings));
> +						(void *)(readings + i),
> +						sizeof(*readings));
>  		if (err)
>  			return 0;
>  	}
> -- 
> 2.34.1
>