Re: [PATCH] perf bpf: 8 byte align bpil data

[Jiri Olsa <olsajiri@gmail.com>]

On Mon, Jun 13, 2022 at 06:47:14PM -0700, Ian Rogers wrote:
> bpil data is accessed assuming 64-bit alignment resulting in undefined
> behavior as the data is just byte aligned. With an -fsanitize=undefined
> build the following errors are observed:
> 
> $ sudo perf record -a sleep 1
> util/bpf-event.c:310:22: runtime error: load of misaligned address 0x55f61084520f for type '__u64', which requires 8 byte alignment
> 0x55f61084520f: note: pointer points here
>  a8 fe ff ff 3c  51 d3 c0 ff ff ff ff 04  84 d3 c0 ff ff ff ff d8  aa d3 c0 ff ff ff ff a4  c0 d3 c0
>              ^
> util/bpf-event.c:311:20: runtime error: load of misaligned address 0x55f61084522f for type '__u32', which requires 4 byte alignment
> 0x55f61084522f: note: pointer points here
>  ff ff ff ff c7  17 00 00 f1 02 00 00 1f  04 00 00 58 04 00 00 00  00 00 00 0f 00 00 00 63  02 00 00
>              ^
> util/bpf-event.c:198:33: runtime error: member access within misaligned address 0x55f61084523f for type 'const struct bpf_func_info', which requires 4 byte alignment
> 0x55f61084523f: note: pointer points here
>  58 04 00 00 00  00 00 00 0f 00 00 00 63  02 00 00 3b 00 00 00 ab  02 00 00 44 00 00 00 14  03 00 00
> 
> Correct this by rouding up the data sizes and aligning the pointers.
> 
> Signed-off-by: Ian Rogers <irogers@google.com>
> ---
>  tools/perf/util/bpf-utils.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/tools/perf/util/bpf-utils.c b/tools/perf/util/bpf-utils.c
> index e271e05e51bc..80b1d2b3729b 100644
> --- a/tools/perf/util/bpf-utils.c
> +++ b/tools/perf/util/bpf-utils.c
> @@ -149,11 +149,10 @@ get_bpf_prog_info_linear(int fd, __u64 arrays)
>  		count = bpf_prog_info_read_offset_u32(&info, desc->count_offset);
>  		size  = bpf_prog_info_read_offset_u32(&info, desc->size_offset);
>  
> -		data_len += count * size;
> +		data_len += roundup(count * size, sizeof(__u64));
>  	}
>  
>  	/* step 3: allocate continuous memory */
> -	data_len = roundup(data_len, sizeof(__u64));
>  	info_linear = malloc(sizeof(struct perf_bpil) + data_len);
>  	if (!info_linear)
>  		return ERR_PTR(-ENOMEM);
> @@ -180,7 +179,7 @@ get_bpf_prog_info_linear(int fd, __u64 arrays)
>  		bpf_prog_info_set_offset_u64(&info_linear->info,
>  					     desc->array_offset,
>  					     ptr_to_u64(ptr));
> -		ptr += count * size;
> +		ptr += roundup(count * size, sizeof(__u64));

this one depends on info_linear->data being alligned(8), right?

should we make sure it's allways the case like in the patch
below, or it's superfluous?

thanks,
jirka


---
diff --git a/tools/perf/util/bpf-utils.h b/tools/perf/util/bpf-utils.h
index 86a5055cdfad..1aba76c44116 100644
--- a/tools/perf/util/bpf-utils.h
+++ b/tools/perf/util/bpf-utils.h
@@ -60,7 +60,7 @@ struct perf_bpil {
 	/* which arrays are included in data */
 	__u64			arrays;
 	struct bpf_prog_info	info;
-	__u8			data[];
+	__u8			data[] __attribute__((aligned(8)));
 };
 
 struct perf_bpil *