netdev - Re: [PATCH bpf-next 07/10] bpftool: add `gen bpfo` command to perform BPF static linking

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Date:   Thu, 11 Mar 2021 11:31:31 +0000
From:   Quentin Monnet <quentin@...valent.com>
To:     Andrii Nakryiko <andrii@...nel.org>, bpf@...r.kernel.org,
        netdev@...r.kernel.org, ast@...com, daniel@...earbox.net
Cc:     kernel-team@...com
Subject: Re: [PATCH bpf-next 07/10] bpftool: add `gen bpfo` command to perform
 BPF static linking

2021-03-09 20:04 UTC-0800 ~ Andrii Nakryiko <andrii@...nel.org>
> Add `bpftool gen bpfo <output-file> <input_file>...` command to statically
> link multiple BPF object files into a single output BPF object file.
> 
> Similarly to existing '*.o' convention, bpftool is establishing a '*.bpfo'
> convention for statically-linked BPF object files. Both .o and .bpfo suffixes
> will be stripped out during BPF skeleton generation to infer BPF object name.
> 
> Signed-off-by: Andrii Nakryiko <andrii@...nel.org>
> ---
>  tools/bpf/bpftool/gen.c | 46 ++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 45 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/bpf/bpftool/gen.c b/tools/bpf/bpftool/gen.c
> index 4033c46d83e7..8b1ed6c0a62f 100644
> --- a/tools/bpf/bpftool/gen.c
> +++ b/tools/bpf/bpftool/gen.c
> +static int do_bpfo(int argc, char **argv)

> +{
> +	struct bpf_linker *linker;
> +	const char *output_file, *file;
> +	int err;
> +
> +	if (!REQ_ARGS(2)) {
> +		usage();
> +		return -1;
> +	}
> +
> +	output_file = GET_ARG();
> +
> +	linker = bpf_linker__new(output_file, NULL);
> +	if (!linker) {
> +		p_err("failed to create BPF linker instance");
> +		return -1;
> +	}
> +
> +	while (argc) {
> +		file = GET_ARG();
> +
> +		err = bpf_linker__add_file(linker, file);
> +		if (err) {
> +			p_err("failed to link '%s': %d", file, err);

I think you mentioned before that your preference was for having just
the error code instead of using strerror(), but I think it would be more
user-friendly for the majority of users who don't know the error codes
if we had something more verbose? How about having both strerror()
output and the error code?

> +			goto err_out;
> +		}
> +	}
> +
> +	err = bpf_linker__finalize(linker);
> +	if (err) {
> +		p_err("failed to finalize ELF file: %d", err);
> +		goto err_out;
> +	}
> +
> +	return 0;
> +err_out:
> +	bpf_linker__free(linker);
> +	return -1;

Should you call bpf_linker__free() even on success? I see that
bpf_linker__finalize() frees some of the resources, but it seems that
bpf_linker__free() does a more thorough job?

> +}
> +
>  static int do_help(int argc, char **argv)
>  {
>  	if (json_output) {
> @@ -611,6 +654,7 @@ static int do_help(int argc, char **argv)
>  
>  static const struct cmd cmds[] = {
>  	{ "skeleton",	do_skeleton },
> +	{ "bpfo",	do_bpfo },
>  	{ "help",	do_help },
>  	{ 0 }
>  };
> 

Please update the usage help message, man page, and bash completion,
thanks. Especially because what "bpftool gen bpfo" does is not intuitive
(but I don't have a better name suggestion at the moment).

Great work!

Quentin