netdev - Re: [PATCH bpf-next v1 01/15] bpf: Factor out fd returning from bpf_btf_find_by_name

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Date:   Mon, 21 Feb 2022 21:28:11 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Kumar Kartikeya Dwivedi <memxor@...il.com>
Cc:     bpf@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        netfilter-devel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH bpf-next v1 01/15] bpf: Factor out fd returning from
 bpf_btf_find_by_name_kind

On Sun, Feb 20, 2022 at 07:17:59PM +0530, Kumar Kartikeya Dwivedi wrote:
> In next few patches, we need a helper that searches all kernel BTFs
> (vmlinux and module BTFs), and finds the type denoted by 'name' and
> 'kind'. Turns out bpf_btf_find_by_name_kind already does the same thing,
> but it instead returns a BTF ID and optionally fd (if module BTF). This
> is used for relocating ksyms in BPF loader code (bpftool gen skel -L).
> 
> We extract the core code out into a new helper
> btf_find_by_name_kind_all, which returns the BTF ID and BTF pointer in
> an out parameter. The reference for the returned BTF pointer is only
> bumped if it is a module BTF, this needs to be kept in mind when using
> this helper.
> 
> Hence, the user must release the BTF reference iff btf_is_module is
> true, otherwise transfer the ownership to e.g. an fd.
> 
> In case of the helper, the fd is only allocated for module BTFs, so no
> extra handling for btf_vmlinux case is required.
> 
> Signed-off-by: Kumar Kartikeya Dwivedi <memxor@...il.com>
> ---
>  kernel/bpf/btf.c | 47 +++++++++++++++++++++++++++++++----------------
>  1 file changed, 31 insertions(+), 16 deletions(-)
> 
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index 2c4c5dbe2abe..3645d8c14a18 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -6545,16 +6545,10 @@ static struct btf *btf_get_module_btf(const struct module *module)
>  	return btf;
>  }
>  
> -BPF_CALL_4(bpf_btf_find_by_name_kind, char *, name, int, name_sz, u32, kind, int, flags)
> +static s32 btf_find_by_name_kind_all(const char *name, u32 kind, struct btf **btfp)

The name is getting too long.
How about bpf_find_btf_id() ?

>  {
>  	struct btf *btf;
> -	long ret;
> -
> -	if (flags)
> -		return -EINVAL;
> -
> -	if (name_sz <= 1 || name[name_sz - 1])
> -		return -EINVAL;
> +	s32 ret;
>  
>  	btf = bpf_get_btf_vmlinux();
>  	if (IS_ERR(btf))
> @@ -6580,19 +6574,40 @@ BPF_CALL_4(bpf_btf_find_by_name_kind, char *, name, int, name_sz, u32, kind, int
>  			spin_unlock_bh(&btf_idr_lock);
>  			ret = btf_find_by_name_kind(mod_btf, name, kind);
>  			if (ret > 0) {
> -				int btf_obj_fd;
> -
> -				btf_obj_fd = __btf_new_fd(mod_btf);
> -				if (btf_obj_fd < 0) {
> -					btf_put(mod_btf);
> -					return btf_obj_fd;
> -				}
> -				return ret | (((u64)btf_obj_fd) << 32);
> +				*btfp = mod_btf;
> +				return ret;
>  			}
>  			spin_lock_bh(&btf_idr_lock);
>  			btf_put(mod_btf);
>  		}
>  		spin_unlock_bh(&btf_idr_lock);
> +	} else {
> +		*btfp = btf;
> +	}

Since we're refactoring let's drop the indent.
How about
  if (ret > 0) {
    *btfp = btf;
    return ret;
  }
  idr_for_each_entry().

and move the func right after btf_find_by_name_kind(),
so that later patch doesn't need to do:
static s32 bpf_find_btf_id();
Eventually this helper might become global with this name.

Also may be do btf_get() for vmlinux_btf too?
In case it reduces 'if (btf_is_module())' checks.