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

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

On Tue, Feb 22, 2022 at 10:58:11AM IST, Alexei Starovoitov wrote:
> 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@gmail.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.
>

Ok, will change.

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

Right, should also change this for btf_get_module_btf then, to make things
consistent.

--
Kartikeya