Subject: Re: [PATCH v2 bpf-next 6/9] libbpf: use negative fd to specify missing BTF

From: Stanislav Fomichev <[email-masked]>
Date: Tue, 2 Jul 2019 16:18:26 -0700

On 07/02, Stanislav Fomichev wrote:
> On 05/29, Andrii Nakryiko wrote:
> > 0 is a valid FD, so it's better to initialize it to -1, as is done in
> > other places. Also, technically, BTF type ID 0 is valid (it's a VOID
> > type), so it's more reliable to check btf_fd, instead of
> > btf_key_type_id, to determine if there is any BTF associated with a map.
> > 
> > Acked-by: Song Liu <[email-masked]>
> > Signed-off-by: Andrii Nakryiko <[email-masked]>
> > ---
> >  tools/lib/bpf/libbpf.c | 13 +++++++------
> >  1 file changed, 7 insertions(+), 6 deletions(-)
> > 
> > diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> > index c972fa10271f..a27a0351e595 100644
> > --- a/tools/lib/bpf/libbpf.c
> > +++ b/tools/lib/bpf/libbpf.c
> > @@ -1751,7 +1751,7 @@ bpf_object__create_maps(struct bpf_object *obj)
> >  		create_attr.key_size = def->key_size;
> >  		create_attr.value_size = def->value_size;
> >  		create_attr.max_entries = def->max_entries;
> > -		create_attr.btf_fd = 0;
> > +		create_attr.btf_fd = -1;
> >  		create_attr.btf_key_type_id = 0;
> >  		create_attr.btf_value_type_id = 0;
> >  		if (bpf_map_type__is_map_in_map(def->type) &&
> > @@ -1765,11 +1765,11 @@ bpf_object__create_maps(struct bpf_object *obj)
> >  		}
> >  
> >  		*pfd = bpf_create_map_xattr(&create_attr);
> > -		if (*pfd < 0 && create_attr.btf_key_type_id) {
> > +		if (*pfd < 0 && create_attr.btf_fd >= 0) {
> >  			cp = libbpf_strerror_r(errno, errmsg, sizeof(errmsg));
> >  			pr_warning("Error in bpf_create_map_xattr(%s):%s(%d). Retrying without BTF.\n",
> >  				   map->name, cp, errno);
> > -			create_attr.btf_fd = 0;
> > +			create_attr.btf_fd = -1;
> This breaks libbpf compatibility with the older kernels. If the kernel
> doesn't know about btf_fd and we set it to -1, then CHECK_ATTR
> fails :-(
> 
> Any objections to converting BTF retries to bpf_capabilities and then
> knowingly passing bft_fd==0 or proper fd?
Oh, nevermind, it looks like you fixed it already in e55d54f43d3f.

> >  			create_attr.btf_key_type_id = 0;
> >  			create_attr.btf_value_type_id = 0;
> >  			map->btf_key_type_id = 0;
> > @@ -2053,6 +2053,9 @@ load_program(struct bpf_program *prog, struct bpf_insn *insns, int insns_cnt,
> >  	char *log_buf;
> >  	int ret;
> >  
> > +	if (!insns || !insns_cnt)
> > +		return -EINVAL;
> > +
> >  	memset(&load_attr, 0, sizeof(struct bpf_load_program_attr));
> >  	load_attr.prog_type = prog->type;
> >  	load_attr.expected_attach_type = prog->expected_attach_type;
> > @@ -2063,7 +2066,7 @@ load_program(struct bpf_program *prog, struct bpf_insn *insns, int insns_cnt,
> >  	load_attr.license = license;
> >  	load_attr.kern_version = kern_version;
> >  	load_attr.prog_ifindex = prog->prog_ifindex;
> > -	load_attr.prog_btf_fd = prog->btf_fd >= 0 ? prog->btf_fd : 0;
> > +	load_attr.prog_btf_fd = prog->btf_fd;
> >  	load_attr.func_info = prog->func_info;
> >  	load_attr.func_info_rec_size = prog->func_info_rec_size;
> >  	load_attr.func_info_cnt = prog->func_info_cnt;
> > @@ -2072,8 +2075,6 @@ load_program(struct bpf_program *prog, struct bpf_insn *insns, int insns_cnt,
> >  	load_attr.line_info_cnt = prog->line_info_cnt;
> >  	load_attr.log_level = prog->log_level;
> >  	load_attr.prog_flags = prog->prog_flags;
> > -	if (!load_attr.insns || !load_attr.insns_cnt)
> > -		return -EINVAL;
> >  
> >  retry_load:
> >  	log_buf = malloc(log_buf_size);
> > -- 
> > 2.17.1
> >