Re: [PATCH bpf 1/2] libbpf: fix removal of inner map in bpf_object__create_map

From: John Fastabend <john.fastabend@gmail.com>
To: John Fastabend <john.fastabend@gmail.com>,
	Martynas Pumputis <m@lambda.lt>,
	John Fastabend <john.fastabend@gmail.com>,
	bpf@vger.kernel.org
Cc: ast@kernel.org, daniel@iogearbox.net, andrii@kernel.org
Subject: Re: [PATCH bpf 1/2] libbpf: fix removal of inner map in bpf_object__create_map
Date: Thu, 15 Jul 2021 20:06:54 -0700	[thread overview]
Message-ID: <60f0f7ceb5382_2a57208ed@john-XPS-13-9370.notmuch> (raw)
In-Reply-To: <60f0938e8e7f6_7a66920818@john-XPS-13-9370.notmuch>

John Fastabend wrote:
> Martynas Pumputis wrote:
> > 
> > 
> > On 7/15/21 2:30 AM, John Fastabend wrote:
> > > Martynas Pumputis wrote:
> > >> If creating an outer map of a BTF-defined map-in-map fails (via
> > >> bpf_object__create_map()), then the previously created its inner map
> > >> won't be destroyed.
> > >>
> > >> Fix this by ensuring that the destroy routines are not bypassed in the
> > >> case of a failure.
> > >>
> > >> Fixes: 646f02ffdd49c ("libbpf: Add BTF-defined map-in-map support")
> > >> Reported-by: Andrii Nakryiko <andrii@kernel.org>
> > >> Signed-off-by: Martynas Pumputis <m@lambda.lt>
> > >> ---
> > >>   tools/lib/bpf/libbpf.c | 5 +++--
> > >>   1 file changed, 3 insertions(+), 2 deletions(-)
> > >>
> > >> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> > >> index 6f5e2757bb3c..1a840e81ea0a 100644
> > >> --- a/tools/lib/bpf/libbpf.c
> > >> +++ b/tools/lib/bpf/libbpf.c
> > >> @@ -4479,6 +4479,7 @@ static int bpf_object__create_map(struct bpf_object *obj, struct bpf_map *map, b
> > >>   {
> > >>   	struct bpf_create_map_attr create_attr;
> > >>   	struct bpf_map_def *def = &map->def;
> > >> +	int ret = 0;
> > >>   
> > >>   	memset(&create_attr, 0, sizeof(create_attr));
> > >>   
> > >> @@ -4561,7 +4562,7 @@ static int bpf_object__create_map(struct bpf_object *obj, struct bpf_map *map, b
> > >>   	}
> > >>   
> > >>   	if (map->fd < 0)
> > >> -		return -errno;
> > >> +		ret = -errno;
> > >>   
> > > 
> > > I'm trying to track this down, not being overly familiar with this bit of
> > > code.
> > > 
> > > We entered bpf_object__create_map with map->inner_map potentially set and
> > > then here we are going to zfree(&map->inner_map). I'm trying to track
> > > down if this is problematic, I guess not? But seems like we could
> > > also free a map here that we didn't create from this call in the above
> > > logic.
> > > 
> > 
> > Keep in mind that we free the inner map anyway if the creation of the 
> > outer map was successful. Also, we don't try to recreate the map if any 
> > of the steps has failed. So I think it should not be problematic.
> 
> Maybe not problematic, but I'm still missing a bit of detail here.
> We create an inner map then immediately destroy it? I'll need to
> reread to understand the why part.

OK understand now ;)