Re: [PATCH] bpf, libbpf: Smatch: Fix potential NULL pointer dereference

[Leo Yan <leo.yan@linaro.org>]

On Wed, Jul 03, 2019 at 12:23:05PM +0200, Daniel Borkmann wrote:
> On 07/02/2019 12:25 PM, Leo Yan wrote:
> > Based on the following report from Smatch, fix the potential
> > NULL pointer dereference check.
> > 
> >   tools/lib/bpf/libbpf.c:3493
> >   bpf_prog_load_xattr() warn: variable dereferenced before check 'attr'
> >   (see line 3483)
> > 
> > 3479 int bpf_prog_load_xattr(const struct bpf_prog_load_attr *attr,
> > 3480                         struct bpf_object **pobj, int *prog_fd)
> > 3481 {
> > 3482         struct bpf_object_open_attr open_attr = {
> > 3483                 .file           = attr->file,
> > 3484                 .prog_type      = attr->prog_type,
> >                                        ^^^^^^
> > 3485         };
> > 
> > At the head of function, it directly access 'attr' without checking if
> > it's NULL pointer.  This patch moves the values assignment after
> > validating 'attr' and 'attr->file'.
> > 
> > Signed-off-by: Leo Yan <leo.yan@linaro.org>
> > ---
> >  tools/lib/bpf/libbpf.c | 8 ++++----
> >  1 file changed, 4 insertions(+), 4 deletions(-)
> > 
> > diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> > index 197b574406b3..809b633fa3d9 100644
> > --- a/tools/lib/bpf/libbpf.c
> > +++ b/tools/lib/bpf/libbpf.c
> > @@ -3479,10 +3479,7 @@ int bpf_prog_load(const char *file, enum bpf_prog_type type,
> >  int bpf_prog_load_xattr(const struct bpf_prog_load_attr *attr,
> >  			struct bpf_object **pobj, int *prog_fd)
> >  {
> > -	struct bpf_object_open_attr open_attr = {
> > -		.file		= attr->file,
> > -		.prog_type	= attr->prog_type,
> > -	};
> 
> Applied, thanks! Fyi, I retained the zeroing of open_attr as otherwise if we ever
> extend struct bpf_object_open_attr in future, we'll easily miss this and pass in
> garbage to bpf_object__open_xattr().

Thanks for the info, Daniel.

I checked the link [1] and thanks for the improvement when applied this
patch.

Thanks,
Leo Yan

[1] https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/commit/?id=33bae185f74d49a0d7b1bfaafb8e959efce0f243