Re: [PATCH 17/39] bpf: resolve_pseudo_ldimm64(): take handling of a single ldimm64 insn into helper

Alexei Starovoitov
2024-08-08T16:51:46Z

On Wed, Aug 7, 2024 at 8:31 AM Andrii Nakryiko
<andrii.nakryiko@gmail.com> wrote:
>
> On Wed, Aug 7, 2024 at 3:30 AM Christian Brauner <brauner@kernel.org> wrote:
> >
> > On Tue, Aug 06, 2024 at 03:32:20PM GMT, Andrii Nakryiko wrote:
> > > On Mon, Jul 29, 2024 at 10:20 PM <viro@kernel.org> wrote:
> > > >
> > > > From: Al Viro <viro@zeniv.linux.org.uk>
> > > >
> > > > Equivalent transformation. For one thing, it's easier to follow that way.
> > > > For another, that simplifies the control flow in the vicinity of struct fd
> > > > handling in there, which will allow a switch to CLASS(fd) and make the
> > > > thing much easier to verify wrt leaks.
> > > >
> > > > Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
> > > > ---
> > > > kernel/bpf/verifier.c | 342 +++++++++++++++++++++---------------------
> > > > 1 file changed, 172 insertions(+), 170 deletions(-)
> > > >
> > >
> > > This looks unnecessarily intrusive. I think it's best to extract the
> > > logic of fetching and adding bpf_map by fd into a helper and that way
> > > contain fdget + fdput logic nicely. Something like below, which I can
> > > send to bpf-next.
> > >
> > > commit b5eec08241cc0263e560551de91eda73ccc5987d
> > > Author: Andrii Nakryiko <andrii@kernel.org>
> > > Date: Tue Aug 6 14:31:34 2024 -0700
> > >
> > > bpf: factor out fetching bpf_map from FD and adding it to used_maps list
> > >
> > > Factor out the logic to extract bpf_map instances from FD embedded in
> > > bpf_insns, adding it to the list of used_maps (unless it's already
> > > there, in which case we just reuse map's index). This simplifies the
> > > logic in resolve_pseudo_ldimm64(), especially around `struct fd`
> > > handling, as all that is now neatly contained in the helper and doesn't
> > > leak into a dozen error handling paths.
> > >
> > > Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
> > >
> > > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > > index df3be12096cf..14e4ef687a59 100644
> > > --- a/kernel/bpf/verifier.c
> > > +++ b/kernel/bpf/verifier.c
> > > @@ -18865,6 +18865,58 @@ static bool bpf_map_is_cgroup_storage(struct
> > > bpf_map *map)
> > > map->map_type == BPF_MAP_TYPE_PERCPU_CGROUP_STORAGE);
> > > }
> > >
> > > +/* Add map behind fd to used maps list, if it's not already there, and return
> > > + * its index. Also set *reused to true if this map was already in the list of
> > > + * used maps.
> > > + * Returns <0 on error, or >= 0 index, on success.
> > > + */
> > > +static int add_used_map_from_fd(struct bpf_verifier_env *env, int fd,
> > > bool *reused)
> > > +{
> > > + struct fd f = fdget(fd);
> >
> > Use CLASS(fd, f)(fd) and you can avoid all that fdput() stuff.
>
> That was the point of Al's next patch in the series, so I didn't want
> to do it in this one that just refactored the logic of adding maps.
> But I can fold that in and send it to bpf-next.

+1.

The bpf changes look ok and Andrii's approach is easier to grasp.
It's better to route bpf conversion to CLASS(fd,..) via bpf-next,
so it goes through bpf CI and our other testing.

bpf patches don't seem to depend on newly added CLASS(fd_pos, ...
and fderr, so pretty much independent from other patches.