RE: [PATCH v4 bpf 2/3] libbpf: restore umem state after socket create failure

["Loftus, Ciara" <ciara.loftus@intel.com>]

> On Tue, Mar 30, 2021 at 11:45 PM Ciara Loftus <ciara.loftus@intel.com>
> wrote:
> >
> > If the call to xsk_socket__create fails, the user may want to retry the
> > socket creation using the same umem. Ensure that the umem is in the
> > same state on exit if the call fails by:
> > 1. ensuring the umem _save pointers are unmodified.
> > 2. not unmapping the set of umem rings that were set up with the umem
> > during xsk_umem__create, since those maps existed before the call to
> > xsk_socket__create and should remain in tact even in the event of
> > failure.
> >
> > Fixes: 2f6324a3937f ("libbpf: Support shared umems between queues and
> devices")
> >
> > Signed-off-by: Ciara Loftus <ciara.loftus@intel.com>
> > ---
> >  tools/lib/bpf/xsk.c | 41 +++++++++++++++++++++++------------------
> >  1 file changed, 23 insertions(+), 18 deletions(-)
> >
> > diff --git a/tools/lib/bpf/xsk.c b/tools/lib/bpf/xsk.c
> > index 443b0cfb45e8..5098d9e3b55a 100644
> > --- a/tools/lib/bpf/xsk.c
> > +++ b/tools/lib/bpf/xsk.c
> > @@ -743,26 +743,30 @@ static struct xsk_ctx *xsk_get_ctx(struct
> xsk_umem *umem, int ifindex,
> >         return NULL;
> >  }
> >
> > -static void xsk_put_ctx(struct xsk_ctx *ctx)
> > +static void xsk_put_ctx(struct xsk_ctx *ctx, bool unmap)
> >  {
> >         struct xsk_umem *umem = ctx->umem;
> >         struct xdp_mmap_offsets off;
> >         int err;
> >
> > -       if (--ctx->refcount == 0) {
> > -               err = xsk_get_mmap_offsets(umem->fd, &off);
> > -               if (!err) {
> > -                       munmap(ctx->fill->ring - off.fr.desc,
> > -                              off.fr.desc + umem->config.fill_size *
> > -                              sizeof(__u64));
> > -                       munmap(ctx->comp->ring - off.cr.desc,
> > -                              off.cr.desc + umem->config.comp_size *
> > -                              sizeof(__u64));
> > -               }
> > +       if (--ctx->refcount)
> > +               return;
> >
> > -               list_del(&ctx->list);
> > -               free(ctx);
> > -       }
> > +       if (!unmap)
> > +               goto out_free;
> > +
> > +       err = xsk_get_mmap_offsets(umem->fd, &off);
> > +       if (err)
> > +               goto out_free;
> > +
> > +       munmap(ctx->fill->ring - off.fr.desc, off.fr.desc + umem-
> >config.fill_size *
> > +              sizeof(__u64));
> > +       munmap(ctx->comp->ring - off.cr.desc, off.cr.desc + umem-
> >config.comp_size *
> > +              sizeof(__u64));
> > +
> > +out_free:
> > +       list_del(&ctx->list);
> > +       free(ctx);
> >  }
> >
> >  static struct xsk_ctx *xsk_create_ctx(struct xsk_socket *xsk,
> > @@ -797,8 +801,6 @@ static struct xsk_ctx *xsk_create_ctx(struct
> xsk_socket *xsk,
> >         memcpy(ctx->ifname, ifname, IFNAMSIZ - 1);
> >         ctx->ifname[IFNAMSIZ - 1] = '\0';
> >
> > -       umem->fill_save = NULL;
> > -       umem->comp_save = NULL;
> >         ctx->fill = fill;
> >         ctx->comp = comp;
> >         list_add(&ctx->list, &umem->ctx_list);
> > @@ -854,6 +856,7 @@ int xsk_socket__create_shared(struct xsk_socket
> **xsk_ptr,
> >         struct xsk_socket *xsk;
> >         struct xsk_ctx *ctx;
> >         int err, ifindex;
> > +       bool unmap = umem->fill_save != fill;
> >
> 
> we are checking !umem only on the next line, so here it can be still
> NULL. Please send a fix, thanks.

Thank you for catching this. I've sent a fix.

Ciara

> 
> >         if (!umem || !xsk_ptr || !(rx || tx))
> >                 return -EFAULT;
> > @@ -994,6 +997,8 @@ int xsk_socket__create_shared(struct xsk_socket
> **xsk_ptr,
> >         }
> >
> >         *xsk_ptr = xsk;
> > +       umem->fill_save = NULL;
> > +       umem->comp_save = NULL;
> >         return 0;
> >
> >  out_mmap_tx:
> > @@ -1005,7 +1010,7 @@ int xsk_socket__create_shared(struct xsk_socket
> **xsk_ptr,
> >                 munmap(rx_map, off.rx.desc +
> >                        xsk->config.rx_size * sizeof(struct xdp_desc));
> >  out_put_ctx:
> > -       xsk_put_ctx(ctx);
> > +       xsk_put_ctx(ctx, unmap);
> >  out_socket:
> >         if (--umem->refcount)
> >                 close(xsk->fd);
> > @@ -1071,7 +1076,7 @@ void xsk_socket__delete(struct xsk_socket *xsk)
> >                 }
> >         }
> >
> > -       xsk_put_ctx(ctx);
> > +       xsk_put_ctx(ctx, true);
> >
> >         umem->refcount--;
> >         /* Do not close an fd that also has an associated umem connected
> > --
> > 2.17.1
> >