Re: [PATCH bpf-next v1 4/9] bpf: Replace PTR_TO_XXX_OR_NULL with PTR_TO_XXX | PTR_MAYBE_NULL

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Tue, Dec 7, 2021 at 7:37 PM Hao Luo <haoluo@google.com> wrote:
>
> On Mon, Dec 6, 2021 at 10:09 PM Andrii Nakryiko
> <andrii.nakryiko@gmail.com> wrote:
> >
> > On Mon, Dec 6, 2021 at 3:22 PM Hao Luo <haoluo@google.com> wrote:
> > >
> > > We have introduced a new type to make bpf_reg composable, by
> > > allocating bits in the type to represent flags.
> > >
> > > One of the flags is PTR_MAYBE_NULL which indicates a pointer
> > > may be NULL. This patch switches the qualified reg_types to
> > > use this flag. The reg_types changed in this patch include:
> > >
> > > 1. PTR_TO_MAP_VALUE_OR_NULL
> > > 2. PTR_TO_SOCKET_OR_NULL
> > > 3. PTR_TO_SOCK_COMMON_OR_NULL
> > > 4. PTR_TO_TCP_SOCK_OR_NULL
> > > 5. PTR_TO_BTF_ID_OR_NULL
> > > 6. PTR_TO_MEM_OR_NULL
> > > 7. PTR_TO_RDONLY_BUF_OR_NULL
> > > 8. PTR_TO_RDWR_BUF_OR_NULL
> > >
> > > Signed-off-by: Hao Luo <haoluo@google.com>
> > > ---
> > >  drivers/net/ethernet/netronome/nfp/bpf/fw.h |   4 +-
> > >  include/linux/bpf.h                         |  16 +-
> > >  kernel/bpf/btf.c                            |   7 +-
> > >  kernel/bpf/map_iter.c                       |   4 +-
> > >  kernel/bpf/verifier.c                       | 278 ++++++++------------
> > >  net/core/bpf_sk_storage.c                   |   2 +-
> > >  net/core/sock_map.c                         |   2 +-
> > >  7 files changed, 126 insertions(+), 187 deletions(-)
> > >
> >
> > [...]
> >
> > > @@ -535,38 +520,35 @@ static bool is_cmpxchg_insn(const struct bpf_insn *insn)
> > >  }
> > >
> > >  /* string representation of 'enum bpf_reg_type' */
> > > -static const char * const reg_type_str[] = {
> > > -       [NOT_INIT]              = "?",
> > > -       [SCALAR_VALUE]          = "inv",
> > > -       [PTR_TO_CTX]            = "ctx",
> > > -       [CONST_PTR_TO_MAP]      = "map_ptr",
> > > -       [PTR_TO_MAP_VALUE]      = "map_value",
> > > -       [PTR_TO_MAP_VALUE_OR_NULL] = "map_value_or_null",
> > > -       [PTR_TO_STACK]          = "fp",
> > > -       [PTR_TO_PACKET]         = "pkt",
> > > -       [PTR_TO_PACKET_META]    = "pkt_meta",
> > > -       [PTR_TO_PACKET_END]     = "pkt_end",
> > > -       [PTR_TO_FLOW_KEYS]      = "flow_keys",
> > > -       [PTR_TO_SOCKET]         = "sock",
> > > -       [PTR_TO_SOCKET_OR_NULL] = "sock_or_null",
> > > -       [PTR_TO_SOCK_COMMON]    = "sock_common",
> > > -       [PTR_TO_SOCK_COMMON_OR_NULL] = "sock_common_or_null",
> > > -       [PTR_TO_TCP_SOCK]       = "tcp_sock",
> > > -       [PTR_TO_TCP_SOCK_OR_NULL] = "tcp_sock_or_null",
> > > -       [PTR_TO_TP_BUFFER]      = "tp_buffer",
> > > -       [PTR_TO_XDP_SOCK]       = "xdp_sock",
> > > -       [PTR_TO_BTF_ID]         = "ptr_",
> > > -       [PTR_TO_BTF_ID_OR_NULL] = "ptr_or_null_",
> > > -       [PTR_TO_PERCPU_BTF_ID]  = "percpu_ptr_",
> > > -       [PTR_TO_MEM]            = "mem",
> > > -       [PTR_TO_MEM_OR_NULL]    = "mem_or_null",
> > > -       [PTR_TO_RDONLY_BUF]     = "rdonly_buf",
> > > -       [PTR_TO_RDONLY_BUF_OR_NULL] = "rdonly_buf_or_null",
> > > -       [PTR_TO_RDWR_BUF]       = "rdwr_buf",
> > > -       [PTR_TO_RDWR_BUF_OR_NULL] = "rdwr_buf_or_null",
> > > -       [PTR_TO_FUNC]           = "func",
> > > -       [PTR_TO_MAP_KEY]        = "map_key",
> > > -};
> > > +static const char *reg_type_str(enum bpf_reg_type type)
> > > +{
> > > +       static const char * const str[] = {
> > > +               [NOT_INIT]              = "?",
> > > +               [SCALAR_VALUE]          = "inv",
> > > +               [PTR_TO_CTX]            = "ctx",
> > > +               [CONST_PTR_TO_MAP]      = "map_ptr",
> > > +               [PTR_TO_MAP_VALUE]      = "map_value",
> > > +               [PTR_TO_STACK]          = "fp",
> > > +               [PTR_TO_PACKET]         = "pkt",
> > > +               [PTR_TO_PACKET_META]    = "pkt_meta",
> > > +               [PTR_TO_PACKET_END]     = "pkt_end",
> > > +               [PTR_TO_FLOW_KEYS]      = "flow_keys",
> > > +               [PTR_TO_SOCKET]         = "sock",
> > > +               [PTR_TO_SOCK_COMMON]    = "sock_common",
> > > +               [PTR_TO_TCP_SOCK]       = "tcp_sock",
> > > +               [PTR_TO_TP_BUFFER]      = "tp_buffer",
> > > +               [PTR_TO_XDP_SOCK]       = "xdp_sock",
> > > +               [PTR_TO_BTF_ID]         = "ptr_",
> > > +               [PTR_TO_PERCPU_BTF_ID]  = "percpu_ptr_",
> > > +               [PTR_TO_MEM]            = "mem",
> > > +               [PTR_TO_RDONLY_BUF]     = "rdonly_buf",
> > > +               [PTR_TO_RDWR_BUF]       = "rdwr_buf",
> > > +               [PTR_TO_FUNC]           = "func",
> > > +               [PTR_TO_MAP_KEY]        = "map_key",
> > > +       };
> > > +
> > > +       return str[base_type(type)];
> >
> > well... now we lose the "_or_null" part, that can be pretty important.
> > Same will happen with RDONLY flag, right?
> >
> > What can we do about that?
> >
>
> We can format the string into a global buffer and return the buffer to
> the caller. A little overhead on string formatting.

Can't use global buffer, because multiple BPF verifications can
proceed at the same time.

>
> I assume there is already synchronization around the verifier to
> prevent race on the buffer. If not, we have to ask the caller of
> reg_type_str() to pass in a buffer.

I think passing temp buffer is the best (even if annoying) solution.

>
> > > +}
> > >
> > >  static char slot_type_char[] = {
> > >         [STACK_INVALID] = '?',
> > > @@ -631,7 +613,7 @@ static void print_verifier_state(struct bpf_verifier_env *env,
> > >                         continue;
> > >                 verbose(env, " R%d", i);
> > >                 print_liveness(env, reg->live);
> > > -               verbose(env, "=%s", reg_type_str[t]);
> > > +               verbose(env, "=%s", reg_type_str(t));
> > >                 if (t == SCALAR_VALUE && reg->precise)
> > >                         verbose(env, "P");
> > >                 if ((t == SCALAR_VALUE || t == PTR_TO_STACK) &&
> >
> > [...]
> >
> > >         if (smin >= BPF_MAX_VAR_OFF || smin <= -BPF_MAX_VAR_OFF) {
> > >                 verbose(env, "value %lld makes %s pointer be out of bounds\n",
> > > -                       smin, reg_type_str[type]);
> > > +                       smin, reg_type_str(type));
> > >                 return false;
> > >         }
> > >
> > > @@ -7209,11 +7151,13 @@ static int adjust_ptr_min_max_vals(struct bpf_verifier_env *env,
> > >                 return -EACCES;
> > >         }
> > >
> > > -       switch (ptr_reg->type) {
> > > -       case PTR_TO_MAP_VALUE_OR_NULL:
> > > +       if (ptr_reg->type == PTR_TO_MAP_VALUE_OR_NULL) {
> > >                 verbose(env, "R%d pointer arithmetic on %s prohibited, null-check it first\n",
> >
> > the same message can be output for other pointer types, so instead of
> > special-casing, let's just combine them with more helpful message
> >
>
> Ok. Just want to confirm my understanding, do you mean checking for
> NULL? Like the following replacement:
>
> - if (ptr_reg->type == PTR_TO_MAP_VALUE_OR_NULL) {
> + if (ptr_reg->type & PTR_MAYBE_NULL) {

no, I meant combine PTR_TO_MAP_VALUE_OR_NULL with all the other cases
(PTR_TO_PACKET_END and others), but update their message to suggest
"null-check it first".

>
> > > -                       dst, reg_type_str[ptr_reg->type]);
> > > +                       dst, reg_type_str(ptr_reg->type));
> > >                 return -EACCES;
> > > +       }
> > > +
> > > +       switch (base_type(ptr_reg->type)) {
> > >         case CONST_PTR_TO_MAP:
> > >                 /* smin_val represents the known value */
> > >                 if (known && smin_val == 0 && opcode == BPF_ADD)
> > > @@ -7221,14 +7165,11 @@ static int adjust_ptr_min_max_vals(struct bpf_verifier_env *env,
> > >                 fallthrough;
> > >         case PTR_TO_PACKET_END:
> > >         case PTR_TO_SOCKET:
> > > -       case PTR_TO_SOCKET_OR_NULL:
> > >         case PTR_TO_SOCK_COMMON:
> > > -       case PTR_TO_SOCK_COMMON_OR_NULL:
> > >         case PTR_TO_TCP_SOCK:
> > > -       case PTR_TO_TCP_SOCK_OR_NULL:
> > >         case PTR_TO_XDP_SOCK:
> > >                 verbose(env, "R%d pointer arithmetic on %s prohibited\n",
> > > -                       dst, reg_type_str[ptr_reg->type]);
> > > +                       dst, reg_type_str(ptr_reg->type));
> > >                 return -EACCES;
> > >         default:
> > >                 break;
> >
> > [...]