On Sun, Mar 20, 2022 at 12:22:51AM +0530, Kumar Kartikeya Dwivedi wrote:
> On Sat, Mar 19, 2022 at 11:45:38PM IST, Alexei Starovoitov wrote:
> > On Thu, Mar 17, 2022 at 05:29:45PM +0530, Kumar Kartikeya Dwivedi wrote:
> > > This commit introduces a new pointer type 'kptr' which can be embedded
> > > in a map value as holds a PTR_TO_BTF_ID stored by a BPF program during
> > > its invocation. Storing to such a kptr, BPF program's PTR_TO_BTF_ID
> > > register must have the same type as in the map value's BTF, and loading
> > > a kptr marks the destination register as PTR_TO_BTF_ID with the correct
> > > kernel BTF and BTF ID.
> > >
> > > Such kptr are unreferenced, i.e. by the time another invocation of the
> > > BPF program loads this pointer, the object which the pointer points to
> > > may not longer exist. Since PTR_TO_BTF_ID loads (using BPF_LDX) are
> > > patched to PROBE_MEM loads by the verifier, it would safe to allow user
> > > to still access such invalid pointer, but passing such pointers into
> > > BPF helpers and kfuncs should not be permitted. A future patch in this
> > > series will close this gap.
> > >
> > > The flexibility offered by allowing programs to dereference such invalid
> > > pointers while being safe at runtime frees the verifier from doing
> > > complex lifetime tracking. As long as the user may ensure that the
> > > object remains valid, it can ensure data read by it from the kernel
> > > object is valid.
> > >
> > > The user indicates that a certain pointer must be treated as kptr
> > > capable of accepting stores of PTR_TO_BTF_ID of a certain type, by using
> > > a BTF type tag 'kptr' on the pointed to type of the pointer. Then, this
> > > information is recorded in the object BTF which will be passed into the
> > > kernel by way of map's BTF information. The name and kind from the map
> > > value BTF is used to look up the in-kernel type, and the actual BTF and
> > > BTF ID is recorded in the map struct in a new kptr_off_tab member. For
> > > now, only storing pointers to structs is permitted.
> > >
> > > An example of this specification is shown below:
> > >
> > > #define __kptr __attribute__((btf_type_tag("kptr")))
> > >
> > > struct map_value {
> > > ...
> > > struct task_struct __kptr *task;
> > > ...
> > > };
> > >
> > > Then, in a BPF program, user may store PTR_TO_BTF_ID with the type
> > > task_struct into the map, and then load it later.
> > >
> > > Note that the destination register is marked PTR_TO_BTF_ID_OR_NULL, as
> > > the verifier cannot know whether the value is NULL or not statically, it
> > > must treat all potential loads at that map value offset as loading a
> > > possibly NULL pointer.
> > >
> > > Only BPF_LDX, BPF_STX, and BPF_ST with insn->imm = 0 (to denote NULL)
> > > are allowed instructions that can access such a pointer. On BPF_LDX, the
> > > destination register is updated to be a PTR_TO_BTF_ID, and on BPF_STX,
> > > it is checked whether the source register type is a PTR_TO_BTF_ID with
> > > same BTF type as specified in the map BTF. The access size must always
> > > be BPF_DW.
> > >
> > > For the map in map support, the kptr_off_tab for outer map is copied
> > > from the inner map's kptr_off_tab. It was chosen to do a deep copy
> > > instead of introducing a refcount to kptr_off_tab, because the copy only
> > > needs to be done when paramterizing using inner_map_fd in the map in map
> > > case, hence would be unnecessary for all other users.
> > >
> > > It is not permitted to use MAP_FREEZE command and mmap for BPF map
> > > having kptr, similar to the bpf_timer case.
> > >
> > > Signed-off-by: Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx>
> > > ---
> > > include/linux/bpf.h | 29 +++++-
> > > include/linux/btf.h | 2 +
> > > kernel/bpf/btf.c | 151 +++++++++++++++++++++++++----
> > > kernel/bpf/map_in_map.c | 5 +-
> > > kernel/bpf/syscall.c | 110 ++++++++++++++++++++-
> > > kernel/bpf/verifier.c | 207 ++++++++++++++++++++++++++++++++--------
> > > 6 files changed, 442 insertions(+), 62 deletions(-)
> > >
> > > diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> > > index 88449fbbe063..f35920d279dd 100644
> > > --- a/include/linux/bpf.h
> > > +++ b/include/linux/bpf.h
> > > @@ -155,6 +155,22 @@ struct bpf_map_ops {
> > > const struct bpf_iter_seq_info *iter_seq_info;
> > > };
> > >
> > > +enum {
> > > + /* Support at most 8 pointers in a BPF map value */
> > > + BPF_MAP_VALUE_OFF_MAX = 8,
> > > +};
> > > +
> > > +struct bpf_map_value_off_desc {
> > > + u32 offset;
> > > + u32 btf_id;
> > > + struct btf *btf;
> > > +};
> > > +
> > > +struct bpf_map_value_off {
> > > + u32 nr_off;
> > > + struct bpf_map_value_off_desc off[];
> > > +};
> > > +
> > > struct bpf_map {
> > > /* The first two cachelines with read-mostly members of which some
> > > * are also accessed in fast-path (e.g. ops, max_entries).
> > > @@ -171,6 +187,7 @@ struct bpf_map {
> > > u64 map_extra; /* any per-map-type extra fields */
> > > u32 map_flags;
> > > int spin_lock_off; /* >=0 valid offset, <0 error */
> > > + struct bpf_map_value_off *kptr_off_tab;
> > > int timer_off; /* >=0 valid offset, <0 error */
> > > u32 id;
> > > int numa_node;
> > > @@ -184,7 +201,7 @@ struct bpf_map {
> > > char name[BPF_OBJ_NAME_LEN];
> > > bool bypass_spec_v1;
> > > bool frozen; /* write-once; write-protected by freeze_mutex */
> > > - /* 14 bytes hole */
> > > + /* 6 bytes hole */
> > >
> > > /* The 3rd and 4th cacheline with misc members to avoid false sharing
> > > * particularly with refcounting.
> > > @@ -217,6 +234,11 @@ static inline bool map_value_has_timer(const struct bpf_map *map)
> > > return map->timer_off >= 0;
> > > }
> > >
> > > +static inline bool map_value_has_kptr(const struct bpf_map *map)
> > > +{
> > > + return !IS_ERR_OR_NULL(map->kptr_off_tab);
> > > +}
> > > +
> > > static inline void check_and_init_map_value(struct bpf_map *map, void *dst)
> > > {
> > > if (unlikely(map_value_has_spin_lock(map)))
> > > @@ -1497,6 +1519,11 @@ void bpf_prog_put(struct bpf_prog *prog);
> > > void bpf_prog_free_id(struct bpf_prog *prog, bool do_idr_lock);
> > > void bpf_map_free_id(struct bpf_map *map, bool do_idr_lock);
> > >
> > > +struct bpf_map_value_off_desc *bpf_map_kptr_off_contains(struct bpf_map *map, u32 offset);
> > > +void bpf_map_free_kptr_off_tab(struct bpf_map *map);
> > > +struct bpf_map_value_off *bpf_map_copy_kptr_off_tab(const struct bpf_map *map);
> > > +bool bpf_map_equal_kptr_off_tab(const struct bpf_map *map_a, const struct bpf_map *map_b);
> > > +
> > > struct bpf_map *bpf_map_get(u32 ufd);
> > > struct bpf_map *bpf_map_get_with_uref(u32 ufd);
> > > struct bpf_map *__bpf_map_get(struct fd f);
> > > diff --git a/include/linux/btf.h b/include/linux/btf.h
> > > index 36bc09b8e890..5b578dc81c04 100644
> > > --- a/include/linux/btf.h
> > > +++ b/include/linux/btf.h
> > > @@ -123,6 +123,8 @@ bool btf_member_is_reg_int(const struct btf *btf, const struct btf_type *s,
> > > u32 expected_offset, u32 expected_size);
> > > int btf_find_spin_lock(const struct btf *btf, const struct btf_type *t);
> > > int btf_find_timer(const struct btf *btf, const struct btf_type *t);
> > > +struct bpf_map_value_off *btf_find_kptr(const struct btf *btf,
> > > + const struct btf_type *t);
> > > bool btf_type_is_void(const struct btf_type *t);
> > > s32 btf_find_by_name_kind(const struct btf *btf, const char *name, u8 kind);
> > > const struct btf_type *btf_type_skip_modifiers(const struct btf *btf,
> > > diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> > > index 5b2824332880..9ac9364ef533 100644
> > > --- a/kernel/bpf/btf.c
> > > +++ b/kernel/bpf/btf.c
> > > @@ -3164,33 +3164,79 @@ static void btf_struct_log(struct btf_verifier_env *env,
> > > enum {
> > > BTF_FIELD_SPIN_LOCK,
> > > BTF_FIELD_TIMER,
> > > + BTF_FIELD_KPTR,
> > > +};
> > > +
> > > +enum {
> > > + BTF_FIELD_IGNORE = 0,
> > > + BTF_FIELD_FOUND = 1,
> > > };
> > >
> > > struct btf_field_info {
> > > + const struct btf_type *type;
> > > u32 off;
> > > };
> > >
> > > static int btf_find_field_struct(const struct btf *btf, const struct btf_type *t,
> > > - u32 off, int sz, struct btf_field_info *info)
> > > + u32 off, int sz, struct btf_field_info *info,
> > > + int info_cnt, int idx)
> > > {
> > > if (!__btf_type_is_struct(t))
> > > - return 0;
> > > + return BTF_FIELD_IGNORE;
> > > if (t->size != sz)
> > > - return 0;
> > > - if (info->off != -ENOENT)
> > > - /* only one such field is allowed */
> > > + return BTF_FIELD_IGNORE;
> > > + if (idx >= info_cnt)
> >
> > No need to pass info_cnt, idx into this function.
> > Move idx >= info_cnt check into the caller and let
> > caller do 'info++' and pass that.
>
> That was what I did initially, but this check actually needs to happen after we
> see that the field is of interest (i.e. not ignored by btf_find_field_*). Doing
> it in caller limits total fields to info_cnt. Moving those checks out into the
> caller may be the other option, but I didn't like that. I can add a comment if
> it makes things clear.
don't increment info unconditionally?
only when field is found.
>
> > This function will simply write into 'info'.
> >
> > > return -E2BIG;
> > > + info[idx].off = off;
> > > info->off = off;
> >
> > This can't be right.
> >
>
> Ouch, thanks for catching this.
>
> > > - return 0;
> > > + return BTF_FIELD_FOUND;
> > > +}
> > > +
> > > +static int btf_find_field_kptr(const struct btf *btf, const struct btf_type *t,
> > > + u32 off, int sz, struct btf_field_info *info,
> > > + int info_cnt, int idx)
> > > +{
> > > + bool kptr_tag = false;
> > > +
> > > + /* For PTR, sz is always == 8 */
> > > + if (!btf_type_is_ptr(t))
> > > + return BTF_FIELD_IGNORE;
> > > + t = btf_type_by_id(btf, t->type);
> > > +
> > > + while (btf_type_is_type_tag(t)) {
> > > + if (!strcmp("kptr", __btf_name_by_offset(btf, t->name_off))) {
> > > + /* repeated tag */
> > > + if (kptr_tag)
> > > + return -EEXIST;
> > > + kptr_tag = true;
> > > + }
> > > + /* Look for next tag */
> > > + t = btf_type_by_id(btf, t->type);
> > > + }
> >
> > There is no need for while() loop and 4 bool kptr_*_tag checks.
> > Just do:
> > if (!btf_type_is_type_tag(t))
> > return BTF_FIELD_IGNORE;
> > /* check next tag */
> > if (btf_type_is_type_tag(btf_type_by_id(btf, t->type))
> > return -EINVAL;
>
> But there may be other tags also in the future? Then on older kernels it would
> return an error, instead of skipping over them and ignoring them.
and that would be correct behavior.
If there is a tag it should be meaningful. The kernel shouldn't ignore them.
