Re: [PATCH bpf-next v5 02/17] bpf/verifier: allow kfunc to return an allocated mem

Benjamin Tissoires
2022-05-19T12:06:26Z

Hi,

thanks a lot for the quick review of these patches.

On Wed, May 18, 2022 at 11:59 PM Kumar Kartikeya Dwivedi
<memxor@gmail.com> wrote:
>
> On Thu, May 19, 2022 at 02:29:09AM IST, Benjamin Tissoires wrote:
> > When a kfunc is not returning a pointer to a struct but to a plain type,
> > we can consider it is a valid allocated memory assuming that:
> > - one of the arguments is called rdonly_buf_size
> > - or one of the arguments is called rdwr_buf_size
> > - and this argument is a const from the caller point of view
> >
> > We can then use this parameter as the size of the allocated memory.
> >
> > The memory is either read-only or read-write based on the name
> > of the size parameter.
> >
> > Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
> >
> > ---
> >
> > changes in v5:
> > - updated PTR_TO_MEM comment in btf.c to match upstream
> > - make it read-only or read-write based on the name of size
> >
> > new in v4
> > ---
> > include/linux/btf.h | 7 +++++
> > kernel/bpf/btf.c | 41 +++++++++++++++++++++++-
> > kernel/bpf/verifier.c | 72 +++++++++++++++++++++++++++++++++----------
> > 3 files changed, 102 insertions(+), 18 deletions(-)
> >
> > diff --git a/include/linux/btf.h b/include/linux/btf.h
> > index 2611cea2c2b6..2a4feafc083e 100644
> > --- a/include/linux/btf.h
> > +++ b/include/linux/btf.h
> > @@ -343,6 +343,13 @@ static inline struct btf_param *btf_params(const struct btf_type *t)
> > return (struct btf_param *)(t + 1);
> > }
> >
> > +struct bpf_reg_state;
> > +
> > +bool btf_is_kfunc_arg_mem_size(const struct btf *btf,
> > + const struct btf_param *arg,
> > + const struct bpf_reg_state *reg,
> > + const char *name);
> > +
> > #ifdef CONFIG_BPF_SYSCALL
> > struct bpf_prog;
> >
> > diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> > index 7bccaa4646e5..2d11d178807c 100644
> > --- a/kernel/bpf/btf.c
> > +++ b/kernel/bpf/btf.c
> > @@ -6049,6 +6049,31 @@ static bool is_kfunc_arg_mem_size(const struct btf *btf,
> > return true;
> > }
> >
> > +bool btf_is_kfunc_arg_mem_size(const struct btf *btf,
> > + const struct btf_param *arg,
> > + const struct bpf_reg_state *reg,
> > + const char *name)
> > +{
> > + int len, target_len = strlen(name);
> > + const struct btf_type *t;
> > + const char *param_name;
> > +
> > + t = btf_type_skip_modifiers(btf, arg->type, NULL);
> > + if (!btf_type_is_scalar(t) || reg->type != SCALAR_VALUE)
> > + return false;
> > +
> > + param_name = btf_name_by_offset(btf, arg->name_off);
> > + if (str_is_empty(param_name))
> > + return false;
> > + len = strlen(param_name);
> > + if (len != target_len)
> > + return false;
> > + if (strncmp(param_name, name, target_len))
> > + return false;
> > +
> > + return true;
> > +}
>
> I think you don't need these checks. btf_check_kfunc_arg_match would have
> already made sure scalar arguments receive scalar. The rest is just matching on
> the argument name, which you can directly strcmp when setting up R0's type.

OK.

>
> > +
> > static int btf_check_func_arg_match(struct bpf_verifier_env *env,
> > const struct btf *btf, u32 func_id,
> > struct bpf_reg_state *regs,
> > @@ -6198,7 +6223,7 @@ static int btf_check_func_arg_match(struct bpf_verifier_env *env,
> > if (reg->type == PTR_TO_BTF_ID) {
> > reg_btf = reg->btf;
> > reg_ref_id = reg->btf_id;
> > - /* Ensure only one argument is referenced PTR_TO_BTF_ID */
> > + /* Ensure only one argument is reference PTR_TO_BTF_ID or PTR_TO_MEM */
>
> But this part of the code would never be reached for PTR_TO_MEM, so the comment
> would be false?

Right, I mostly duplicated the code and the comment, so I'll drop it, thanks.

>
> > if (reg->ref_obj_id) {
> > if (ref_obj_id) {
> > bpf_log(log, "verifier internal error: more than one arg with ref_obj_id R%d %u %u\n",
> > @@ -6258,6 +6283,20 @@ static int btf_check_func_arg_match(struct bpf_verifier_env *env,
> > i++;
> > continue;
> > }
> > +
> > + if (rel && reg->ref_obj_id) {
> > + /* Ensure only one argument is referenced PTR_TO_BTF_ID or PTR_TO_MEM */
> > + if (ref_obj_id) {
> > + bpf_log(log,
> > + "verifier internal error: more than one arg with ref_obj_id R%d %u %u\n",
> > + regno,
> > + reg->ref_obj_id,
> > + ref_obj_id);
> > + return -EFAULT;
> > + }
> > + ref_regno = regno;
> > + ref_obj_id = reg->ref_obj_id;
> > + }
>
> Why do we need this part? I don't see any code passing that __u8 * back into a
> release function. The only release function I see that you are adding is
> releasing a struct, which should be PTR_TO_BTF_ID and already supported.

In my mind, we should have been able to acquire/release PTR_TO_MEM in
the same way we are doing with PTR_TO_BTF_ID. But after fully writing
down the code, it was not required, so maybe we can keep
acquire/release only for PTR_TO_BTF_ID.

>
> Also acquire function should not return non-struct pointer. Can you also update
> the if (acq && !btf_type_is_ptr(t)) check in check_kfunc_call to instead check
> for btf_type_is_struct? The verbose log would be misleading now, but it was
> based on the assumption only PTR_TO_BTF_ID as return pointer is supported.

OK.

>
> > }
> >
> > resolve_ret = btf_resolve_size(btf, ref_t, &type_size);
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 9b59581026f8..084319073064 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -7219,13 +7219,14 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
> > int *insn_idx_p)
> > {
> > const struct btf_type *t, *func, *func_proto, *ptr_type;
> > - struct bpf_reg_state *regs = cur_regs(env);
> > + struct bpf_reg_state *reg, *regs = cur_regs(env);
> > const char *func_name, *ptr_type_name;
> > - u32 i, nargs, func_id, ptr_type_id;
> > + u32 i, nargs, func_id, ptr_type_id, regno;
> > int err, insn_idx = *insn_idx_p;
> > const struct btf_param *args;
> > struct btf *desc_btf;
> > bool acq;
> > + size_t reg_rw_size = 0, reg_ro_size = 0;
>
> Not reverse X-mas tree.

Oh, I didn't realize this was the applied convention. I'll amend
(though the code refactoring from your comment below will probably
change that hunk above).

>
> >
> > /* skip for now, but return error when we find this in fixup_kfunc_call */
> > if (!insn->imm)
> > @@ -7266,8 +7267,8 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
> > }
> > }
> >
> > - for (i = 0; i < CALLER_SAVED_REGS; i++)
> > - mark_reg_not_init(env, regs, caller_saved[i]);
> > + /* reset REG_0 */
> > + mark_reg_not_init(env, regs, BPF_REG_0);
> >
> > /* Check return type */
> > t = btf_type_skip_modifiers(desc_btf, func_proto->type, NULL);
> > @@ -7277,6 +7278,9 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
> > return -EINVAL;
> > }
> >
> > + nargs = btf_type_vlen(func_proto);
> > + args = btf_params(func_proto);
> > +
> > if (btf_type_is_scalar(t)) {
> > mark_reg_unknown(env, regs, BPF_REG_0);
> > mark_btf_func_reg_size(env, BPF_REG_0, t->size);
> > @@ -7284,24 +7288,57 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
> > ptr_type = btf_type_skip_modifiers(desc_btf, t->type,
> > &ptr_type_id);
> > if (!btf_type_is_struct(ptr_type)) {
> > - ptr_type_name = btf_name_by_offset(desc_btf,
> > - ptr_type->name_off);
> > - verbose(env, "kernel function %s returns pointer type %s %s is not supported\n",
> > - func_name, btf_type_str(ptr_type),
> > - ptr_type_name);
> > - return -EINVAL;
> > + /* if we have an array, look for the arguments */
> > + for (i = 0; i < nargs; i++) {
> > + regno = i + BPF_REG_1;
> > + reg = &regs[regno];
> > +
> > + /* look for any const scalar parameter of name "rdonly_buf_size"
> > + * or "rdwr_buf_size"
> > + */
> > + if (!check_reg_arg(env, regno, SRC_OP) &&
> > + tnum_is_const(regs[regno].var_off)) {
>
> Instead of this, we should probably just check the argument that has its name as
> rdonly/rdwr_buf_size inside btf_check_kfunc_arg_match and ensure there is only
> one of those. No need for check_reg_arg, and just this tnum_is_const can also be
> enforced inside btf_check_kfunc_arg_match. You can pass a struct like so:
>
> struct bpf_kfunc_arg_meta {
> u64 r0_size;
> bool r0_rdonly;
> };
>
> and set its value to reg->var_off.value from inside the function in the argument
> checking loop. Then you don't have to change the mark_reg_not_init order here.
> All your code can be inside the if (btf_type_is_scalar(t)) branch.

OK. I think I get it. Not sure I'll be able to get to it by the end of
the week or next week, but I'll work on that cleanup for sure.

>
> Also, it would be nice to use this struct to signal the register that is being
> released. Right now it's done using a > 0 return value (the if (err)) which is a
> bit ugly. But up to you if you want to do that tiny cleanup.

Should be easy enough to do, yes.

>
> > + if (btf_is_kfunc_arg_mem_size(desc_btf, &args[i], reg,
> > + "rdonly_buf_size"))
> > + reg_ro_size = regs[regno].var_off.value;
> > + else if (btf_is_kfunc_arg_mem_size(desc_btf, &args[i], reg,
> > + "rdwr_buf_size"))
> > + reg_rw_size = regs[regno].var_off.value;
> > + }
> > + }
> > +
> > + if (!reg_rw_size && !reg_ro_size) {
> > + ptr_type_name = btf_name_by_offset(desc_btf,
> > + ptr_type->name_off);
> > + verbose(env,
> > + "kernel function %s returns pointer type %s %s is not supported\n",
> > + func_name,
> > + btf_type_str(ptr_type),
> > + ptr_type_name);
> > + return -EINVAL;
> > + }
> > +
> > + mark_reg_known_zero(env, regs, BPF_REG_0);
> > + regs[BPF_REG_0].type = PTR_TO_MEM;
> > + regs[BPF_REG_0].mem_size = reg_ro_size + reg_rw_size;
> > +
> > + if (reg_ro_size)
> > + regs[BPF_REG_0].type |= MEM_RDONLY;
> > + } else {
> > + mark_reg_known_zero(env, regs, BPF_REG_0);
> > + regs[BPF_REG_0].type = PTR_TO_BTF_ID;
> > + regs[BPF_REG_0].btf = desc_btf;
> > + regs[BPF_REG_0].btf_id = ptr_type_id;
> > + mark_btf_func_reg_size(env, BPF_REG_0, sizeof(void *));
> > }
> > - mark_reg_known_zero(env, regs, BPF_REG_0);
> > - regs[BPF_REG_0].btf = desc_btf;
> > - regs[BPF_REG_0].type = PTR_TO_BTF_ID;
> > - regs[BPF_REG_0].btf_id = ptr_type_id;
> > +
> > if (btf_kfunc_id_set_contains(desc_btf, resolve_prog_type(env->prog),
> > BTF_KFUNC_TYPE_RET_NULL, func_id)) {
> > regs[BPF_REG_0].type |= PTR_MAYBE_NULL;
> > /* For mark_ptr_or_null_reg, see 93c230e3f5bd6 */
> > regs[BPF_REG_0].id = ++env->id_gen;
> > }
> > - mark_btf_func_reg_size(env, BPF_REG_0, sizeof(void *));
> > +
>
> Any reason to do this call only for PTR_TO_BTF_ID and not for PTR_TO_MEM?

I must confess I am doing part of the things blindly, and it kind of
worked, passed the tests and I was fine. So no, no reasons except that
maybe at some point it broke what I was trying to do. I'll try to
re-evaluate this line in the next version.

Cheers,
Benjamin

>
> > if (acq) {
> > int id = acquire_reference_state(env, insn_idx);
> >
> > @@ -7312,8 +7349,9 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
> > }
> > } /* else { add_kfunc_call() ensures it is btf_type_is_void(t) } */
> >
> > - nargs = btf_type_vlen(func_proto);
> > - args = (const struct btf_param *)(func_proto + 1);
> > + for (i = 1 ; i < CALLER_SAVED_REGS; i++)
> > + mark_reg_not_init(env, regs, caller_saved[i]);
> > +
> > for (i = 0; i < nargs; i++) {
> > u32 regno = i + 1;
> >
> > --
> > 2.36.1
> >
>
> --
> Kartikeya
>