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bpf: Add a kfunc for generic type cast

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Implement bpf_rdonly_cast() which tries to cast the object
to a specified type. This tries to support use case like below:
  #define skb_shinfo(SKB) ((struct skb_shared_info *)(skb_end_pointer(SKB)))
where skb_end_pointer(SKB) is a 'unsigned char *' and needs to
be casted to 'struct skb_shared_info *'.

The signature of bpf_rdonly_cast() looks like
   void *bpf_rdonly_cast(void *obj, __u32 btf_id)
The function returns the same 'obj' but with PTR_TO_BTF_ID with
btf_id. The verifier will ensure btf_id being a struct type.

Since the supported type cast may not reflect what the 'obj'
represents, the returned btf_id is marked as PTR_UNTRUSTED, so
the return value and subsequent pointer chasing cannot be
used as helper/kfunc arguments.

Signed-off-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20221120195437.3114585-1-yhs@fb.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Yonghong Song 2022-11-20 11:54:37 -08:00 committed by Alexei Starovoitov
parent fd264ca020
commit a35b9af4ec
2 changed files with 24 additions and 2 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

6
kernel/bpf/helpers.c
View File

@ -1884,6 +1884,11 @@ void *bpf_cast_to_kern_ctx(void *obj)
return obj;
}
void *bpf_rdonly_cast(void *obj__ign, u32 btf_id__k)
{
return obj__ign;
}
__diag_pop();
BTF_SET8_START(generic_btf_ids)
@ -1913,6 +1918,7 @@ BTF_ID(func, bpf_task_release)
BTF_SET8_START(common_btf_ids)
BTF_ID_FLAGS(func, bpf_cast_to_kern_ctx)
BTF_ID_FLAGS(func, bpf_rdonly_cast)
BTF_SET8_END(common_btf_ids)
static const struct btf_kfunc_id_set common_kfunc_set = {

20
kernel/bpf/verifier.c
View File

@ -8153,6 +8153,7 @@ enum special_kfunc_type {
KF_bpf_list_pop_front,
KF_bpf_list_pop_back,
KF_bpf_cast_to_kern_ctx,
KF_bpf_rdonly_cast,
};
BTF_SET_START(special_kfunc_set)
@ -8163,6 +8164,7 @@ BTF_ID(func, bpf_list_push_back)
BTF_ID(func, bpf_list_pop_front)
BTF_ID(func, bpf_list_pop_back)
BTF_ID(func, bpf_cast_to_kern_ctx)
BTF_ID(func, bpf_rdonly_cast)
BTF_SET_END(special_kfunc_set)
BTF_ID_LIST(special_kfunc_list)
@ -8173,6 +8175,7 @@ BTF_ID(func, bpf_list_push_back)
BTF_ID(func, bpf_list_pop_front)
BTF_ID(func, bpf_list_pop_back)
BTF_ID(func, bpf_cast_to_kern_ctx)
BTF_ID(func, bpf_rdonly_cast)
static enum kfunc_ptr_arg_type
get_kfunc_ptr_arg_type(struct bpf_verifier_env *env,
@ -8809,6 +8812,7 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
u32 i, nargs, func_id, ptr_type_id;
int err, insn_idx = *insn_idx_p;
const struct btf_param *args;
const struct btf_type *ret_t;
struct btf *desc_btf;
u32 *kfunc_flags;
@ -8888,7 +8892,6 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
if (meta.btf == btf_vmlinux && btf_id_set_contains(&special_kfunc_set, meta.func_id)) {
if (meta.func_id == special_kfunc_list[KF_bpf_obj_new_impl]) {
const struct btf_type *ret_t;
struct btf *ret_btf;
u32 ret_btf_id;
@ -8941,6 +8944,18 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
regs[BPF_REG_0].type = PTR_TO_BTF_ID | PTR_TRUSTED;
regs[BPF_REG_0].btf = desc_btf;
regs[BPF_REG_0].btf_id = meta.ret_btf_id;
} else if (meta.func_id == special_kfunc_list[KF_bpf_rdonly_cast]) {
ret_t = btf_type_by_id(desc_btf, meta.arg_constant.value);
if (!ret_t || !btf_type_is_struct(ret_t)) {
verbose(env,
"kfunc bpf_rdonly_cast type ID argument must be of a struct\n");
return -EINVAL;
}
mark_reg_known_zero(env, regs, BPF_REG_0);
regs[BPF_REG_0].type = PTR_TO_BTF_ID | PTR_UNTRUSTED;
regs[BPF_REG_0].btf = desc_btf;
regs[BPF_REG_0].btf_id = meta.arg_constant.value;
} else {
verbose(env, "kernel function %s unhandled dynamic return type\n",
meta.func_name);
@ -15194,7 +15209,8 @@ static int fixup_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
insn_buf[1] = addr[1];
insn_buf[2] = *insn;
*cnt = 3;
} else if (desc->func_id == special_kfunc_list[KF_bpf_cast_to_kern_ctx]) {
} else if (desc->func_id == special_kfunc_list[KF_bpf_cast_to_kern_ctx] ||
desc->func_id == special_kfunc_list[KF_bpf_rdonly_cast]) {
insn_buf[0] = BPF_MOV64_REG(BPF_REG_0, BPF_REG_1);
*cnt = 1;
}