Re: [PATCH bpf-next v2 4/6] bpf: Introduce bpf_per_cpu_ptr()

From: Andrii Nakryiko
Date: Fri Sep 04 2020 - 16:05:03 EST

On Thu, Sep 3, 2020 at 3:35 PM Hao Luo <haoluo@xxxxxxxxxx> wrote:
>
> Add bpf_per_cpu_ptr() to help bpf programs access percpu vars.
> bpf_per_cpu_ptr() has the same semantic as per_cpu_ptr() in the kernel
> except that it may return NULL. This happens when the cpu parameter is
> out of range. So the caller must check the returned value.
>
> Acked-by: Andrii Nakryiko <andriin@xxxxxx>
> Signed-off-by: Hao Luo <haoluo@xxxxxxxxxx>
> ---
> include/linux/bpf.h | 3 ++
> include/linux/btf.h | 11 ++++++
> include/uapi/linux/bpf.h | 17 +++++++++
> kernel/bpf/btf.c | 10 ------
> kernel/bpf/verifier.c | 66 +++++++++++++++++++++++++++++++---
> kernel/trace/bpf_trace.c | 18 ++++++++++
> tools/include/uapi/linux/bpf.h | 17 +++++++++
> 7 files changed, 128 insertions(+), 14 deletions(-)
>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index c6d9f2c444f4..6b2034f7665e 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -292,6 +292,7 @@ enum bpf_arg_type {
> ARG_PTR_TO_ALLOC_MEM, /* pointer to dynamically allocated memory */
> ARG_PTR_TO_ALLOC_MEM_OR_NULL, /* pointer to dynamically allocated memory or NULL */
> ARG_CONST_ALLOC_SIZE_OR_ZERO, /* number of allocated bytes requested */
> + ARG_PTR_TO_PERCPU_BTF_ID, /* pointer to in-kernel percpu type */
> };
>
> /* type of values returned from helper functions */
> @@ -305,6 +306,7 @@ enum bpf_return_type {
> RET_PTR_TO_SOCK_COMMON_OR_NULL, /* returns a pointer to a sock_common or NULL */
> RET_PTR_TO_ALLOC_MEM_OR_NULL, /* returns a pointer to dynamically allocated memory or NULL */
> RET_PTR_TO_BTF_ID_OR_NULL, /* returns a pointer to a btf_id or NULL */
> + RET_PTR_TO_MEM_OR_BTF_ID_OR_NULL, /* returns a pointer to a valid memory or a btf_id or NULL */
> };
>
> /* eBPF function prototype used by verifier to allow BPF_CALLs from eBPF programs
> @@ -385,6 +387,7 @@ enum bpf_reg_type {
> PTR_TO_RDONLY_BUF_OR_NULL, /* reg points to a readonly buffer or NULL */
> PTR_TO_RDWR_BUF, /* reg points to a read/write buffer */
> PTR_TO_RDWR_BUF_OR_NULL, /* reg points to a read/write buffer or NULL */
> + PTR_TO_PERCPU_BTF_ID, /* reg points to percpu kernel type */
> };
>
> /* The information passed from prog-specific *_is_valid_access
> diff --git a/include/linux/btf.h b/include/linux/btf.h
> index 592373d359b9..07b7de1c05b0 100644
> --- a/include/linux/btf.h
> +++ b/include/linux/btf.h
> @@ -71,6 +71,11 @@ btf_resolve_size(const struct btf *btf, const struct btf_type *type,
> i < btf_type_vlen(struct_type); \
> i++, member++)
>
> +#define for_each_vsi(i, struct_type, member) \

datasec_type?

> + for (i = 0, member = btf_type_var_secinfo(struct_type); \
> + i < btf_type_vlen(struct_type); \
> + i++, member++)
> +
> static inline bool btf_type_is_ptr(const struct btf_type *t)
> {
> return BTF_INFO_KIND(t->info) == BTF_KIND_PTR;
> @@ -155,6 +160,12 @@ static inline const struct btf_member *btf_type_member(const struct btf_type *t)
> return (const struct btf_member *)(t + 1);
> }
>
> +static inline const struct btf_var_secinfo *btf_type_var_secinfo(
> + const struct btf_type *t)
> +{
> + return (const struct btf_var_secinfo *)(t + 1);
> +}
> +
> #ifdef CONFIG_BPF_SYSCALL
> const struct btf_type *btf_type_by_id(const struct btf *btf, u32 type_id);
> const char *btf_name_by_offset(const struct btf *btf, u32 offset);
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index ab00ad9b32e5..d0ec94d5bdbf 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -3596,6 +3596,22 @@ union bpf_attr {
> * the data in *dst*. This is a wrapper of copy_from_user().
> * Return
> * 0 on success, or a negative error in case of failure.
> + *
> + * void *bpf_per_cpu_ptr(const void *percpu_ptr, u32 cpu)
> + * Description
> + * Take a pointer to a percpu ksym, *percpu_ptr*, and return a
> + * pointer to the percpu kernel variable on *cpu*. A ksym is an
> + * extern variable decorated with '__ksym'. For ksym, there is a
> + * global var (either static or global) defined of the same name
> + * in the kernel. The ksym is percpu if the global var is percpu.
> + * The returned pointer points to the global percpu var on *cpu*.
> + *
> + * bpf_per_cpu_ptr() has the same semantic as per_cpu_ptr() in the
> + * kernel, except that bpf_per_cpu_ptr() may return NULL. This
> + * happens if *cpu* is larger than nr_cpu_ids. The caller of
> + * bpf_per_cpu_ptr() must check the returned value.
> + * Return
> + * A generic pointer pointing to the kernel percpu variable on *cpu*.

Or NULL, if *cpu* is invalid.

> */
> #define __BPF_FUNC_MAPPER(FN) \
> FN(unspec), \
> @@ -3747,6 +3763,7 @@ union bpf_attr {
> FN(inode_storage_delete), \
> FN(d_path), \
> FN(copy_from_user), \
> + FN(bpf_per_cpu_ptr), \
> /* */
>

[...]

> @@ -4003,6 +4008,15 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
> if (type != expected_type)
> goto err_type;
> }
> + } else if (arg_type == ARG_PTR_TO_PERCPU_BTF_ID) {
> + expected_type = PTR_TO_PERCPU_BTF_ID;
> + if (type != expected_type)
> + goto err_type;
> + if (!reg->btf_id) {
> + verbose(env, "Helper has invalid btf_id in R%d\n", regno);
> + return -EACCES;
> + }
> + meta->ret_btf_id = reg->btf_id;
> } else if (arg_type == ARG_PTR_TO_BTF_ID) {
> bool ids_match = false;
>
> @@ -5002,6 +5016,30 @@ static int check_helper_call(struct bpf_verifier_env *env, int func_id, int insn
> regs[BPF_REG_0].type = PTR_TO_MEM_OR_NULL;
> regs[BPF_REG_0].id = ++env->id_gen;
> regs[BPF_REG_0].mem_size = meta.mem_size;
> + } else if (fn->ret_type == RET_PTR_TO_MEM_OR_BTF_ID_OR_NULL) {

Given this is internal implementation detail, this return type is
fine, but I'm wondering if it would be better to just make
PTR_TO_BTF_ID to allow not just structs? E.g., if we have an int, just
allow reading those 4 bytes.

Not sure what the implications are in terms of implementation, but
conceptually that shouldn't be a problem, given we do have BTF type ID
describing size and all.

> + const struct btf_type *t;
> +
> + mark_reg_known_zero(env, regs, BPF_REG_0);
> + t = btf_type_skip_modifiers(btf_vmlinux, meta.ret_btf_id, NULL);
> + if (!btf_type_is_struct(t)) {
> + u32 tsize;
> + const struct btf_type *ret;
> + const char *tname;
> +
> + /* resolve the type size of ksym. */
> + ret = btf_resolve_size(btf_vmlinux, t, &tsize);
> + if (IS_ERR(ret)) {
> + tname = btf_name_by_offset(btf_vmlinux, t->name_off);
> + verbose(env, "unable to resolve the size of type '%s': %ld\n",
> + tname, PTR_ERR(ret));
> + return -EINVAL;
> + }
> + regs[BPF_REG_0].type = PTR_TO_MEM_OR_NULL;
> + regs[BPF_REG_0].mem_size = tsize;
> + } else {
> + regs[BPF_REG_0].type = PTR_TO_BTF_ID_OR_NULL;
> + regs[BPF_REG_0].btf_id = meta.ret_btf_id;
> + }
> } else if (fn->ret_type == RET_PTR_TO_BTF_ID_OR_NULL) {
> int ret_btf_id;
>

[...]

> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index b2a5380eb187..d474c1530f87 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -1144,6 +1144,22 @@ static const struct bpf_func_proto bpf_d_path_proto = {
> .allowed = bpf_d_path_allowed,
> };
>
> +BPF_CALL_2(bpf_per_cpu_ptr, const void *, ptr, u32, cpu)
> +{
> + if (cpu >= nr_cpu_ids)
> + return 0;
> +
> + return (u64)per_cpu_ptr(ptr, cpu);

not sure, but on 32-bit arches this might cause compilation warning,
case to (unsigned long) instead?

> +}
> +
> +static const struct bpf_func_proto bpf_per_cpu_ptr_proto = {
> + .func = bpf_per_cpu_ptr,
> + .gpl_only = false,
> + .ret_type = RET_PTR_TO_MEM_OR_BTF_ID_OR_NULL,
> + .arg1_type = ARG_PTR_TO_PERCPU_BTF_ID,
> + .arg2_type = ARG_ANYTHING,
> +};
> +

[...]