Re: [PATCH bpf-next v2 1/6] bpf: Introduce pseudo_btf_id

On Thu, Sep 3, 2020 at 3:34 PM Hao Luo haoluo@google.com wrote:

Pseudo_btf_id is a type of ld_imm insn that associates a btf_id to a ksym so that further dereferences on the ksym can use the BTF info to validate accesses. Internally, when seeing a pseudo_btf_id ld insn, the verifier reads the btf_id stored in the insn[0]'s imm field and marks the dst_reg as PTR_TO_BTF_ID. The btf_id points to a VAR_KIND, which is encoded in btf_vminux by pahole. If the VAR is not of a struct type, the dst reg will be marked as PTR_TO_MEM instead of PTR_TO_BTF_ID and the mem_size is resolved to the size of the VAR's type.

From the VAR btf_id, the verifier can also read the address of the ksym's corresponding kernel var from kallsyms and use that to fill dst_reg.

Therefore, the proper functionality of pseudo_btf_id depends on (1) kallsyms and (2) the encoding of kernel global VARs in pahole, which should be available since pahole v1.18.

Signed-off-by: Hao Luo haoluo@google.com

Logic looks correct, but I have a few suggestions for naming things and verifier logs. Please see below.

include/linux/bpf_verifier.h | 4 ++ include/linux/btf.h | 15 +++++ include/uapi/linux/bpf.h | 38 ++++++++--- kernel/bpf/btf.c | 15 ----- kernel/bpf/verifier.c | 112 ++++++++++++++++++++++++++++++--- tools/include/uapi/linux/bpf.h | 38 ++++++++--- 6 files changed, 182 insertions(+), 40 deletions(-)

diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h index 53c7bd568c5d..a14063f64d96 100644 --- a/include/linux/bpf_verifier.h +++ b/include/linux/bpf_verifier.h @@ -308,6 +308,10 @@ struct bpf_insn_aux_data { u32 map_index; /* index into used_maps[] */ u32 map_off; /* offset from value base address */ };

•           struct {
  

•                   u32 pseudo_btf_id_type; /* type of pseudo_btf_id */
  

•                   u32 pseudo_btf_id_meta; /* memsize or btf_id */
  

a bit misleading names, not clear at all in the code what's in there. This section is for ldimm64 insns that are loading BTF variables, right? so how about this:

struct { u32 reg_type; union { u32 btf_id; u32 memsize; }; } btf_var;

In case someone hates non-anonymous structs, I'd still go with btf_var_reg_type, btf_var_btf_id and btf_var_memsize.

•           };
    };
    u64 map_key_state; /* constant (32 bit) key tracking for maps */
    int ctx_field_size; /* the ctx field size for load insn, maybe 0 */
  

diff --git a/include/linux/btf.h b/include/linux/btf.h index a9af5e7a7ece..592373d359b9 100644 --- a/include/linux/btf.h +++ b/include/linux/btf.h @@ -106,6 +106,21 @@ static inline bool btf_type_is_func_proto(const struct btf_type *t) return BTF_INFO_KIND(t->info) == BTF_KIND_FUNC_PROTO; }

[...]

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index b4e9c56b8b32..3b382c080cfd 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -7398,6 +7398,24 @@ static int check_ld_imm(struct bpf_verifier_env *env, struct bpf_insn *insn) return 0; }

•   if (insn->src_reg == BPF_PSEUDO_BTF_ID) {
  

•           u32 type = aux->pseudo_btf_id_type;
  

•           u32 meta = aux->pseudo_btf_id_meta;
  

•           mark_reg_known_zero(env, regs, insn->dst_reg);
  

•           regs[insn->dst_reg].type = type;
  

•           if (type == PTR_TO_MEM) {
  

•                   regs[insn->dst_reg].mem_size = meta;
  

•           } else if (type == PTR_TO_BTF_ID) {
  

•                   regs[insn->dst_reg].btf_id = meta;
  

nit: probably worthwhile to introduce a local variable (dst_reg) to capture pointer to regs[insn->dst_reg] in this entire function. Then no reall need for type and meta local vars above, everything is going to be short and sweet.

•           } else {
  

•                   verbose(env, "bpf verifier is misconfigured\n");
  

•                   return -EFAULT;
  

•           }
  

•           return 0;
  

•   }
  

•   map = env->used_maps[aux->map_index];
    mark_reg_known_zero(env, regs, insn->dst_reg);
    regs[insn->dst_reg].map_ptr = map;
  

@@ -9284,6 +9302,74 @@ static int do_check(struct bpf_verifier_env *env) return 0; }

+/* replace pseudo btf_id with kernel symbol address */ +static int check_pseudo_btf_id(struct bpf_verifier_env *env,

•                          struct bpf_insn *insn,
  

•                          struct bpf_insn_aux_data *aux)
  

+{

•   u32 type, id = insn->imm;
  

•   const struct btf_type *t;
  

•   const char *sym_name;
  

•   u64 addr;
  

•   if (!btf_vmlinux) {
  

•           verbose(env, "%s: btf not available. verifier misconfigured.\n",
  

•                   __func__);
  

verifier logs so far hasn't used __func__ and it's not all that meaningful to users and might change due to later refactorings.

Also, in this particular case, it's not a verifier misconfiguration, but rather that the kernel doesn't have a built-in BTF, so suggest enabling CONFIG_DEBUG_INFO_BTF=y.

"kernel is missing BTF, make sure CONFIG_DEBUG_INFO_BTF=y is specified in Kconfig"

•           return -EINVAL;
  

•   }
  

•   t = btf_type_by_id(btf_vmlinux, id);
  

•   if (!t) {
  

•           verbose(env, "%s: invalid btf_id %d\n", __func__, id);
  

"ldimm64 insn specifies invalid btf_id %d"? add similar context below

•           return -ENOENT;
  

•   }
  

•   if (insn[1].imm != 0) {
  

•           verbose(env, "%s: BPF_PSEUDO_BTF_ID uses reserved fields\n",
  

•                   __func__);
  

•           return -EINVAL;
  

•   }
  

•   if (!btf_type_is_var(t)) {
  

•           verbose(env, "%s: btf_id %d isn't KIND_VAR\n", __func__, id);
  

•           return -EINVAL;
  

•   }
  

•   sym_name = btf_name_by_offset(btf_vmlinux, t->name_off);
  

•   addr = kallsyms_lookup_name(sym_name);
  

•   if (!addr) {
  

•           verbose(env, "%s: failed to find the address of symbol '%s'.\n",
  

•                   __func__, sym_name);
  

•           return -ENOENT;
  

•   }
  

[...]

@@ -9394,10 +9480,14 @@ static bool bpf_map_is_cgroup_storage(struct bpf_map *map) map->map_type == BPF_MAP_TYPE_PERCPU_CGROUP_STORAGE); }

-/* look for pseudo eBPF instructions that access map FDs and

• • replace them with actual map pointers

+/* find and rewrite pseudo imm in ld_imm64 instructions:

• • 1. if it accesses map FD, replace it with actual map pointer.
• • 2. if it accesses btf_id of a VAR, replace it with pointer to the var.
• • NOTE: btf_vmlinux is required for converting pseudo btf_id.
  */

-static int replace_map_fd_with_map_ptr(struct bpf_verifier_env *env) +static int replace_pseudo_imm_with_ptr(struct bpf_verifier_env *env)

nit: I'd call this something like "resolve_pseudo_ldimm64" instead. ptr here is an implementation detail of map pointers ldimm64 and doesn't even match what we are doing for pseudo_btf_id

{ struct bpf_insn *insn = env->prog->insnsi; int insn_cnt = env->prog->len; @@ -9438,6 +9528,14 @@ static int replace_map_fd_with_map_ptr(struct bpf_verifier_env *env) /* valid generic load 64-bit imm */ goto next_insn;

•                   if (insn[0].src_reg == BPF_PSEUDO_BTF_ID) {
  

•                           aux = &env->insn_aux_data[i];
  

•                           err = check_pseudo_btf_id(env, insn, aux);
  

•                           if (err)
  

•                                   return err;
  

•                           goto next_insn;
  

•                   }
  

•                   /* In final convert_pseudo_ld_imm64() step, this is
                     * converted into regular 64-bit imm load insn.
                     */
  

[...]