On Thu, Oct 31, 2024 at 8:10 PM Yonghong Song <yonghong.song@xxxxxxxxx> wrote:
>
> For struct_ops progs, whether a particular prog will use private stack
> or not (prog->aux->use_priv_stack) will be set before actual insn-level
> verification for that prog. One particular implementation is to
> piggyback on struct_ops->check_member(). The next patch will have an
> example for this. The struct_ops->check_member() will set
> prog->aux->use_priv_stack to be true which enables private stack
> usage with ignoring BPF_PRIV_STACK_MIN_SIZE limit.
>
> If use_priv_stack is true for a particular struct_ops prog, bpf
> trampoline will need to do recursion checks (one level at this point)
> to avoid stack overwrite. A field (recursion_skipped()) is added to
> bpf_prog_aux structure such that if bpf_prog->aux->recursion_skipped
> is set by the struct_ops subsystem, the function will be called
> to terminate the prog run, collect related info, etc.
>
> Acked-by: Tejun Heo <tj@xxxxxxxxxx>
> Signed-off-by: Yonghong Song <yonghong.song@xxxxxxxxx>
> ---
> include/linux/bpf.h | 1 +
> include/linux/bpf_verifier.h | 1 +
> kernel/bpf/trampoline.c | 4 ++++
> kernel/bpf/verifier.c | 36 ++++++++++++++++++++++++++++++++----
> 4 files changed, 38 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 8a3ea7440a4a..7a34108c6974 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -1528,6 +1528,7 @@ struct bpf_prog_aux {
> u64 prog_array_member_cnt; /* counts how many times as member of prog_array */
> struct mutex ext_mutex; /* mutex for is_extended and prog_array_member_cnt */
> struct bpf_arena *arena;
> + void (*recursion_skipped)(struct bpf_prog *prog); /* callback if recursion is skipped */
The name doesn't fit.
The recursion wasn't skipped.
It's the execution of the program that was skipped.
'recursion_detected' or 'recursion_disallowed' would be a better name.
> /* BTF_KIND_FUNC_PROTO for valid attach_btf_id */
> const struct btf_type *attach_func_proto;
> /* function name for valid attach_btf_id */
> diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
> index bc28ce7996ac..ff0fba935f89 100644
> --- a/include/linux/bpf_verifier.h
> +++ b/include/linux/bpf_verifier.h
> @@ -889,6 +889,7 @@ static inline bool bpf_prog_check_recur(const struct bpf_prog *prog)
> case BPF_PROG_TYPE_TRACING:
> return prog->expected_attach_type != BPF_TRACE_ITER;
> case BPF_PROG_TYPE_STRUCT_OPS:
> + return prog->aux->use_priv_stack;
> case BPF_PROG_TYPE_LSM:
> return false;
> default:
> diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
> index 9f36c049f4c2..a84e60efbf89 100644
> --- a/kernel/bpf/trampoline.c
> +++ b/kernel/bpf/trampoline.c
> @@ -899,6 +899,8 @@ static u64 notrace __bpf_prog_enter_recur(struct bpf_prog *prog, struct bpf_tram
>
> if (unlikely(this_cpu_inc_return(*(prog->active)) != 1)) {
> bpf_prog_inc_misses_counter(prog);
> + if (prog->aux->recursion_skipped)
> + prog->aux->recursion_skipped(prog);
> return 0;
> }
> return bpf_prog_start_time();
> @@ -975,6 +977,8 @@ u64 notrace __bpf_prog_enter_sleepable_recur(struct bpf_prog *prog,
>
> if (unlikely(this_cpu_inc_return(*(prog->active)) != 1)) {
> bpf_prog_inc_misses_counter(prog);
> + if (prog->aux->recursion_skipped)
> + prog->aux->recursion_skipped(prog);
> return 0;
> }
> return bpf_prog_start_time();
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 30e74db6a85f..865191c5d21b 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -6023,17 +6023,31 @@ static int check_ptr_alignment(struct bpf_verifier_env *env,
>
> static int bpf_enable_priv_stack(struct bpf_verifier_env *env)
> {
> + bool force_priv_stack = env->prog->aux->use_priv_stack;
> struct bpf_subprog_info *si;
> + int ret;
> +
> + if (!bpf_jit_supports_private_stack()) {
> + if (force_priv_stack) {
> + verbose(env, "Private stack not supported by jit\n");
> + return -EACCES;
> + }
This logic would fit better in the patch 2.
Less code churn and the whole approach is easier to understand.
I don't like this inband signaling.
Now I see why you had that weird <0 check in patch 2 :(
This is ugly.
May be it should be a separate bool request_priv_stack:1
that struct_ops callback will set and it will clean up
this logic.
>
> - if (!bpf_jit_supports_private_stack())
> return NO_PRIV_STACK;
> + }
>
> + ret = PRIV_STACK_ADAPTIVE;
> switch (env->prog->type) {
> case BPF_PROG_TYPE_KPROBE:
> case BPF_PROG_TYPE_TRACEPOINT:
> case BPF_PROG_TYPE_PERF_EVENT:
> case BPF_PROG_TYPE_RAW_TRACEPOINT:
> break;
> + case BPF_PROG_TYPE_STRUCT_OPS:
> + if (!force_priv_stack)
> + return NO_PRIV_STACK;
> + ret = PRIV_STACK_ALWAYS;
> + break;
> case BPF_PROG_TYPE_TRACING:
> if (env->prog->expected_attach_type != BPF_TRACE_ITER)
> break;
> @@ -6044,11 +6058,18 @@ static int bpf_enable_priv_stack(struct bpf_verifier_env *env)
>
> si = env->subprog_info;
> for (int i = 0; i < env->subprog_cnt; i++) {
> - if (si[i].has_tail_call)
> + if (si[i].has_tail_call) {
> + if (ret == PRIV_STACK_ALWAYS) {
> + verbose(env,
> + "Private stack not supported due to tail call presence\n");
> + return -EACCES;
> + }
> +
> return NO_PRIV_STACK;
> + }
> }
>
> - return PRIV_STACK_ADAPTIVE;
> + return ret;
> }
>
> static int round_up_stack_depth(struct bpf_verifier_env *env, int stack_depth)
> @@ -6121,7 +6142,8 @@ static int check_max_stack_depth_subprog(struct bpf_verifier_env *env, int idx,
> idx, subprog_depth);
> return -EACCES;
> }
> - if (subprog_depth >= BPF_PRIV_STACK_MIN_SIZE) {
> + if (priv_stack_supported == PRIV_STACK_ALWAYS ||
> + subprog_depth >= BPF_PRIV_STACK_MIN_SIZE) {
> subprog[idx].use_priv_stack = true;
> subprog_visited[idx] = 1;
> }
> @@ -6271,6 +6293,12 @@ static int check_max_stack_depth(struct bpf_verifier_env *env)
> depth_frame, subtree_depth);
> return -EACCES;
> }
> + if (orig_priv_stack_supported == PRIV_STACK_ALWAYS) {
> + verbose(env,
> + "Private stack not supported due to possible nested subprog run\n");
> + ret = -EACCES;
> + goto out;
> + }
> if (orig_priv_stack_supported == PRIV_STACK_ADAPTIVE) {
> for (int i = 0; i < env->subprog_cnt; i++)
> si[i].use_priv_stack = false;
> --
> 2.43.5
>
