On Mon, Nov 4, 2024 at 11:38 AM Yonghong Song <yonghong.song@xxxxxxxxx> wrote:
>
> In previous patch, tracing progs are enabled for private stack since
> recursion checking ensures there exists no nested same bpf prog run on
> the same cpu.
>
> But it is still possible for nested bpf subprog run on the same cpu
> if the same subprog is called in both main prog and async callback,
> or in different async callbacks. For example,
> main_prog
> bpf_timer_set_callback(timer, timer_cb);
> call sub1
> sub1
> ...
> time_cb
> call sub1
>
> In the above case, nested subprog run for sub1 is possible with one in
> process context and the other in softirq context. If this is the case,
> the verifier will disable private stack for this bpf prog.
>
> Signed-off-by: Yonghong Song <yonghong.song@xxxxxxxxx>
> ---
> include/linux/bpf_verifier.h | 2 ++
> kernel/bpf/verifier.c | 42 +++++++++++++++++++++++++++++++-----
> 2 files changed, 39 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
> index 0622c11a7e19..e921589abc72 100644
> --- a/include/linux/bpf_verifier.h
> +++ b/include/linux/bpf_verifier.h
> @@ -669,6 +669,8 @@ struct bpf_subprog_info {
> /* true if bpf_fastcall stack region is used by functions that can't be inlined */
> bool keep_fastcall_stack: 1;
> bool use_priv_stack: 1;
> + bool visited_with_priv_stack_accum: 1;
> + bool visited_with_priv_stack: 1;
>
> u8 arg_cnt;
> struct bpf_subprog_arg_info args[MAX_BPF_FUNC_REG_ARGS];
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 406195c433ea..e01b3f0fd314 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -6118,8 +6118,12 @@ 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 (subprog_depth >= BPF_PRIV_STACK_MIN_SIZE) {
> subprog[idx].use_priv_stack = true;
> + subprog[idx].visited_with_priv_stack = true;
> + }
> + } else {
> + subprog[idx].visited_with_priv_stack = true;
See suggestion for patch 3.
It's cleaner to rewrite with a single visited_with_priv_stack = true; statement.
> }
> }
> continue_func:
> @@ -6220,10 +6224,12 @@ static int check_max_stack_depth_subprog(struct bpf_verifier_env *env, int idx,
> static int check_max_stack_depth(struct bpf_verifier_env *env)
> {
> struct bpf_subprog_info *si = env->subprog_info;
> + enum priv_stack_mode orig_priv_stack_supported;
> enum priv_stack_mode priv_stack_supported;
> int ret, subtree_depth = 0, depth_frame;
>
> priv_stack_supported = bpf_enable_priv_stack(env->prog);
> + orig_priv_stack_supported = priv_stack_supported;
>
> if (priv_stack_supported != NO_PRIV_STACK) {
> for (int i = 0; i < env->subprog_cnt; i++) {
> @@ -6240,13 +6246,39 @@ static int check_max_stack_depth(struct bpf_verifier_env *env)
> priv_stack_supported);
> if (ret < 0)
> return ret;
> +
> + if (priv_stack_supported != NO_PRIV_STACK) {
> + for (int j = 0; j < env->subprog_cnt; j++) {
> + if (si[j].visited_with_priv_stack_accum &&
> + si[j].visited_with_priv_stack) {
> + /* si[j] is visited by both main/async subprog
> + * and another async subprog.
> + */
> + priv_stack_supported = NO_PRIV_STACK;
> + break;
> + }
> + if (!si[j].visited_with_priv_stack_accum)
> + si[j].visited_with_priv_stack_accum =
> + si[j].visited_with_priv_stack;
> + }
> + }
> + if (priv_stack_supported != NO_PRIV_STACK) {
> + for (int j = 0; j < env->subprog_cnt; j++)
> + si[j].visited_with_priv_stack = false;
> + }
I cannot understand what this algorithm is doing.
What is the meaning of visited_with_priv_stack_accum ?
> }
> }
>
> - if (priv_stack_supported == NO_PRIV_STACK && subtree_depth > MAX_BPF_STACK) {
> - verbose(env, "combined stack size of %d calls is %d. Too large\n",
> - depth_frame, subtree_depth);
> - return -EACCES;
> + if (priv_stack_supported == NO_PRIV_STACK) {
> + if (subtree_depth > MAX_BPF_STACK) {
> + verbose(env, "combined stack size of %d calls is %d. Too large\n",
> + depth_frame, subtree_depth);
> + return -EACCES;
> + }
> + if (orig_priv_stack_supported == PRIV_STACK_ADAPTIVE) {
> + for (int i = 0; i < env->subprog_cnt; i++)
> + si[i].use_priv_stack = false;
> + }
why? This patch suppose clear use_priv_stack from subprogs
that are dual called and only from those subprogs.
All other subprogs are fine.
But it seems the alog attempts to detect one such calling scenario
and disables priv_stack everywhere?
