On 2/4/24 4:02 AM, Kumar Kartikeya Dwivedi wrote:
Currently, calling any helpers, kfuncs, or subprogs except the graph data structure (lists, rbtrees) API kfuncs while holding a bpf_spin_lock is not allowed. One of the original motivations of this decision was to force the BPF programmer's hand into keeping the bpf_spin_lock critical section small, and to ensure the execution time of the program does not increase due to lock waiting times. In addition to this, some of the helpers and kfuncs may be unsafe to call while holding a bpf_spin_lock. However, when it comes to subprog calls, atleast for static subprogs, the verifier is able to explore their instructions during verification. Therefore, it is similar in effect to having the same code inlined into the critical section. Hence, not allowing static subprog calls in the bpf_spin_lock critical section is mostly an annoyance that needs to be worked around, without providing any tangible benefit. Unlike static subprog calls, global subprog calls are not safe to permit within the critical section, as the verifier does not explore them during verification, therefore whether the same lock will be taken again, or unlocked, cannot be ascertained. Therefore, allow calling static subprogs within a bpf_spin_lock critical section, and only reject it in case the subprog linkage is global. Signed-off-by: Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx>
SGTM with a small nit below. Acked-by: Yonghong Song <yonghong.song@xxxxxxxxx>
--- kernel/bpf/verifier.c | 10 +++++++--- tools/testing/selftests/bpf/progs/verifier_spin_lock.c | 2 +- 2 files changed, 8 insertions(+), 4 deletions(-) diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index 64fa188d00ad..f858c959753b 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -9493,6 +9493,12 @@ static int check_func_call(struct bpf_verifier_env *env, struct bpf_insn *insn, if (subprog_is_global(env, subprog)) { const char *sub_name = subprog_name(env, subprog);+ /* Only global subprogs cannot be called with a lock held. */+ if (env->cur_state->active_lock.ptr) { + verbose(env, "function calls are not allowed while holding a lock\n");
Maybe explicit to mention "global function calls are not allowed ..."?
+ return -EINVAL; + } + if (err) { verbose(env, "Caller passes invalid args into func#%d ('%s')\n", subprog, sub_name); @@ -17644,7 +17650,6 @@ static int do_check(struct bpf_verifier_env *env)if (env->cur_state->active_lock.ptr) {if ((insn->src_reg == BPF_REG_0 && insn->imm != BPF_FUNC_spin_unlock) || - (insn->src_reg == BPF_PSEUDO_CALL) || (insn->src_reg == BPF_PSEUDO_KFUNC_CALL && (insn->off != 0 || !is_bpf_graph_api_kfunc(insn->imm)))) { verbose(env, "function calls are not allowed while holding a lock\n"); @@ -17692,8 +17697,7 @@ static int do_check(struct bpf_verifier_env *env) return -EINVAL; } process_bpf_exit_full: - if (env->cur_state->active_lock.ptr && - !in_rbtree_lock_required_cb(env)) { + if (env->cur_state->active_lock.ptr && !env->cur_state->curframe) { verbose(env, "bpf_spin_unlock is missing\n"); return -EINVAL; }
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