Attempt to fix unwinder errors from lua JIT stacks that aren't on top

parca-dev · Aug 7, 2024 · 21ab75a · 21ab75a
1 parent 000ef13
commit 21ab75a
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Showing 2 changed files with 22 additions and 1 deletion.
diff --git a/bpf/unwinders/lua.bpf.c b/bpf/unwinders/lua.bpf.c
@@ -606,7 +606,7 @@ static __always_inline lua_State *fish_for_L(u64 sp, int cur_L_offset) {
     struct ngx_http_lua_co_ctx_s *lua_co_ctx;
     lua_State *Lmaybe = NULL;
 
-    for (int i = 1; i <= 4; i++) {
+    for (int i = 1; i <= 6; i++) {
         bpf_probe_read_user(&lua_co_ctx, sizeof(lua_co_ctx), (void **)sp - i);
         if (lua_co_ctx == NULL) {
             continue;

diff --git a/bpf/unwinders/native.bpf.c b/bpf/unwinders/native.bpf.c
@@ -887,6 +887,7 @@ SEC("perf_event")
 int native_unwind(struct bpf_perf_event_data *ctx) {
     u64 pid_tgid = bpf_get_current_pid_tgid();
     int per_process_id = pid_tgid >> 32;
+    int per_thread_id = pid_tgid;
 
     int err = 0;
     bool reached_bottom_of_stack = false;
@@ -932,6 +933,26 @@ int native_unwind(struct bpf_perf_event_data *ctx) {
 
         if (unwind_table_result == FIND_UNWIND_JITTED) {
             LOG("[debug] Unwinding JITed stacks");
+            if (unwind_state->unwinder_type == RUNTIME_UNWINDER_TYPE_LUA) {
+                LOG("[info] lua: JIT pc 0x%llx encountered, tail calling lua unwinder", unwind_state->ip);
+                lua_uprobe_state_t *ls = bpf_map_lookup_elem(&tid_to_lua_state, &per_thread_id);
+                if (ls != NULL && ls->sp != 0) {
+                    LOG("[info] lua: saved lua entry state found, tail calling native unwinder from there");
+                    // copy saved lua state to native unwind state and tail call native unwinder
+                    unwind_state->bp = ls->bp;
+                    unwind_state->sp = ls->sp;
+                    unwind_state->ip = ls->ip;
+                    ls->bp = 0;
+                    ls->sp = 0;
+                    ls->ip = 0;
+                    // remove luajit frame
+                    unwind_state->stack.len = 0;
+                    bpf_tail_call(ctx, &programs, NATIVE_UNWINDER_PROGRAM_ID);
+                } else {
+                    LOG("[info] lua: no saved lua entry state found, JIT'd stacks won't be connected to native stack, enable LUA uprobes");
+                    bpf_tail_call(ctx, &programs, LUA_UNWINDER_PROGRAM_ID);
+                }
+            }
 
             unwind_state->unwinding_jit = true;
             if (dwarf_to_jit) {