feat(ebpf): use bpf_loop for local d_path implementation

Part of ROX-32059.

The local implementation of d_path now uses bpf_loop, which enables it
to iterate further than the previously implemented loop that did at most
16 levels of path.

Author: Molter73

fact-ebpf/src/bpf/d_path.h

@@ -8,6 +8,64 @@
 #include <bpf/bpf_core_read.h>
 // clang-format on
 
+struct d_path_ctx {
+  struct helper_t* helper;
+  struct path root;
+  struct mount* mnt;
+  struct dentry* dentry;
+  int offset;
+  int buflen;
+  bool success;
+};
+
+static long __d_path_inner(uint32_t index, void* _ctx) {
+  struct d_path_ctx* ctx = (struct d_path_ctx*)_ctx;
+  struct dentry* dentry = ctx->dentry;
+  struct dentry* parent = BPF_CORE_READ(dentry, d_parent);
+  struct mount* mnt = ctx->mnt;
+  struct dentry* mnt_root = BPF_CORE_READ(mnt, mnt.mnt_root);
+
+  if (dentry == mnt_root) {
+    struct mount* m = BPF_CORE_READ(mnt, mnt_parent);
+    if (m != mnt) {
+      ctx->dentry = BPF_CORE_READ(mnt, mnt_mountpoint);
+      ctx->mnt = m;
+      return 0;
+    }
+    ctx->success = true;
+    return 1;
+  }
+
+  if (dentry == parent) {
+    return 1;
+  }
+
+  struct qstr d_name;
+  BPF_CORE_READ_INTO(&d_name, dentry, d_name);
+  int len = d_name.len;
+  if (len <= 0 || len >= ctx->buflen) {
+    return 1;
+  }
+
+  ctx->offset -= len;
+  if (ctx->offset <= 0) {
+    return 1;
+  }
+
+  if (bpf_probe_read_kernel(&ctx->helper->buf[ctx->offset], len, d_name.name) != 0) {
+    return 1;
+  }
+
+  ctx->offset--;
+  if (ctx->offset <= 0) {
+    return 1;
+  }
+  ctx->helper->buf[ctx->offset] = '/';
+
+  ctx->dentry = parent;
+  return 0;
+}
+
 /**
  * Reimplementation of the kernel d_path function.
  *
@@ -19,65 +77,27 @@ __always_inline static long __d_path(const struct path* path, char* buf, int buf
     return -1;
   }
 
-  struct helper_t* helper = get_helper();
-  if (helper == NULL) {
+  struct d_path_ctx ctx = {0};
+
+  ctx.helper = get_helper();
+  if (ctx.helper == NULL) {
     return -1;
   }
 
   struct task_struct* task = (struct task_struct*)bpf_get_current_task();
   int offset = (buflen - 1) & (PATH_MAX - 1);
-  helper->buf[offset] = '\0';  // Ensure null termination
-
-  struct path root;
-  BPF_CORE_READ_INTO(&root, task, fs, root);
-  struct mount* mnt = container_of(path->mnt, struct mount, mnt);
-  struct dentry* dentry;
-  BPF_CORE_READ_INTO(&dentry, path, dentry);
-
-  for (int i = 0; i < 16 && (dentry != root.dentry || &mnt->mnt != root.mnt); i++) {
-    struct dentry* parent = BPF_CORE_READ(dentry, d_parent);
-    struct dentry* mnt_root = BPF_CORE_READ(mnt, mnt.mnt_root);
-
-    if (dentry == mnt_root) {
-      struct mount* m = BPF_CORE_READ(mnt, mnt_parent);
-      if (m != mnt) {
-        dentry = BPF_CORE_READ(mnt, mnt_mountpoint);
-        mnt = m;
-        continue;
-      }
-      break;
-    }
-
-    if (dentry == parent) {
-      return -1;
-    }
+  ctx.helper->buf[offset] = '\0';  // Ensure null termination
 
-    struct qstr d_name;
-    BPF_CORE_READ_INTO(&d_name, dentry, d_name);
-    int len = d_name.len;
-    if (len <= 0 || len >= buflen) {
-      return -1;
-    }
-
-    offset -= len;
-    if (offset <= 0) {
-      return -1;
-    }
-
-    if (bpf_probe_read_kernel(&helper->buf[offset], len, d_name.name) != 0) {
-      return -1;
-    }
-
-    offset--;
-    if (offset <= 0) {
-      return -1;
-    }
-    helper->buf[offset] = '/';
+  BPF_CORE_READ_INTO(&ctx.root, task, fs, root);
+  ctx.mnt = container_of(path->mnt, struct mount, mnt);
+  BPF_CORE_READ_INTO(&ctx.dentry, path, dentry);
 
-    dentry = parent;
+  long res = bpf_loop(PATH_MAX, __d_path_inner, &ctx, 0);
+  if (res <= 0 || !ctx.success) {
+    return -1;
   }
 
-  bpf_probe_read_str(buf, buflen, &helper->buf[offset]);
+  bpf_probe_read_str(buf, buflen, &ctx.helper->buf[offset]);
   return buflen - offset;
 }