Use syscall when name_to_handle_at is not available

This makes cgroupid function to work fine when building with alpine.

src/CMakeLists.txt

@@ -8,6 +8,7 @@ add_executable(bpftrace
   map.cpp
   mapkey.cpp
   printf.cpp
+  resolve_cgroupid.cpp
   tracepoint_format_parser.cpp
   types.cpp
   list.cpp

src/act_helpers.h

@@ -9,6 +9,7 @@
 // Here are some helper macros helping to ensure if the C++
 // counterpart has members of the same type and offset.
 
+#include <cstddef>
 #include <type_traits>
 
 namespace act_helpers

src/bpftrace.cpp

@@ -14,11 +14,11 @@
 #include "bcc_syms.h"
 #include "perf_reader.h"
 
-#include "act_helpers.h"
 #include "bpforc.h"
 #include "bpftrace.h"
 #include "attached_probe.h"
 #include "triggers.h"
+#include "resolve_cgroupid.h"
 
 namespace bpftrace {
 
@@ -1288,63 +1288,11 @@ uint64_t BPFtrace::resolve_kname(const std::string &name)
   return addr;
 }
 
-#ifdef HAVE_NAME_TO_HANDLE_AT
-
-namespace
-{
-
-// Not embedding file_handle directly in cgid_file_handle, because C++
-// has problems with zero-sized array as struct members and
-// file_handle's f_handle is a zero-sized array.
-//
-// Also, not embedding file_handle through the public inheritance,
-// since checking for member offsets with offsetof within a
-// non-standard-layout type is conditionally-supported (so compilers
-// are not required to support it). And we want to do it to make sure
-// we got the wrapper right.
-//
-// Hence open coding the file_handle members directly in
-// cgid_file_handle and the static asserts following it.
-struct cgid_file_handle
-{
-  file_handle *as_file_handle_ptr()
-  {
-    return reinterpret_cast<file_handle*>(this);
-  }
-
-  unsigned int handle_bytes = sizeof(uint64_t);
-  int handle_type;
-  uint64_t cgid;
-};
-
-ACTH_ASSERT_SAME_SIZE(cgid_file_handle, file_handle, uint64_t);
-ACTH_ASSERT_SAME_MEMBER(cgid_file_handle, handle_bytes, file_handle, handle_bytes);
-ACTH_ASSERT_SAME_MEMBER(cgid_file_handle, handle_type, file_handle, handle_type);
-ACTH_ASSERT_SAME_OFFSET(cgid_file_handle, cgid, file_handle, f_handle);
-
-}
-
 uint64_t BPFtrace::resolve_cgroupid(const std::string &path)
 {
-  cgid_file_handle cfh;
-  int mount_id;
-  auto err = name_to_handle_at(AT_FDCWD, path.c_str(), cfh.as_file_handle_ptr(), &mount_id, 0);
-  if (err < 0) {
-    throw std::runtime_error("cgroup '" + path + "' not found");
-  }
-
-  return cfh.cgid;
+  return bpftrace_linux::resolve_cgroupid(path);
 }
 
-#else
-
-uint64_t BPFtrace::resolve_cgroupid(const std::string &path)
-{
-  throw std::runtime_error("cgroupid is not supported on this system");
-}
-
-#endif
-
 uint64_t BPFtrace::resolve_uname(const std::string &name, const std::string &path)
 {
   uint64_t addr = 0;

src/resolve_cgroupid.cpp

+#ifndef _GNU_SOURCE
+# define _GNU_SOURCE
+#endif
+
+#ifdef HAVE_NAME_TO_HANDLE_AT
+
+# include <sys/types.h>
+# include <sys/stat.h>
+
+#else
+
+# include <unistd.h>
+# include <sys/syscall.h>
+
+#endif
+
+#include <fcntl.h>
+
+#include <stdexcept>
+
+#include "act_helpers.h"
+#include "resolve_cgroupid.h"
+
+namespace
+{
+
+#ifndef HAVE_NAME_TO_HANDLE_AT
+
+struct file_handle
+{
+  unsigned int handle_bytes;
+  int handle_type;
+  char f_handle[0];
+};
+
+int name_to_handle_at(int dirfd, const char *pathname,
+                      struct file_handle *handle,
+                      int *mount_id, int flags)
+{
+  return (int)syscall(SYS_name_to_handle_at, dirfd, pathname, handle, mount_id, flags);
+}
+
+#endif
+
+// Not embedding file_handle directly in cgid_file_handle, because C++
+// has problems with zero-sized array as struct members and
+// file_handle's f_handle is a zero-sized array.
+//
+// Also, not embedding file_handle through the public inheritance,
+// since checking for member offsets with offsetof within a
+// non-standard-layout type is conditionally-supported (so compilers
+// are not required to support it). And we want to do it to make sure
+// we got the wrapper right.
+//
+// Hence open coding the file_handle members directly in
+// cgid_file_handle and the static asserts following it.
+struct cgid_file_handle
+{
+  file_handle *as_file_handle_ptr()
+  {
+    return reinterpret_cast<file_handle*>(this);
+  }
+
+  unsigned int handle_bytes = sizeof(std::uint64_t);
+  int handle_type;
+  std::uint64_t cgid;
+};
+
+ACTH_ASSERT_SAME_SIZE(cgid_file_handle, file_handle, std::uint64_t);
+ACTH_ASSERT_SAME_MEMBER(cgid_file_handle, handle_bytes, file_handle, handle_bytes);
+ACTH_ASSERT_SAME_MEMBER(cgid_file_handle, handle_type, file_handle, handle_type);
+ACTH_ASSERT_SAME_OFFSET(cgid_file_handle, cgid, file_handle, f_handle);
+
+}
+
+namespace bpftrace_linux
+{
+
+std::uint64_t resolve_cgroupid(const std::string &path)
+{
+  cgid_file_handle cfh;
+  int mount_id;
+  auto err = name_to_handle_at(AT_FDCWD, path.c_str(), cfh.as_file_handle_ptr(), &mount_id, 0);
+  if (err < 0) {
+    throw std::runtime_error("cgroup '" + path + "' not found");
+  }
+
+  return cfh.cgid;
+}
+
+}

src/resolve_cgroupid.h

+#pragma once
+
+#include <cstdint>
+#include <string>
+
+namespace bpftrace_linux
+{
+
+std::uint64_t resolve_cgroupid(const std::string &path);
+
+}

tests/CMakeLists.txt

@@ -20,6 +20,7 @@ add_executable(bpftrace_test
   ${CMAKE_SOURCE_DIR}/src/map.cpp
   ${CMAKE_SOURCE_DIR}/src/mapkey.cpp
   ${CMAKE_SOURCE_DIR}/src/printf.cpp
+  ${CMAKE_SOURCE_DIR}/src/resolve_cgroupid.cpp
   ${CMAKE_SOURCE_DIR}/src/tracepoint_format_parser.cpp
   ${CMAKE_SOURCE_DIR}/src/types.cpp
 )