--- /dev/null
+/*
+ * SPDX-License-Identifier: LGPL-2.1-only
+ *
+ * Copyright (C) 2009-2012 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
+ * Copyright (C) 2019 Michael Jeanson <mjeanson@efficios.com>
+ *
+ * LTTng UST namespaced real group ID context.
+ */
+
+#define _LGPL_SOURCE
+#include <limits.h>
+#include <stddef.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <unistd.h>
+#include <lttng/ust-events.h>
+#include <lttng/ust-tracer.h>
+#include <lttng/ringbuffer-context.h>
+
+#include "context-internal.h"
+#include "creds.h"
+
+
+/*
+ * At the kernel level, user IDs and group IDs are a per-thread attribute.
+ * However, POSIX requires that all threads in a process share the same
+ * credentials. The NPTL threading implementation handles the POSIX
+ * requirements by providing wrapper functions for the various system calls
+ * that change process UIDs and GIDs. These wrapper functions (including those
+ * for setreuid() and setregid()) employ a signal-based technique to ensure
+ * that when one thread changes credentials, all of the other threads in the
+ * process also change their credentials.
+ */
+
+/*
+ * We cache the result to ensure we don't trigger a system call for
+ * each event. User / group IDs are global to the process.
+ */
+static gid_t cached_vgid = INVALID_GID;
+
+static
+gid_t get_vgid(void)
+{
+ gid_t vgid;
+
+ vgid = CMM_LOAD_SHARED(cached_vgid);
+
+ if (caa_unlikely(cached_vgid == (gid_t) -1)) {
+ vgid = getgid();
+ CMM_STORE_SHARED(cached_vgid, vgid);
+ }
+
+ return vgid;
+}
+
+/*
+ * The vgid can change on setuid, setreuid and setresuid.
+ */
+void lttng_context_vgid_reset(void)
+{
+ CMM_STORE_SHARED(cached_vgid, INVALID_GID);
+}
+
+static
+size_t vgid_get_size(void *priv __attribute__((unused)),
+ size_t offset)
+{
+ size_t size = 0;
+
+ size += lttng_ust_lib_ring_buffer_align(offset, lttng_ust_rb_alignof(gid_t));
+ size += sizeof(gid_t);
+ return size;
+}
+
+static
+void vgid_record(void *priv __attribute__((unused)),
+ struct lttng_ust_lib_ring_buffer_ctx *ctx,
+ struct lttng_ust_channel_buffer *chan)
+{
+ gid_t vgid;
+
+ vgid = get_vgid();
+ chan->ops->event_write(ctx, &vgid, sizeof(vgid), lttng_ust_rb_alignof(vgid));
+}
+
+static
+void vgid_get_value(void *priv __attribute__((unused)),
+ struct lttng_ust_ctx_value *value)
+{
+ value->u.s64 = get_vgid();
+}
+
+static const struct lttng_ust_ctx_field *ctx_field = lttng_ust_static_ctx_field(
+ lttng_ust_static_event_field("vgid",
+ lttng_ust_static_type_integer(sizeof(gid_t) * CHAR_BIT,
+ lttng_ust_rb_alignof(gid_t) * CHAR_BIT,
+ lttng_ust_is_signed_type(gid_t),
+ BYTE_ORDER, 10),
+ false, false),
+ vgid_get_size,
+ vgid_record,
+ vgid_get_value,
+ NULL, NULL);
+
+int lttng_add_vgid_to_ctx(struct lttng_ust_ctx **ctx)
+{
+ int ret;
+
+ if (lttng_find_context(*ctx, ctx_field->event_field->name)) {
+ ret = -EEXIST;
+ goto error_find_context;
+ }
+ ret = lttng_ust_context_append(ctx, ctx_field);
+ if (ret)
+ return ret;
+ return 0;
+
+error_find_context:
+ return ret;
+}