liblttng-ust/lttng-context-vgid.c

   1 /*
   2  * SPDX-License-Identifier: LGPL-2.1-only
   3  *
   4  * Copyright (C) 2009-2012 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
   5  * Copyright (C) 2019 Michael Jeanson <mjeanson@efficios.com>
   6  *
   7  * LTTng UST namespaced real group ID context.
   8  */
   9
  10 #define _LGPL_SOURCE
  11 #include <stddef.h>
  12 #include <sys/types.h>
  13 #include <sys/stat.h>
  14 #include <unistd.h>
  15 #include <lttng/ust-events.h>
  16 #include <lttng/ust-tracer.h>
  17 #include <lttng/ringbuffer-context.h>
  18
  19 #include "context-internal.h"
  20 #include "creds.h"
  21
  22
  23 /*
  24  * At the kernel level, user IDs and group IDs are a per-thread attribute.
  25  * However, POSIX requires that all threads in a process share the same
  26  * credentials. The NPTL threading implementation handles the POSIX
  27  * requirements by providing wrapper functions for the various system calls
  28  * that change process UIDs and GIDs. These wrapper functions (including those
  29  * for setreuid() and setregid()) employ a signal-based technique to ensure
  30  * that when one thread changes credentials, all of the other threads in the
  31  * process also change their credentials.
  32  */
  33
  34 /*
  35  * We cache the result to ensure we don't trigger a system call for
  36  * each event. User / group IDs are global to the process.
  37  */
  38 static gid_t cached_vgid = INVALID_GID;
  39
  40 static
  41 gid_t get_vgid(void)
  42 {
  43         gid_t vgid;
  44
  45         vgid = CMM_LOAD_SHARED(cached_vgid);
  46
  47         if (caa_unlikely(cached_vgid == (gid_t) -1)) {
  48                 vgid = getgid();
  49                 CMM_STORE_SHARED(cached_vgid, vgid);
  50         }
  51
  52         return vgid;
  53 }
  54
  55 /*
  56  * The vgid can change on setuid, setreuid and setresuid.
  57  */
  58 void lttng_context_vgid_reset(void)
  59 {
  60         CMM_STORE_SHARED(cached_vgid, INVALID_GID);
  61 }
  62
  63 static
  64 size_t vgid_get_size(struct lttng_ust_ctx_field *field, size_t offset)
  65 {
  66         size_t size = 0;
  67
  68         size += lib_ring_buffer_align(offset, lttng_alignof(gid_t));
  69         size += sizeof(gid_t);
  70         return size;
  71 }
  72
  73 static
  74 void vgid_record(struct lttng_ust_ctx_field *field,
  75                  struct lttng_ust_lib_ring_buffer_ctx *ctx,
  76                  struct lttng_ust_channel_buffer *chan)
  77 {
  78         gid_t vgid;
  79
  80         vgid = get_vgid();
  81         lib_ring_buffer_align_ctx(ctx, lttng_alignof(vgid));
  82         chan->ops->event_write(ctx, &vgid, sizeof(vgid));
  83 }
  84
  85 static
  86 void vgid_get_value(struct lttng_ust_ctx_field *field,
  87                 struct lttng_ust_ctx_value *value)
  88 {
  89         value->u.s64 = get_vgid();
  90 }
  91
  92 int lttng_add_vgid_to_ctx(struct lttng_ust_ctx **ctx)
  93 {
  94         struct lttng_ust_ctx_field *field;
  95         struct lttng_ust_type_common *type;
  96         int ret;
  97
  98         type = lttng_ust_create_type_integer(sizeof(gid_t) * CHAR_BIT,
  99                         lttng_alignof(gid_t) * CHAR_BIT,
 100                         lttng_is_signed_type(gid_t),
 101                         BYTE_ORDER, 10);
 102         if (!type)
 103                 return -ENOMEM;
 104         field = lttng_append_context(ctx);
 105         if (!field) {
 106                 ret = -ENOMEM;
 107                 goto error_context;
 108         }
 109         if (lttng_find_context(*ctx, "vgid")) {
 110                 ret = -EEXIST;
 111                 goto error_find_context;
 112         }
 113         field->event_field->name = strdup("vgid");
 114         if (!field->event_field->name) {
 115                 ret = -ENOMEM;
 116                 goto error_name;
 117         }
 118         field->event_field->type = type;
 119         field->get_size = vgid_get_size;
 120         field->record = vgid_record;
 121         field->get_value = vgid_get_value;
 122         lttng_context_update(*ctx);
 123         return 0;
 124
 125 error_name:
 126 error_find_context:
 127         lttng_remove_context_field(ctx, field);
 128 error_context:
 129         lttng_ust_destroy_type(type);
 130         return ret;
 131 }