src/lib/lttng-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 <limits.h>
  12 #include <stddef.h>
  13 #include <sys/types.h>
  14 #include <sys/stat.h>
  15 #include <unistd.h>
  16 #include <lttng/ust-events.h>
  17 #include <lttng/ust-tracer.h>
  18 #include <lttng/ust-ringbuffer-context.h>
  19
  20 #include "context-internal.h"
  21 #include "common/creds.h"
  22
  23
  24 /*
  25  * At the kernel level, user IDs and group IDs are a per-thread attribute.
  26  * However, POSIX requires that all threads in a process share the same
  27  * credentials. The NPTL threading implementation handles the POSIX
  28  * requirements by providing wrapper functions for the various system calls
  29  * that change process UIDs and GIDs. These wrapper functions (including those
  30  * for setreuid() and setregid()) employ a signal-based technique to ensure
  31  * that when one thread changes credentials, all of the other threads in the
  32  * process also change their credentials.
  33  */
  34
  35 /*
  36  * We cache the result to ensure we don't trigger a system call for
  37  * each event. User / group IDs are global to the process.
  38  */
  39 static gid_t cached_vgid = INVALID_GID;
  40
  41 static
  42 gid_t get_vgid(void)
  43 {
  44         gid_t vgid;
  45
  46         vgid = CMM_LOAD_SHARED(cached_vgid);
  47
  48         if (caa_unlikely(cached_vgid == (gid_t) -1)) {
  49                 vgid = getgid();
  50                 CMM_STORE_SHARED(cached_vgid, vgid);
  51         }
  52
  53         return vgid;
  54 }
  55
  56 /*
  57  * The vgid can change on setuid, setreuid and setresuid.
  58  */
  59 void lttng_context_vgid_reset(void)
  60 {
  61         CMM_STORE_SHARED(cached_vgid, INVALID_GID);
  62 }
  63
  64 static
  65 size_t vgid_get_size(void *priv __attribute__((unused)),
  66                 size_t offset)
  67 {
  68         size_t size = 0;
  69
  70         size += lttng_ust_ring_buffer_align(offset, lttng_ust_rb_alignof(gid_t));
  71         size += sizeof(gid_t);
  72         return size;
  73 }
  74
  75 static
  76 void vgid_record(void *priv __attribute__((unused)),
  77                  struct lttng_ust_ring_buffer_ctx *ctx,
  78                  struct lttng_ust_channel_buffer *chan)
  79 {
  80         gid_t vgid;
  81
  82         vgid = get_vgid();
  83         chan->ops->event_write(ctx, &vgid, sizeof(vgid), lttng_ust_rb_alignof(vgid));
  84 }
  85
  86 static
  87 void vgid_get_value(void *priv __attribute__((unused)),
  88                 struct lttng_ust_ctx_value *value)
  89 {
  90         value->u.s64 = get_vgid();
  91 }
  92
  93 static const struct lttng_ust_ctx_field *ctx_field = lttng_ust_static_ctx_field(
  94         lttng_ust_static_event_field("vgid",
  95                 lttng_ust_static_type_integer(sizeof(gid_t) * CHAR_BIT,
  96                                 lttng_ust_rb_alignof(gid_t) * CHAR_BIT,
  97                                 lttng_ust_is_signed_type(gid_t),
  98                                 LTTNG_UST_BYTE_ORDER, 10),
  99                 false, false),
 100         vgid_get_size,
 101         vgid_record,
 102         vgid_get_value,
 103         NULL, NULL);
 104
 105 int lttng_add_vgid_to_ctx(struct lttng_ust_ctx **ctx)
 106 {
 107         int ret;
 108
 109         if (lttng_find_context(*ctx, ctx_field->event_field->name)) {
 110                 ret = -EEXIST;
 111                 goto error_find_context;
 112         }
 113         ret = lttng_ust_context_append(ctx, ctx_field);
 114         if (ret)
 115                 return ret;
 116         return 0;
 117
 118 error_find_context:
 119         return ret;
 120 }