src/lib/lttng-ust/lttng-context-vuid.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 user 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 uid_t cached_vuid = INVALID_UID;
  40
  41 static
  42 uid_t get_vuid(void)
  43 {
  44         uid_t vuid;
  45
  46         vuid = CMM_LOAD_SHARED(cached_vuid);
  47
  48         if (caa_unlikely(vuid == INVALID_UID)) {
  49                 vuid = getuid();
  50                 CMM_STORE_SHARED(cached_vuid, vuid);
  51         }
  52
  53         return vuid;
  54 }
  55
  56 /*
  57  * The vuid can change on setuid, setreuid and setresuid.
  58  */
  59 void lttng_context_vuid_reset(void)
  60 {
  61         CMM_STORE_SHARED(cached_vuid, INVALID_UID);
  62 }
  63
  64 static
  65 size_t vuid_get_size(void *priv __attribute__((unused)),
  66                 struct lttng_ust_probe_ctx *probe_ctx __attribute__((unused)),
  67                 size_t offset)
  68 {
  69         size_t size = 0;
  70
  71         size += lttng_ust_ring_buffer_align(offset, lttng_ust_rb_alignof(uid_t));
  72         size += sizeof(uid_t);
  73         return size;
  74 }
  75
  76 static
  77 void vuid_record(void *priv __attribute__((unused)),
  78                 struct lttng_ust_probe_ctx *probe_ctx __attribute__((unused)),
  79                 struct lttng_ust_ring_buffer_ctx *ctx,
  80                 struct lttng_ust_channel_buffer *chan)
  81 {
  82         uid_t vuid;
  83
  84         vuid = get_vuid();
  85         chan->ops->event_write(ctx, &vuid, sizeof(vuid), lttng_ust_rb_alignof(vuid));
  86 }
  87
  88 static
  89 void vuid_get_value(void *priv __attribute__((unused)),
  90                 struct lttng_ust_probe_ctx *probe_ctx __attribute__((unused)),
  91                 struct lttng_ust_ctx_value *value)
  92 {
  93         value->u.u64 = get_vuid();
  94 }
  95
  96 static const struct lttng_ust_ctx_field *ctx_field = lttng_ust_static_ctx_field(
  97         lttng_ust_static_event_field("vuid",
  98                 lttng_ust_static_type_integer(sizeof(uid_t) * CHAR_BIT,
  99                                 lttng_ust_rb_alignof(uid_t) * CHAR_BIT,
 100                                 lttng_ust_is_signed_type(uid_t),
 101                                 LTTNG_UST_BYTE_ORDER, 10),
 102                 false, false),
 103         vuid_get_size,
 104         vuid_record,
 105         vuid_get_value,
 106         NULL, NULL);
 107
 108 int lttng_add_vuid_to_ctx(struct lttng_ust_ctx **ctx)
 109 {
 110         int ret;
 111
 112         if (lttng_find_context(*ctx, ctx_field->event_field->name)) {
 113                 ret = -EEXIST;
 114                 goto error_find_context;
 115         }
 116         ret = lttng_ust_context_append(ctx, ctx_field);
 117         if (ret)
 118                 return ret;
 119         return 0;
 120
 121 error_find_context:
 122         return ret;
 123 }