liblttng-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 <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-config.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 uid_t cached_vuid = INVALID_UID;
  39
  40 static
  41 uid_t get_vuid(void)
  42 {
  43         uid_t vuid;
  44
  45         vuid = CMM_LOAD_SHARED(cached_vuid);
  46
  47         if (caa_unlikely(vuid == INVALID_UID)) {
  48                 vuid = getuid();
  49                 CMM_STORE_SHARED(cached_vuid, vuid);
  50         }
  51
  52         return vuid;
  53 }
  54
  55 /*
  56  * The vuid can change on setuid, setreuid and setresuid.
  57  */
  58 void lttng_context_vuid_reset(void)
  59 {
  60         CMM_STORE_SHARED(cached_vuid, INVALID_UID);
  61 }
  62
  63 static
  64 size_t vuid_get_size(struct lttng_ctx_field *field, size_t offset)
  65 {
  66         size_t size = 0;
  67
  68         size += lib_ring_buffer_align(offset, lttng_alignof(uid_t));
  69         size += sizeof(uid_t);
  70         return size;
  71 }
  72
  73 static
  74 void vuid_record(struct lttng_ctx_field *field,
  75                  struct lttng_ust_lib_ring_buffer_ctx *ctx,
  76                  struct lttng_channel *chan)
  77 {
  78         uid_t vuid;
  79
  80         vuid = get_vuid();
  81         lib_ring_buffer_align_ctx(ctx, lttng_alignof(vuid));
  82         chan->ops->event_write(ctx, &vuid, sizeof(vuid));
  83 }
  84
  85 static
  86 void vuid_get_value(struct lttng_ctx_field *field,
  87                 struct lttng_ctx_value *value)
  88 {
  89         value->u.s64 = get_vuid();
  90 }
  91
  92 int lttng_add_vuid_to_ctx(struct lttng_ctx **ctx)
  93 {
  94         struct lttng_ctx_field *field;
  95
  96         field = lttng_append_context(ctx);
  97         if (!field)
  98                 return -ENOMEM;
  99         if (lttng_find_context(*ctx, "vuid")) {
 100                 lttng_remove_context_field(ctx, field);
 101                 return -EEXIST;
 102         }
 103         field->event_field.name = "vuid";
 104         field->event_field.type.atype = atype_integer;
 105         field->event_field.type.u.integer.size = sizeof(uid_t) * CHAR_BIT;
 106         field->event_field.type.u.integer.alignment = lttng_alignof(uid_t) * CHAR_BIT;
 107         field->event_field.type.u.integer.signedness = lttng_is_signed_type(uid_t);
 108         field->event_field.type.u.integer.reverse_byte_order = 0;
 109         field->event_field.type.u.integer.base = 10;
 110         field->event_field.type.u.integer.encoding = lttng_encode_none;
 111         field->get_size = vuid_get_size;
 112         field->record = vuid_record;
 113         field->get_value = vuid_get_value;
 114         lttng_context_update(*ctx);
 115         return 0;
 116 }