liblttng-ust/lttng-context-vsgid.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 saved set-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_vsgid = INVALID_GID;
  39
  40 static
  41 gid_t get_vsgid(void)
  42 {
  43         gid_t vsgid;
  44
  45         vsgid = CMM_LOAD_SHARED(cached_vsgid);
  46
  47         if (caa_unlikely(vsgid == INVALID_GID)) {
  48                 gid_t gid, egid, sgid;
  49
  50                 if (getresgid(&gid, &egid, &sgid) == 0) {
  51                         vsgid = sgid;
  52                         CMM_STORE_SHARED(cached_vsgid, vsgid);
  53                 }
  54         }
  55
  56         return vsgid;
  57 }
  58
  59 /*
  60  * The vsgid can change on setuid, setreuid and setresuid.
  61  */
  62 void lttng_context_vsgid_reset(void)
  63 {
  64         CMM_STORE_SHARED(cached_vsgid, INVALID_GID);
  65 }
  66
  67 static
  68 size_t vsgid_get_size(struct lttng_ust_ctx_field *field, size_t offset)
  69 {
  70         size_t size = 0;
  71
  72         size += lib_ring_buffer_align(offset, lttng_alignof(gid_t));
  73         size += sizeof(gid_t);
  74         return size;
  75 }
  76
  77 static
  78 void vsgid_record(struct lttng_ust_ctx_field *field,
  79                  struct lttng_ust_lib_ring_buffer_ctx *ctx,
  80                  struct lttng_channel *chan)
  81 {
  82         gid_t vsgid;
  83
  84         vsgid = get_vsgid();
  85         lib_ring_buffer_align_ctx(ctx, lttng_alignof(vsgid));
  86         chan->ops->event_write(ctx, &vsgid, sizeof(vsgid));
  87 }
  88
  89 static
  90 void vsgid_get_value(struct lttng_ust_ctx_field *field,
  91                 struct lttng_ust_ctx_value *value)
  92 {
  93         value->u.s64 = get_vsgid();
  94 }
  95
  96 int lttng_add_vsgid_to_ctx(struct lttng_ust_ctx **ctx)
  97 {
  98         struct lttng_ust_ctx_field *field;
  99         struct lttng_ust_type_common *type;
 100         int ret;
 101
 102         type = lttng_ust_create_type_integer(sizeof(gid_t) * CHAR_BIT,
 103                         lttng_alignof(gid_t) * CHAR_BIT,
 104                         lttng_is_signed_type(gid_t),
 105                         BYTE_ORDER, 10);
 106         if (!type)
 107                 return -ENOMEM;
 108         field = lttng_append_context(ctx);
 109         if (!field) {
 110                 ret = -ENOMEM;
 111                 goto error_context;
 112         }
 113         if (lttng_find_context(*ctx, "vsgid")) {
 114                 ret = -EEXIST;
 115                 goto error_find_context;
 116         }
 117         field->event_field->name = strdup("vsgid");
 118         if (!field->event_field->name) {
 119                 ret = -ENOMEM;
 120                 goto error_name;
 121         }
 122         field->event_field->type = type;
 123         field->get_size = vsgid_get_size;
 124         field->record = vsgid_record;
 125         field->get_value = vsgid_get_value;
 126         lttng_context_update(*ctx);
 127         return 0;
 128
 129 error_name:
 130 error_find_context:
 131         lttng_remove_context_field(ctx, field);
 132 error_context:
 133         lttng_ust_destroy_type(type);
 134         return ret;
 135 }