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-config.h>
  18 #include "creds.h"
  19
  20
  21 /*
  22  * At the kernel level, user IDs and group IDs are a per-thread attribute.
  23  * However, POSIX requires that all threads in a process share the same
  24  * credentials. The NPTL threading implementation handles the POSIX
  25  * requirements by providing wrapper functions for the various system calls
  26  * that change process UIDs and GIDs. These wrapper functions (including those
  27  * for setreuid() and setregid()) employ a signal-based technique to ensure
  28  * that when one thread changes credentials, all of the other threads in the
  29  * process also change their credentials.
  30  */
  31
  32 /*
  33  * We cache the result to ensure we don't trigger a system call for
  34  * each event. User / group IDs are global to the process.
  35  */
  36 static gid_t cached_vsgid = INVALID_GID;
  37
  38 static
  39 gid_t get_vsgid(void)
  40 {
  41         gid_t vsgid;
  42
  43         vsgid = CMM_LOAD_SHARED(cached_vsgid);
  44
  45         if (caa_unlikely(vsgid == INVALID_GID)) {
  46                 gid_t gid, egid, sgid;
  47
  48                 if (getresgid(&gid, &egid, &sgid) == 0) {
  49                         vsgid = sgid;
  50                         CMM_STORE_SHARED(cached_vsgid, vsgid);
  51                 }
  52         }
  53
  54         return vsgid;
  55 }
  56
  57 /*
  58  * The vsgid can change on setuid, setreuid and setresuid.
  59  */
  60 void lttng_context_vsgid_reset(void)
  61 {
  62         CMM_STORE_SHARED(cached_vsgid, INVALID_GID);
  63 }
  64
  65 static
  66 size_t vsgid_get_size(struct lttng_ctx_field *field, size_t offset)
  67 {
  68         size_t size = 0;
  69
  70         size += lib_ring_buffer_align(offset, lttng_alignof(gid_t));
  71         size += sizeof(gid_t);
  72         return size;
  73 }
  74
  75 static
  76 void vsgid_record(struct lttng_ctx_field *field,
  77                  struct lttng_ust_lib_ring_buffer_ctx *ctx,
  78                  struct lttng_channel *chan)
  79 {
  80         gid_t vsgid;
  81
  82         vsgid = get_vsgid();
  83         lib_ring_buffer_align_ctx(ctx, lttng_alignof(vsgid));
  84         chan->ops->event_write(ctx, &vsgid, sizeof(vsgid));
  85 }
  86
  87 static
  88 void vsgid_get_value(struct lttng_ctx_field *field,
  89                 struct lttng_ctx_value *value)
  90 {
  91         value->u.s64 = get_vsgid();
  92 }
  93
  94 int lttng_add_vsgid_to_ctx(struct lttng_ctx **ctx)
  95 {
  96         struct lttng_ctx_field *field;
  97
  98         field = lttng_append_context(ctx);
  99         if (!field)
 100                 return -ENOMEM;
 101         if (lttng_find_context(*ctx, "vsgid")) {
 102                 lttng_remove_context_field(ctx, field);
 103                 return -EEXIST;
 104         }
 105         field->event_field.name = "vsgid";
 106         field->event_field.type.atype = atype_integer;
 107         field->event_field.type.u.integer.size = sizeof(gid_t) * CHAR_BIT;
 108         field->event_field.type.u.integer.alignment = lttng_alignof(gid_t) * CHAR_BIT;
 109         field->event_field.type.u.integer.signedness = lttng_is_signed_type(gid_t);
 110         field->event_field.type.u.integer.reverse_byte_order = 0;
 111         field->event_field.type.u.integer.base = 10;
 112         field->event_field.type.u.integer.encoding = lttng_encode_none;
 113         field->get_size = vsgid_get_size;
 114         field->record = vsgid_record;
 115         field->get_value = vsgid_get_value;
 116         lttng_context_update(*ctx);
 117         return 0;
 118 }