liblttng-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 <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_vgid = INVALID_GID;
  37
  38 static
  39 gid_t get_vgid(void)
  40 {
  41         gid_t vgid;
  42
  43         vgid = CMM_LOAD_SHARED(cached_vgid);
  44
  45         if (caa_unlikely(cached_vgid == (gid_t) -1)) {
  46                 vgid = getgid();
  47                 CMM_STORE_SHARED(cached_vgid, vgid);
  48         }
  49
  50         return vgid;
  51 }
  52
  53 /*
  54  * The vgid can change on setuid, setreuid and setresuid.
  55  */
  56 void lttng_context_vgid_reset(void)
  57 {
  58         CMM_STORE_SHARED(cached_vgid, INVALID_GID);
  59 }
  60
  61 static
  62 size_t vgid_get_size(struct lttng_ctx_field *field, size_t offset)
  63 {
  64         size_t size = 0;
  65
  66         size += lib_ring_buffer_align(offset, lttng_alignof(gid_t));
  67         size += sizeof(gid_t);
  68         return size;
  69 }
  70
  71 static
  72 void vgid_record(struct lttng_ctx_field *field,
  73                  struct lttng_ust_lib_ring_buffer_ctx *ctx,
  74                  struct lttng_channel *chan)
  75 {
  76         gid_t vgid;
  77
  78         vgid = get_vgid();
  79         lib_ring_buffer_align_ctx(ctx, lttng_alignof(vgid));
  80         chan->ops->event_write(ctx, &vgid, sizeof(vgid));
  81 }
  82
  83 static
  84 void vgid_get_value(struct lttng_ctx_field *field,
  85                 struct lttng_ctx_value *value)
  86 {
  87         value->u.s64 = get_vgid();
  88 }
  89
  90 int lttng_add_vgid_to_ctx(struct lttng_ctx **ctx)
  91 {
  92         struct lttng_ctx_field *field;
  93
  94         field = lttng_append_context(ctx);
  95         if (!field)
  96                 return -ENOMEM;
  97         if (lttng_find_context(*ctx, "vgid")) {
  98                 lttng_remove_context_field(ctx, field);
  99                 return -EEXIST;
 100         }
 101         field->event_field.name = "vgid";
 102         field->event_field.type.atype = atype_integer;
 103         field->event_field.type.u.integer.size = sizeof(gid_t) * CHAR_BIT;
 104         field->event_field.type.u.integer.alignment = lttng_alignof(gid_t) * CHAR_BIT;
 105         field->event_field.type.u.integer.signedness = lttng_is_signed_type(gid_t);
 106         field->event_field.type.u.integer.reverse_byte_order = 0;
 107         field->event_field.type.u.integer.base = 10;
 108         field->event_field.type.u.integer.encoding = lttng_encode_none;
 109         field->get_size = vgid_get_size;
 110         field->record = vgid_record;
 111         field->get_value = vgid_get_value;
 112         lttng_context_update(*ctx);
 113         return 0;
 114 }