src/lib/lttng-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 <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 gid_t cached_vsgid = INVALID_GID;
  40
  41 static
  42 gid_t get_vsgid(void)
  43 {
  44         gid_t vsgid;
  45
  46         vsgid = CMM_LOAD_SHARED(cached_vsgid);
  47
  48         if (caa_unlikely(vsgid == INVALID_GID)) {
  49                 gid_t gid, egid, sgid;
  50
  51                 if (getresgid(&gid, &egid, &sgid) == 0) {
  52                         vsgid = sgid;
  53                         CMM_STORE_SHARED(cached_vsgid, vsgid);
  54                 }
  55         }
  56
  57         return vsgid;
  58 }
  59
  60 /*
  61  * The vsgid can change on setuid, setreuid and setresuid.
  62  */
  63 void lttng_context_vsgid_reset(void)
  64 {
  65         CMM_STORE_SHARED(cached_vsgid, INVALID_GID);
  66 }
  67
  68 static
  69 size_t vsgid_get_size(void *priv __attribute__((unused)),
  70                 struct lttng_ust_probe_ctx *probe_ctx __attribute__((unused)),
  71                 size_t offset)
  72 {
  73         size_t size = 0;
  74
  75         size += lttng_ust_ring_buffer_align(offset, lttng_ust_rb_alignof(gid_t));
  76         size += sizeof(gid_t);
  77         return size;
  78 }
  79
  80 static
  81 void vsgid_record(void *priv __attribute__((unused)),
  82                 struct lttng_ust_probe_ctx *probe_ctx __attribute__((unused)),
  83                 struct lttng_ust_ring_buffer_ctx *ctx,
  84                 struct lttng_ust_channel_buffer *chan)
  85 {
  86         gid_t vsgid;
  87
  88         vsgid = get_vsgid();
  89         chan->ops->event_write(ctx, &vsgid, sizeof(vsgid), lttng_ust_rb_alignof(vsgid));
  90 }
  91
  92 static
  93 void vsgid_get_value(void *priv __attribute__((unused)),
  94                 struct lttng_ust_probe_ctx *probe_ctx __attribute__((unused)),
  95                 struct lttng_ust_ctx_value *value)
  96 {
  97         value->u.u64 = get_vsgid();
  98 }
  99
 100 static const struct lttng_ust_ctx_field *ctx_field = lttng_ust_static_ctx_field(
 101         lttng_ust_static_event_field("vsgid",
 102                 lttng_ust_static_type_integer(sizeof(gid_t) * CHAR_BIT,
 103                                 lttng_ust_rb_alignof(gid_t) * CHAR_BIT,
 104                                 lttng_ust_is_signed_type(gid_t),
 105                                 LTTNG_UST_BYTE_ORDER, 10),
 106                 false, false),
 107         vsgid_get_size,
 108         vsgid_record,
 109         vsgid_get_value,
 110         NULL, NULL);
 111
 112 int lttng_add_vsgid_to_ctx(struct lttng_ust_ctx **ctx)
 113 {
 114         int ret;
 115
 116         if (lttng_find_context(*ctx, ctx_field->event_field->name)) {
 117                 ret = -EEXIST;
 118                 goto error_find_context;
 119         }
 120         ret = lttng_ust_context_append(ctx, ctx_field);
 121         if (ret)
 122                 return ret;
 123         return 0;
 124
 125 error_find_context:
 126         return ret;
 127 }