src/probes/lttng-probe-user.c

   1 /* SPDX-License-Identifier: (GPL-2.0-only or LGPL-2.1-only)
   2  *
   3  * lttng-probe-user.c
   4  *
   5  * Copyright (C) 2012 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
   6  */
   7
   8 #include <linux/uaccess.h>
   9 #include <linux/module.h>
  10 #include <wrapper/uaccess.h>
  11 #include <lttng/probe-user.h>
  12
  13 #define LTTNG_MAX_USER_STRING_LEN 1048576 /* 1MB */
  14
  15 /*
  16  * Calculate string length. Include final null terminating character if there is
  17  * one, or ends at first fault. Disabling page faults ensures that we can safely
  18  * call this from pretty much any context, including those where the caller
  19  * holds mmap_sem, or any lock which nests in mmap_sem.
  20  */
  21 long lttng_strlen_user_inatomic(const char *addr)
  22 {
  23         long count = 0;
  24         mm_segment_t old_fs;
  25
  26         if (!addr)
  27                 return 0;
  28
  29         old_fs = get_fs();
  30         set_fs(KERNEL_DS);
  31         pagefault_disable();
  32         for (;;) {
  33                 char v;
  34                 unsigned long ret;
  35
  36                 if (unlikely(!lttng_access_ok(VERIFY_READ,
  37                                 (__force const char __user *) addr,
  38                                 sizeof(v))))
  39                         break;
  40                 ret = __copy_from_user_inatomic(&v,
  41                         (__force const char __user *)(addr),
  42                         sizeof(v));
  43                 if (unlikely(ret > 0))
  44                         break;
  45                 count++;
  46                 if (unlikely(count > LTTNG_MAX_USER_STRING_LEN))
  47                         break;
  48                 if (unlikely(!v))
  49                         break;
  50                 addr++;
  51         }
  52         pagefault_enable();
  53         set_fs(old_fs);
  54         return count;
  55 }
  56 EXPORT_SYMBOL_GPL(lttng_strlen_user_inatomic);