libc/src/sys/mman/linux/mmap.cpp

   1 //===---------- Linux implementation of the POSIX mmap function -----------===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8
   9 #include "src/sys/mman/mmap.h"
  10
  11 #include "src/__support/OSUtil/syscall.h" // For internal syscall function.
  12 #include "src/__support/common.h"
  13
  14 #include "src/errno/libc_errno.h"
  15 #include <linux/param.h> // For EXEC_PAGESIZE.
  16 #include <sys/syscall.h> // For syscall numbers.
  17
  18 namespace LIBC_NAMESPACE {
  19
  20 // This function is currently linux only. It has to be refactored suitably if
  21 // mmap is to be supported on non-linux operating systems also.
  22 LLVM_LIBC_FUNCTION(void *, mmap,
  23                    (void *addr, size_t size, int prot, int flags, int fd,
  24                     off_t offset)) {
  25   // A lot of POSIX standard prescribed validation of the parameters is not
  26   // done in this function as modern linux versions do it in the syscall.
  27   // TODO: Perform argument validation not done by the linux syscall.
  28
  29   // EXEC_PAGESIZE is used for the page size. While this is OK for x86_64, it
  30   // might not be correct in general.
  31   // TODO: Use pagesize read from the ELF aux vector instead of EXEC_PAGESIZE.
  32
  33 #ifdef SYS_mmap2
  34   offset /= EXEC_PAGESIZE;
  35   long syscall_number = SYS_mmap2;
  36 #elif defined(SYS_mmap)
  37   long syscall_number = SYS_mmap;
  38 #else
  39 #error "mmap or mmap2 syscalls not available."
  40 #endif
  41
  42   long ret =
  43       LIBC_NAMESPACE::syscall_impl(syscall_number, reinterpret_cast<long>(addr),
  44                                    size, prot, flags, fd, offset);
  45
  46   // The mmap/mmap2 syscalls return negative values on error. These negative
  47   // values are actually the negative values of the error codes. So, fix them
  48   // up in case an error code is detected.
  49   //
  50   // A point to keep in mind for the fix up is that a negative return value
  51   // from the syscall can also be an error-free value returned by the syscall.
  52   // However, since a valid return address cannot be within the last page, a
  53   // return value corresponding to a location in the last page is an error
  54   // value.
  55   if (ret < 0 && ret > -EXEC_PAGESIZE) {
  56     libc_errno = static_cast<int>(-ret);
  57     return MAP_FAILED;
  58   }
  59
  60   return reinterpret_cast<void *>(ret);
  61 }
  62
  63 } // namespace LIBC_NAMESPACE