Mmap and Genode - bug with MAP_ANON?

[Alexander Tormasov]

I want to add that addr below is non-0, like 0xc000000000 - for Linux and for my test compiled on Genode.
So, Anon mapping does  not require address to be 0, and as I understand code in file_operations.cc<http://file_operations.cc> below should be fixed using MAP_ANONYMOUS flag?
Or you think that this is correct check for addr==0 and I need to fix something else?

Отправлено с iPhone

1 сент. 2019 г., в 3:53, Alexander Tormasov via users <users at lists.genode.org<mailto:users at lists.genode.org>> написал(а):

I try to run a program which use the following hidden from me code:
It  call function to reserve some mem area using the following:

func sysReserve(v unsafe.Pointer, n uintptr, reserved *bool) unsafe.Pointer {
   // On 64-bit, people with ulimit -v set complain if we reserve too
   // much address space. Instead, assume that the reservation is okay
   // if we can reserve at least 64K and check the assumption in SysMap.
   // Only user-mode Linux (UML) rejects these requests.
   if sys.PtrSize == 8 && uint64(n) > 1<<32 {
       p, err := mmap_fixed(v, 64<<10, _PROT_NONE, _MAP_ANON|_MAP_PRIVATE, mmapFD, 0)
       if p != v || err != 0 {
           if err == 0 {
               munmap(p, 64<<10)
           }
           return nil
       }
       munmap(p, 64<<10)
       *reserved = false
       return v
   }
In Man page related to mmap I found that this combination of flags ignore FD:
MAP_PRIVATE
             Create a private copy-on-write mapping.  Updates to the
             mapping are not visible to other processes mapping the same
             file, and are not carried through to the underlying file.  It
             is unspecified whether changes made to the file after the
             mmap() call are visible in the mapped region.

MAP_ANONYMOUS
             The mapping is not backed by any file; its contents are
             initialized to zero.  The fd argument is ignored; however,
             some implementations require fd to be -1 if MAP_ANONYMOUS (or
             MAP_ANON) is specified, and portable applications should
             ensure this.  The offset argument should be zero.  The use of
             MAP_ANONYMOUS in conjunction with MAP_SHARED is supported on
             Linux only since kernel 2.4.

The code in Genode mmap wrapper try to find anyway the mapping for fd descriptor (-1 in my case) and fail because of
libc_fd_to_fd(libc_fd, "mmap");:

__SYS_(void *, mmap, (void *addr, ::size_t length,
                     int prot, int flags,
                     int libc_fd, ::off_t offset),
{

   /* handle requests for anonymous memory */
   if (!addr && libc_fd == -1) {
       bool const executable = prot & PROT_EXEC;
       void *start = Libc::mem_alloc(executable)->alloc(length, PAGE_SHIFT);
       if (!start) {
           errno = ENOMEM;
           return MAP_FAILED;
       }
       mmap_registry()->insert(start, length, 0);
       return start;
   }

   /* lookup plugin responsible for file descriptor */
   File_descriptor *fd = libc_fd_to_fd(libc_fd, "mmap");
   if (!fd || !fd->plugin || !fd->plugin->supports_mmap()) {
       Genode::warning("mmap not supported for file descriptor ", libc_fd);
       errno = EBADF;
       return MAP_FAILED;
   }

   void *start = fd->plugin->mmap(addr, length, prot, flags, fd, offset);
   mmap_registry()->insert(start, length, fd->plugin);
   return start;
})

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