Re: [bug-gnulib] Mapping text files

[Bruno Haible]

Bruce Korb wrote:
>   txtmi->txt_data = mmap (NULL, txtmi->txt_size, prot, flags, txtmi->txt_fd, 
> 0);
...
>     /*
>      *  Still here?  We have to append a page of NUL's
>      */
>     txtmi->txt_full_size += pgsz;
>     {
>       void* pNuls;
> 
> #ifdef MAP_ANONYMOUS
>       pNuls = mmap ((void*)(((char*)txtmi->txt_data) + txtmi->txt_size), pgsz,
>                     PROT_READ, MAP_SHARED|MAP_ANONYMOUS|MAP_FIXED, 0, 0);
> #else
>       txtmi->txt_zero_fd = open ("/dev/zero", O_RDONLY);
>       if (txtmi->txt_zero_fd < 0)
>         pNuls = MAP_FAILED;
> 
>       else pNuls =
>              mmap ((void*)(((char*)txtmi->txt_data) + txtmi->txt_size), pgsz,
>                    PROT_READ, MAP_PRIVATE|MAP_FIXED, txtmi->txt_zero_fd, 0);
> #endif
> 
>       /*
>        *  If we couldn't map the page of NULs, then the caller can
>        *  figure this out by checking that full_size == size and
>        *  the txt_errno is not zero.
>        */
>       if (pNuls == MAP_FAILED) {
>         txtmi->txt_errno = errno;
>         txtmi->txt_full_size = txtmi->txt_size;
>       }

This is not reliable. You have no guarantee that a page will be available
after the area covered by the first mmap. If you're unlucky, the second
mmap will supersede a block allocated by malloc or the text segment of a
shared library.

To do it correctly, you need
  - first, an anonymous mmap of the entire address range at once,
  - then, mmap the file with MAP_FIXED to the first portion of this range.

Bruno