CLAM-1531: Fix clamdscan directory scan on Windows

[kang-grace]

Cause: _wopen on Windows doesn't work on directories and gives a Permission Denied error.
Old approach used _wopen to get a file descriptor and gets the
realpath from that. New approach redefines realpath() to _fullpath
on Windows, removing the use of _wopen in clamdscan on directories.

_fullpath: https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/fullpath-wfullpath?view=msvc-160

[micahsnyder]

This fixes the error, but unfortunately _fullpath() only converts relative paths to absolute paths and it doesn't evaluate symlinks.

I spent some time on this today trying to figure out a different solution and found that we could instead replace safe_open() with a call to CreateFileA(), and then convert the handle to a file descriptor before passing it to cli_get_filepath_from_filedesc(). Like this:

    hFile = CreateFileA(file_name,                  // file to open
                        GENERIC_READ,               // open for reading
                        FILE_SHARE_READ,            // share for reading
                        NULL,                       // default security
                        OPEN_EXISTING,              // existing file only
                        FILE_FLAG_BACKUP_SEMANTICS, // may be a directory
                        NULL);                      // no attr. template
    if (hFile == INVALID_HANDLE_VALUE) {
        cli_warnmsg("Can't open file %s: %d\n", file_name, GetLastError());
        status = CL_EOPEN;
        goto done;
    }

    if (-1 == (desc = _open_osfhandle((intptr_t)hFile, O_RDONLY))) {
        cli_warnmsg("Can't convert handle to file descriptor.\n");
        status = CL_EOPEN;
        goto done;
    }
    hFile = INVALID_HANDLE_VALUE; // _open_osfhandle() gives up ownership of the handle to desc.

    status = cli_get_filepath_from_filedesc(desc, &real_file_path);

This works pretty well, though it will still print errors on some devices like RAM disks (see the commit message 3a7a20c for details). That's not so bad though. It's a pretty uncommon situation.

But it's a little bit silly because cli_get_filepath_from_filedesc() will simply convert the file descriptor back to a handle.

I think ideally we could refactor cli_get_filepath_from_filedesc() into two functions, adding one called cli_get_filepath_from_handle(). We could call that one directly from cli_realpath() and then not have to do the switch to an fd & then back to a handle.

@kang-grace would you up for giving that a try?

[micahsnyder]

Please declare this at the top of the function, replacing desc and initialize it to INVALID_HANDLE_VALUE. Then we'll need to clean it up with CloseHandle() at the end after done: instead of desc, checking first if (INVALID_HANDLE_VALUE != hFile) {...}.

[micahsnyder]

Please add a prototype to others.h so it can be used elsewhere in libclamav.

[micahsnyder]

Perfect! Also, it's working really nicely.

[micahsnyder]

I also cherry-picked the fix into the rel/0.104 branch