[Dnsmasq-discuss] Extension to hosts-dir and dhcohosts-dir

[Dominik Derigs]

Hey Andy (and list),

According to the inotify man page, the name buffer will always be
null-terminated. Furthermore, the name buffer seems to be allocated in
chunks of 16 bytes. I have not found an official confirmation for that.

I the way to go would be: char lastchar = in->name[strlen(in->name)-1];

I have extended your test code by the following two lines:

if(in->name == NULL) continue;
printf("DD: strlen: %lu\n", strlen(in->name));
char lastchar = in->name[strlen(in->name)-1];
printf("DD: strlen last char: %c\n", lastchar);

Here are results obtained with your + my debug output:

For a short filename without ~

ADH: len: 16
ADH: name: shortfilename
ADH: last char:
DD: strlen: 13
DD: strlen last char: e

For a short filename with ~

ADH: len: 16
ADH: name: shortfilename~
ADH: last char:
DD: strlen: 14
DD: strlen last char: ~

For a long filename without ~

ADH: len: 32
ADH: name: veryverylongfilename
ADH: last char:
DD: strlen: 20
DD: strlen last char: e

For a long filename with ~

ADH: len: 32
ADH: name: veryverylongfilename~
ADH: last char:
DD: strlen: 21
DD: strlen last char: ~

Best regards,
Dominik


On 11.02.2018 12:57, Andy Hawkins wrote:
> Hi,
>
> In article <slrnp80aau.nkj.andy at xcp-mailnews.gently.org.uk>,
>            Andy Hawkins<andy at gently.org.uk> wrote:
>> In inotify.c, around line 236 is the following code block:
>>
>> 	  /* ignore emacs backups and dotfiles */
>> 	  if (in->len == 0 || 
>> 	      in->name[in->len - 1] == '~' ||
>> 	      (in->name[0] == '#' && in->name[in->len - 1] == '#') ||
>> 	      in->name[0] == '.')
>> 	    continue;
>> 	  
>> However, if I create a file called 'fred~' in the directory I've specified
>> using dhcp-hostsdir I still get an event in syslog that shows this file is
>> being processed:
>>
>> Feb 11 11:14:34 xcp-gateway dnsmasq[1039]: inotify, new or changed file 
>> 	/etc/dnsmasq/dhcp-hosts.d/fred~
> Ok, I've done some debugging. I added the following lines:
>
>           my_syslog(LOG_INFO, "ADH: len: %d", in->len);
>           my_syslog(LOG_INFO, "ADH: name: %s", in->name);
>           my_syslog(LOG_INFO, "ADH: last char: %c", in->name[in->len - 1]);
>
> And I get the following output:
>
> dnsmasq: ADH: len: 16
> dnsmasq: ADH: name: fred4~
> dnsmasq: ADH: last char:
>
> So, it appears that the length in in->len is being interpreted correctly.
>
> According to the inotify man page:
>
>        The len field counts all of the bytes in name, including the null
>        bytes; the length of each inotify_event structure is thus
>        sizeof(struct inotify_event)+len.
>
> So in fact, 'len' seems to be a fixed length, irrespective of the length of
> the file name in the 'name' field.
>
> It looks like the check should actually be something like:
>
>           /* ignore emacs backups and dotfiles */
>           if (in->len == 0 ||
>               in->name[strlen(in->name) - 1] == '~' ||
>               (in->name[0] == '#' && in->name[strlen(in->name) - 1] == '#') ||
>               in->name[0] == '.')
>
> I guess you may need to check that there's a null in the name somewhere
> before using strlen, otherwise you might end up running off the end of the
> string. I don't know inotify well enough to know if there's guaranteed to be
> a null in there somewhere. The manpage does say that the name field is null
> terminated, but I don't know if that's guaranteed or not.
>
> I could have a look at submitting a patch, but my editor is showing some
> very strange indentation of the source, so I suspect I have my tab settings
> incorrect. What is the standard setting for tabs on the dnasmasq source
> files?
>
> Thanks
>
> Andy
>
>
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