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Re: [libvirt] [PATCH v2 2/5] Instantiate comments in ip(6)tables rules




Eric Blake <eblake redhat com> wrote on 09/27/2010 04:52:31 PM:

>
> On 09/27/2010 12:40 PM, Stefan Berger wrote:
> > V2 changes:
> >   following Eric's comments:
> >    - commands in the script are assigned using cmd='...' rather
> than cmd="..."
> >    - invoke commands using eval res=... rather than res=eval ...
> >    - rewrote function escaping ` and single quotes (which became
> more tricky)
> >
> > In this patch I am extending the rule instantiator to create the comment
> > node where supported, which is the case for iptables and ip6tables.
> >
> > Since commands are written in the format
> >
> > cmd='iptables ...-m comment --comment \"\" '
> >
> > certain characters ('`) in the comment need to be escaped to
> > prevent comments from becoming commands themselves or cause other
> > forms of (bash) substitutions. I have tested this with various input and in
> > my tests the input made it straight into the comment. A test case for TCK
> > will be provided separately that tests this.
>
> At first, I failed to see how ` needs escaping inside '', unless you
> aren't uniformly using '' like you think you are.  Then it hit me - yuck
> - we aren't uniformly using '' - we really do have to escape ` inside
> ``, or come up with an alternate solution.  That is:
>
> ret=`iptables -m comment --comment '`'`
>
> is indeed ill-formed.  But if you are going to escape `, then you also
> have to escape \ and $ for the same duration.  Yuck again.



I had tested these also and it doesn't seem to be the case. I looked like really only ` and ' needed special treatment.


>
> But fear not - I have a slicker solution:
>
> comment='comment with lone '\'', ", `, \, $x, and two  spaces'
> cmd='iptables ...-m comment --comment "$comment"'
> eval ret=\`"$cmd"\`


I changed this now for v3.


>
> which at expansion time results in:
>
> eval ret=\`"$cmd"\`
> ret=`iptables ...-m comment --comment "$comment"`
> iptables ...-m comment --comment \
>   'comment with lone '\'', `, ", `, \, $x, and two  spaces'
>
> That is, rather than trying to pass the comment literally through $cmd
> (and thus having to carefully escape $cmd for its eventual use inside
> ``), it might be nicer to stick the comment in an intermediate variable
> (where you only need to escape ') and make $cmd reference the
> intermediate variable (where you won't have any problematic uses of ",
> `, or ' to contend with, and where your only $ is one where you
> intentionally want variable expansion).



It's nicer, true, just a little more changes need for building the final command where
a 'prefix', here the comment='' is stuck in front of the line with the iptables command.


>
> > @@ -52,10 +53,10 @@
> >
> >
> >  #define CMD_SEPARATOR "\n"
> > -#define CMD_DEF_PRE  "cmd=\""
> > -#define CMD_DEF_POST "\""
> > +#define CMD_DEF_PRE  "cmd='"
> > +#define CMD_DEF_POST "'"
> >  #define CMD_DEF(X) CMD_DEF_PRE X CMD_DEF_POST
> > -#define CMD_EXEC   "res=`${cmd}`" CMD_SEPARATOR
> > +#define CMD_EXEC   "eval res=\\`\"${cmd}\"\\`" CMD_SEPARATOR
>
> This part seems okay - the ` is quoted to protect it from evaluation
> until after eval has had a chance to collect its arguments.
>
> > +static char *
> > +escapeComment(const char *buf)
> > +{
> > +    char *res;
> > +    size_t i, j, add = 0, len = strlen(buf);
> > +
> > +    static const char SINGLEQUOTE_REPLACEMENT[12] = "'\\'\\\"\\'\\\"\\''";
>
> That seems rather long to me.  Broken into chunks with C-string escaping
> eliminated:
>
> '    \'\"\'\"\'    '
> end the current single quoting
>       the 5 literal shell chars '"'"'
>                     resume single quoting
>
> I'm not following why we need 5 literal shell characters, instead of 1.
>

That's because I needed to place the ' in between "". Without it would not work. The first ' was to close the string in front and the final ' was to start another string.

> > +
> > +    if (len > IPTABLES_MAX_COMMENT_SIZE)
> > +        len = IPTABLES_MAX_COMMENT_SIZE;
> > +
> > +    for (i = 0; i < len; i++) {
> > +        if (buf[i] == '`')
> > +            add++;
> > +        else if (buf[i] == '\'')
> > +            add += sizeof(SINGLEQUOTE_REPLACEMENT);
> > +    }
>
> Back to my observation that this doesn't help for \ or $, the other two
> characters that need escaping inside ``.  It would be so much easier if
> we could rely on $() instead of ``.  Wait a minute - this is only done
> for Linux hosts, where we are guaranteed a sane shell (it's pretty much
> just Solaris where /bin/sh is too puny to do $()) - using $() instead of
> `` would solve a lot of escaping issues.
>
> > @@ -993,6 +1034,16 @@ iptablesHandleIpHdr(virBufferPtr buf,
> >          }
> >      }
> >
> > +    if (HAS_ENTRY_ITEM(&ipHdr->dataComment)) {
> > +        char *cmt = escapeComment(ipHdr->dataComment.u.string);
> > +        if (!cmt)
> > +            goto err_exit;
> > +        virBufferVSprintf(buf,
> > +                          " -m comment --comment '\\''%s'\\''",
> > +                          cmt);
> > +        VIR_FREE(cmt);
>
> OK, maybe I see why your comment had such a long replacement for ',
> because you aren't adding any escaping to cmt here.  But I still think
> we can come up with a more elegant solution, by using the intermediate
> variable.  Thanks for forcing me to explain myself - it's an interesting
> process thinking about this.
>
>
> [Do I even dare mention that at an even higher layer, it might be nicer
> to avoid /bin/sh in the first place, and instead put effort into my
> pending virCommand API patches to make it easier to directly invoke all
> the iptables commands from C?]


I do also generate some scripts with 'if .. then' statements that won't be able to executed via your virCommand API...

   Stefan


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