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

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.

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"\`

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).

@@ -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_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.

+    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?]

Eric Blake   eblake redhat com    +1-801-349-2682
Libvirt virtualization library http://libvirt.org

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