Re: Crash core dump when parsing krb5_openlog.3

[Ingo Schwarze <schwarze@usta.de>]

Hi Bapt,

Ingo Schwarze wrote on Fri, Oct 13, 2023 at 12:57:37PM +0200:
> Baptiste Daroussin wrote on Fri, Oct 13, 2023 at 09:06:56AM +0200:

>> https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=266882
>> I haven't had time yet to debug, do you actually need more information?

I believe this is fixed by the commit appended below.

I'm also going to update the FreeBSD bug report.

[...]
> Having a quick look at your reproducing input file, i suspect that
> the following input confuses mandoc:
> 
> .\" ouch!
> .ds xx \\*(fP\fR(\fP\\*(lI*\\*(fP
> 
> Notably, you already marked that line with "ouch" in the manual
> page source code, presumably acknowledging that doing such low-level
> gymnastics in a manual page is asking for trouble.  To not handle such
> madness gracefully is still a bug in mandoc though, i do not deny that.
> 
> I suspect that using the extremely special escape sequence "\\"
> inside .ds is confusing mandoc - that sequence is mostly intended
> for being used inside .de.  The pre-parser still sees the "\\" and
> consequently sees no user-defined string replacement escape sequences.
> But when the "xx" string is later used, the "\\*" likely gets resolved
> to "\*", i.e. to a string replacement request, but at the point, the
> pre-parser has already been run so the string does not get replaced
> and makes it through to the formatters, and those cannot handle it.
> Or something like that, i will have to investigate in detail.

That turned out to be true, as far as it goes.

Two aspects are missing from the explanation:

 1. Not only is the "xx" string later used, but it is used
    (1) with delayed expansion (i.e. \\*(xx rather than \*(xx)
    and (2) inside a macro argument.
 2. The macro argument parser already handled delayed expansion.
    What it failed to handle was recursive expansion where both
    the outer and the inner expansion were delayed.

To summarize, as far as i can see, the assertion only triggers
when a manual pages uses delayed expansion recursively inside
a macro argument.

Note that using expansion inside a manual page is already
bad style, even if it is neither recursive nor delayed nor
inside a macro argument...  :-(

Feedback whether the patch works for you is welcome.

Thanks again for the report!
  Ingo


Log Message:
-----------
When parsing a macro argument results in delayed escape sequence 
expansion, re-check for all contained escape sequences whether they 
need delayed expansion, not just for the particular escape sequences
that triggered delayed expansion in the first place.  This is needed
because delayed expansion can result in strings containing nested
escape sequences recursively needing delayed expansion, too.

This fixes an assertion failure in krb5_openlog(3), see:
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=266882

Thanks to Wolfram Schneider <wosch at FreeBSD> for reporting the bug
and to Baptiste Daroussin <bapt at FreeBSD> for forwarding the report.

Modified Files:
--------------
    mandoc:
        mandoc.h
        roff.c

Revision Data
-------------
Index: mandoc.h
===================================================================
RCS file: /home/cvs/mandoc/mandoc/mandoc.h,v
retrieving revision 1.281
retrieving revision 1.282
diff -Lmandoc.h -Lmandoc.h -u -p -r1.281 -r1.282
--- mandoc.h
+++ mandoc.h
@@ -23,7 +23,6 @@
 #define ASCII_NBRZW	 30  /* non-breaking zero-width space */
 #define ASCII_BREAK	 29  /* breakable zero-width space */
 #define ASCII_HYPH	 28  /* breakable hyphen */
-#define ASCII_ESC	 27  /* escape sequence from copy-in processing */
 #define ASCII_TABREF	 26  /* reset tab reference position */
 
 /*
Index: roff.c
===================================================================
RCS file: /home/cvs/mandoc/mandoc/roff.c,v
retrieving revision 1.396
retrieving revision 1.397
diff -Lroff.c -Lroff.c -u -p -r1.396 -r1.397
--- roff.c
+++ roff.c
@@ -1387,7 +1387,7 @@ roff_expand(struct roff *r, struct buf *
 		 */
 
 		if (buf->buf[pos] != ec) {
-			if (ec != ASCII_ESC && buf->buf[pos] == '\\') {
+			if (buf->buf[pos] == '\\') {
 				roff_expand_patch(buf, pos, "\\e", pos + 1);
 				pos++;
 			}
@@ -1632,12 +1632,7 @@ roff_getarg(struct roff *r, char **cpp, 
 				cp++;
 				break;
 			case '\\':
-				/*
-				 * Signal to roff_expand() that an escape
-				 * sequence resulted from copy-in processing
-				 * and needs to be checked or interpolated.
-				 */
-				cp[-pairs] = ASCII_ESC;
+				cp[-pairs] = '\\';
 				newesc = 1;
 				pairs++;
 				cp++;
@@ -1694,7 +1689,7 @@ roff_getarg(struct roff *r, char **cpp, 
 	buf.buf = start;
 	buf.sz = strlen(start) + 1;
 	buf.next = NULL;
-	if (roff_expand(r, &buf, ln, 0, ASCII_ESC) & ROFF_IGN) {
+	if (roff_expand(r, &buf, ln, 0, '\\') & ROFF_IGN) {
 		free(buf.buf);
 		buf.buf = mandoc_strdup("");
 	}
--
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