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Subject: Catching format bugs
From: Olaf Kirch (okircaldera.de)
Date: Sat Sep 16 2000 - 09:36:25 CDT

Hi all,

I just wanted to bounce around an idea for catching formatting bugs
with less drastic measures than lobotomizing libc (i.e. removing %n
completely).

There's a very simple way to tell a constant from a non-constant string,
which is to compare it to _end:

        extern char _end;

        if (format + strlen(format) >= &_end
         && strstr(format, "%n") != NULL) {
                /* non-const format string with %n? looks fishy */
                syslog(LOG_ERR, "fishy non-const format: `%s'", format);
                abort();
        }

Unless you've compiled your program with -fwritable-strings, constant
strings are write-protected.

Maybe there are less destructive ways of dealing with the non-const %n
format case, but the question is whether that's really necessary.

Anyway, this is my proposal. This could be implemented either in glibc
(bad idea because it breaks 100% conformance) or in a libsafe type
of thing.

Does this sound reasonable? Is there anyone who'd like to try adding
this to libsafe?

Olaf

PS: The last program I had to with -fwritable-strings was Taylor UUCP 1.04,
long fixed since... 

-- 
Olaf Kirch         |  --- o --- Nous sommes du soleil we love when we play
okirmonad.swb.de  |    / | \   sol.dhoop.naytheet.ah kin.ir.samse.qurax
okircaldera.de    +-------------------- Why Not?! -----------------------
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