musl - Potential bug in printf

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Message-ID: <23e62796-4efb-3495-d7c8-1e3d85f4ea82@jensenkarlsson.se>
Date: Fri, 6 Aug 2021 15:14:22 +0200
From: Pontus Jensen Karlsson <pontus@...senkarlsson.se>
To: musl@...ts.openwall.com
Subject: Potential bug in printf_core

Hi,

I've been trying to build audit-userspace tools for an ARMv7 SBC using 
musl 1.2.2 as libc.
The tool auditd continously segfaults and I've traced it to a printf 
statement that
I have isolated the issue to this piece of code (simplified for 
debugging purposes):

#include <sys/time.h>
#include <stdio.h>

int main(int argc, char **argv)
{
     struct timeval tv = {
         .tv_sec = 1000,
         .tv_usec = 4177777
     };
     char *str = "Hello World";
     unsigned num = 8062;

     printf("%lu %03u %u %s", tv.tv_sec, (unsigned)(tv.tv_usec), num, str);
}

This code segfaults at memchr (s = 0x3fbf71 <error: Cannot access memory 
at address 0x3fbf71>)
but three frames up we're at src/stdio/vfprintf.c:593.

Here it attempts to read the string length from the arg.p address, the 
problem is that arg.p points
to the int-value of (unsigned)(tv.tv_usec) and not the memory address of 
str.

So, I'm confused as to why this happens? Is it something weird with the 
state-machine in printf_core,
or am I misunderstanding something which needs to be patched into 
audit-userspace?

Best regards,
Pontus Jensen Karlsson

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