qemu m68k/mcf5208: problem with signal handler

[Waldemar Brodkorb]

Dear kernel hackers,

I have a problem with the signal handling under qemu-system-m68k
emulating coldfire mcf5208 evalboard. Following example code
provided by Busybox maintainer Denys Vlasenko
shows the problem when running on qemu:

#include <unistd.h>                                                                                                                                                    
#include <signal.h>                                                                                                                                                    
#include <sys/types.h>                                                                                                                                                 
#include <sys/wait.h>                                                                                                                                                  
static void sighandler(int sig)                                                                                                                                        
{                                                                                                                                                                      
        write(1, "SIGNAL\n", 7);                                                                                                                                       
}                                                                                                                                                                      
int main()                                                                                                                                                             
{                                                                                                                                                                      
        int pid;                                                                                                                                                       
                                                                                                                                                                       
        write(1, "VFORK1\n", 7);                                                                                                                                       
        pid = vfork();                                                                                                                                                 
        if (pid == 0) {                                                                                                                                                
                write(1, "EXIT1\n", 6);                                                                                                                                
                _exit(1);                                                                                                                                              
        }                                                                                                                                                              
        wait(NULL);                                                                                                                                                    
        signal(SIGCHLD, sighandler);                                                                                                                                   
        write(1, "VFORK2\n", 7);                                                                                                                                       
        pid = vfork();                                                                                                                                                 
        if (pid == 0) {                                                                                                                                                
                write(1, "EXIT2\n", 6);                                                                                                                                
                _exit(1);                                                                                                                                              
        }                                                                                                                                                              
        wait(NULL);                                                                                                                                                    
        write(1, "EXIT\n", 5);                                                                                                                                         
        return 0;                                                                                                                                                      
}                                                                                                                                                                      

You can generate a bootable image with latest buildroot, which shows the issue:
$ git clone git://git.buildroot.net/buildroot
$ cd buildroot; make qemu_m68k_mcf5208_defconfig; make
$ qemu-system-m68k -M mcf5208evb -cpu m5208 -kernel output/images/vmlinux -nographic

Every command forked from busybox hush shell will lead into a segmentation fault.

I added following printk to start investigating the problem:
diff -Nur linux-4.5.3.orig/arch/m68k/kernel/signal.c linux-4.5.3/arch/m68k/kernel/signal.c
--- linux-4.5.3.orig/arch/m68k/kernel/signal.c	2016-05-04 23:50:38.000000000 +0200
+++ linux-4.5.3/arch/m68k/kernel/signal.c	2016-05-09 04:24:53.885199544 +0200
@@ -595,6 +595,7 @@
 			       void __user *fp)
 {
 	int fsize = frame_extra_sizes(formatvec >> 12);
+	printk("avoid broken signal handler...\n");
 	if (fsize < 0) {
 		/*
 		 * user process trying to return with weird frame format

But now the problem disappeared. :/

What do you think? Is it a Kernel bug or a C library problem?

Busybox hush otherwise works fine for other noMMU targets as stm32
evalboard with cortex-m4. It also works in Qemu M68k emulating Q800
full MMU system.

Thanks for any ideas,
 Waldemar

http://lists.busybox.net/pipermail/busybox/2014-September/081659.html