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From: Alex Barcelo <abarcelo@ac.upc.edu>
Date: Fri, 10 Feb 2012 10:57:05 +0100
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Subject: Re: [Qemu-devel] [TRIVIAL v2] Bad zero comparison for sas_ss_flags
	on powerpc
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To: qemu-devel <qemu-devel@nongnu.org>
Cc: qemu-trivial@nongnu.org, Riku Voipio <riku.voipio@iki.fi>

// Test source and desired /real output:

#include <signal.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>

void handler(int sig)
{
   unsigned int a;
   // to prevent uninitialized stack, normally a = 0
   if ( a>10 ) a = 0;
   a = a + 1;
   printf ("new value: %d\n" , a );
   if (a > 7) _exit(a);
   return;
}

int main()
{
   int ret;
   char * stackA = malloc(SIGSTKSZ);
   char * stackB = malloc(SIGSTKSZ);
   stack_t ssA = {
       .ss_size = SIGSTKSZ,
       .ss_sp = stackA,
   };
   stack_t ssB = {
       .ss_size = SIGSTKSZ,
       .ss_sp = stackB,
   };
   struct sigaction sa = {
       .sa_handler = handler,
       .sa_flags = SA_ONSTACK
   };

   // no error checking, only debug output
   ret = sigfillset(&sa.sa_mask);
   printf ( "Sigfillset: %d\n" , ret );
   ret = sigaction(SIGUSR1, &sa, 0);
   printf ( "Sigaction: %d\n" , ret );

   while (1) {
       printf ("On stack A -- " );
       ret = sigaltstack(&ssA, 0);
       printf ( "sigaltstack return: %d -- " , ret );
       kill(0, SIGUSR1);
       sleep(1);
       printf ("                                    -- " );
       kill(0, SIGUSR1);
       sleep(1);

       printf ("On stack B -- " );
       ret = sigaltstack(&ssB, 0);
       printf ( "sigaltstack return: %d -- " , ret );
       kill(0, SIGUSR1);
       sleep(1);
   }
}

/* Desired output:
Sigfillset: 0
Sigaction: 0
On stack A -- sigaltstack return: 0 -- new value: 1
                                   -- new value: 2
On stack B -- sigaltstack return: 0 -- new value: 1
On stack A -- sigaltstack return: 0 -- new value: 3
                                   -- new value: 4
On stack B -- sigaltstack return: 0 -- new value: 2
On stack A -- sigaltstack return: 0 -- new value: 5
                                   -- new value: 6
On stack B -- sigaltstack return: 0 -- new value: 3
On stack A -- sigaltstack return: 0 -- new value: 7
                                   -- new value: 8

Output for ppc without patch:
Sigfillset: 0
Sigaction: 0
On stack A -- sigaltstack return: 0 -- new value: 1
                                   -- new value: 2
On stack B -- sigaltstack return: 0 -- new value: 3 // WRONG!!
On stack A -- sigaltstack return: 0 -- new value: 4
                                   -- new value: 5 // WRONG AGAIN!
...
*/