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List:       freebsd-hackers
Subject:    Re: mprotect(2) won't make shared memory read-only
From:       Brooks Davis <brooks () one-eyed-alien ! net>
Date:       2000-02-29 1:04:43
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On Mon, Feb 28, 2000 at 12:50:13PM -0800, Brooks Davis wrote:
> On a -current system as of a week or two ago (as well as a 3.3-RC and a
> 2.2.8-STABLE box) I've found that mprotect fails with with EACCES when
> trying to make a shared memory segment that was created user read/write
> read-only.  It works find if I malloc the memory instead and making the
> shm segment write-only or inaccessible works fine as well.  Is this
> expected behavior?  If so it's pretty weird.

Following up on this.  Charles Randall suggested I try shmctl to make
the segment read-only.  Unfortunatly it appears that shared memory
premissions are screwed up as well.  You can create a read-only shared
memory segment either via shmget or via shmctl on an existing read-write
segment and it will appear via ipcs, but it doesn't appear to have any
effect.  The following program creates a read-only segment and then
proceds to write to it twice, both of which work under FreeBSD -current.
When run on a Solaris 2.6 machine it segfaults in the first write.


--<cut>--
/* Test read-only shared memory segments */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/mman.h>
#include <sys/ipc.h>
#include <sys/shm.h>

#define SIZE 1024*sizeof(int)

int main(int argc, char **argv)
{
	int *array;

	int shmid;
	shmid = shmget(IPC_PRIVATE, SIZE, SHM_R);
	array = shmat(shmid, NULL, 0);

	bzero(array, SIZE);

	fprintf(stderr, "array[0] = %d\n", array[0]);

	array[0] = 1;

	fprintf(stderr, "array[0] = %d\n", array[0]);

	shmctl(shmid, IPC_RMID, NULL);

	return 0;
}
--<cut>--

Thanks,
Brooks

-- 
Any statement of the form "X is the one, true Y" is FALSE.


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