vfork implementation...

vfork implementation...

[Abu M. Muttalib]

Hi,

This mail is intended for Robert Love, I hope I can find him on the list.

Please refer to Pg 24 of chapter 2 of Linux Kernel Development.

As mentioned in the description of vfork call, it is said that child is not
allowed to write to the address space, but in the following example its not
so. The child is able to write to the process address space. This program
was tested with Linux Kernel 2.6.9. Why is it so?

fork.c
----------------------------------------------------------------------------
---------------------------------------
#include <stdio.h>

unsigned char *glob_var = NULL;

void main()
{
	int pid = -8,i;
	pid = vfork();

	if(pid < 0)
		printf("\n FORK ERROR \n");

	if(pid == 0)
	{
		unsigned char * local_var = NULL;
		local_var = (unsigned char *)malloc(5);
		strcpy(local_var,"ABCD");
		glob_var = local_var;
		printf("\nCHILD :Value of glob_var is  %X local_var is %X glob_var is %c
\n",glob_var,local_var,*glob_var);
		for(i=0;i<4;i++)
		{
			printf("\n CHAR is %c \n",glob_var[i]);
		}
		printf("\nCHILD1 :Value of glob_var is %X %c\n",glob_var,*(glob_var));
	}

	if(pid > 0)
	{
		printf("\nParent : Value of glob_var is  %X %c\n",glob_var,*(glob_var));
		free(glob_var);
		printf("\nParent : Value of glob_var is %X %c\n",glob_var,*(glob_var));
		exit(0);
	}
}
----------------------------------------------------------------------------
---------------------------------------

Regards,
Abu.

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Re: vfork implementation...

[Gleb Natapov]

On Wed, Jun 14, 2006 at 03:11:58PM +0530, Abu M. Muttalib wrote:
> Hi,
> 
> This mail is intended for Robert Love, I hope I can find him on the list.
> 
> Please refer to Pg 24 of chapter 2 of Linux Kernel Development.
> 
> As mentioned in the description of vfork call, it is said that child is not
> allowed to write to the address space, but in the following example its not
> so. The child is able to write to the process address space. This program
> was tested with Linux Kernel 2.6.9. Why is it so?
> 
Not allowed and not able are two different things.

> fork.c
> ----------------------------------------------------------------------------
> ---------------------------------------
> #include <stdio.h>
> 
> unsigned char *glob_var = NULL;
> 
> void main()
> {
> 	int pid = -8,i;
> 	pid = vfork();
> 
> 	if(pid < 0)
> 		printf("\n FORK ERROR \n");
> 
> 	if(pid == 0)
> 	{
> 		unsigned char * local_var = NULL;
> 		local_var = (unsigned char *)malloc(5);
> 		strcpy(local_var,"ABCD");
> 		glob_var = local_var;
> 		printf("\nCHILD :Value of glob_var is  %X local_var is %X glob_var is %c
> \n",glob_var,local_var,*glob_var);
> 		for(i=0;i<4;i++)
> 		{
> 			printf("\n CHAR is %c \n",glob_var[i]);
> 		}
> 		printf("\nCHILD1 :Value of glob_var is %X %c\n",glob_var,*(glob_var));
> 	}
> 
> 	if(pid > 0)
> 	{
> 		printf("\nParent : Value of glob_var is  %X %c\n",glob_var,*(glob_var));
> 		free(glob_var);
> 		printf("\nParent : Value of glob_var is %X %c\n",glob_var,*(glob_var));
> 		exit(0);
> 	}
> }
> ----------------------------------------------------------------------------
> ---------------------------------------
> 
> Regards,
> Abu.
> 
> --
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--
			Gleb.

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RE: vfork implementation...

[Abu M. Muttalib]

I meant, its NOT ALLOWED, still program is ABLE to do so.. hence the
wordings..

~Abu.

-----Original Message-----
From: owner-linux-mm@kvack.org [mailto:owner-linux-mm@kvack.org]On
Behalf Of Gleb Natapov
Sent: Wednesday, June 14, 2006 3:16 PM
To: Abu M. Muttalib
Cc: linux-mm@kvack.org
Subject: Re: vfork implementation...


On Wed, Jun 14, 2006 at 03:11:58PM +0530, Abu M. Muttalib wrote:
> Hi,
>
> This mail is intended for Robert Love, I hope I can find him on the list.
>
> Please refer to Pg 24 of chapter 2 of Linux Kernel Development.
>
> As mentioned in the description of vfork call, it is said that child is
not
> allowed to write to the address space, but in the following example its
not
> so. The child is able to write to the process address space. This program
> was tested with Linux Kernel 2.6.9. Why is it so?
>
Not allowed and not able are two different things.

> fork.c
> --------------------------------------------------------------------------
--
> ---------------------------------------
> #include <stdio.h>
>
> unsigned char *glob_var = NULL;
>
> void main()
> {
> 	int pid = -8,i;
> 	pid = vfork();
>
> 	if(pid < 0)
> 		printf("\n FORK ERROR \n");
>
> 	if(pid == 0)
> 	{
> 		unsigned char * local_var = NULL;
> 		local_var = (unsigned char *)malloc(5);
> 		strcpy(local_var,"ABCD");
> 		glob_var = local_var;
> 		printf("\nCHILD :Value of glob_var is  %X local_var is %X glob_var is %c
> \n",glob_var,local_var,*glob_var);
> 		for(i=0;i<4;i++)
> 		{
> 			printf("\n CHAR is %c \n",glob_var[i]);
> 		}
> 		printf("\nCHILD1 :Value of glob_var is %X %c\n",glob_var,*(glob_var));
> 	}
>
> 	if(pid > 0)
> 	{
> 		printf("\nParent : Value of glob_var is  %X %c\n",glob_var,*(glob_var));
> 		free(glob_var);
> 		printf("\nParent : Value of glob_var is %X %c\n",glob_var,*(glob_var));
> 		exit(0);
> 	}
> }
> --------------------------------------------------------------------------
--
> ---------------------------------------
>
> Regards,
> Abu.
>
> --
> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> the body to majordomo@kvack.org.  For more info on Linux MM,
> see: http://www.linux-mm.org/ .
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--
			Gleb.

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Re: vfork implementation...

[Gleb Natapov]

On Wed, Jun 14, 2006 at 03:33:27PM +0530, Abu M. Muttalib wrote:
> I meant, its NOT ALLOWED, still program is ABLE to do so.. hence the
> wordings..
> 
The only thing a process ALLOWED to do after vfork() is exec(), but it is
ABLE to do whatever it wants to do. I am not ALLOWED to drive while drunk,
but I am ABLE to.


> ~Abu.
> 
> -----Original Message-----
> From: owner-linux-mm@kvack.org [mailto:owner-linux-mm@kvack.org]On
> Behalf Of Gleb Natapov
> Sent: Wednesday, June 14, 2006 3:16 PM
> To: Abu M. Muttalib
> Cc: linux-mm@kvack.org
> Subject: Re: vfork implementation...
> 
> 
> On Wed, Jun 14, 2006 at 03:11:58PM +0530, Abu M. Muttalib wrote:
> > Hi,
> >
> > This mail is intended for Robert Love, I hope I can find him on the list.
> >
> > Please refer to Pg 24 of chapter 2 of Linux Kernel Development.
> >
> > As mentioned in the description of vfork call, it is said that child is
> not
> > allowed to write to the address space, but in the following example its
> not
> > so. The child is able to write to the process address space. This program
> > was tested with Linux Kernel 2.6.9. Why is it so?
> >
> Not allowed and not able are two different things.
> 
> > fork.c
> > --------------------------------------------------------------------------
> --
> > ---------------------------------------
> > #include <stdio.h>
> >
> > unsigned char *glob_var = NULL;
> >
> > void main()
> > {
> > 	int pid = -8,i;
> > 	pid = vfork();
> >
> > 	if(pid < 0)
> > 		printf("\n FORK ERROR \n");
> >
> > 	if(pid == 0)
> > 	{
> > 		unsigned char * local_var = NULL;
> > 		local_var = (unsigned char *)malloc(5);
> > 		strcpy(local_var,"ABCD");
> > 		glob_var = local_var;
> > 		printf("\nCHILD :Value of glob_var is  %X local_var is %X glob_var is %c
> > \n",glob_var,local_var,*glob_var);
> > 		for(i=0;i<4;i++)
> > 		{
> > 			printf("\n CHAR is %c \n",glob_var[i]);
> > 		}
> > 		printf("\nCHILD1 :Value of glob_var is %X %c\n",glob_var,*(glob_var));
> > 	}
> >
> > 	if(pid > 0)
> > 	{
> > 		printf("\nParent : Value of glob_var is  %X %c\n",glob_var,*(glob_var));
> > 		free(glob_var);
> > 		printf("\nParent : Value of glob_var is %X %c\n",glob_var,*(glob_var));
> > 		exit(0);
> > 	}
> > }
> > --------------------------------------------------------------------------
> --
> > ---------------------------------------
> >
> > Regards,
> > Abu.
> >
> > --
> > To unsubscribe, send a message with 'unsubscribe linux-mm' in
> > the body to majordomo@kvack.org.  For more info on Linux MM,
> > see: http://www.linux-mm.org/ .
> > Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
> 
> --
> 			Gleb.
> 
> --
> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> the body to majordomo@kvack.org.  For more info on Linux MM,
> see: http://www.linux-mm.org/ .
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--
			Gleb.

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RE: vfork implementation...

[Abu M. Muttalib]

HI,

Should the wording be interpreted as "should not write"?

~Abu.

-----Original Message-----
From: Gleb Natapov [mailto:gleb@minantech.com]
Sent: Wednesday, June 14, 2006 3:37 PM
To: Abu M. Muttalib
Cc: linux-mm@kvack.org
Subject: Re: vfork implementation...


On Wed, Jun 14, 2006 at 03:33:27PM +0530, Abu M. Muttalib wrote:
> I meant, its NOT ALLOWED, still program is ABLE to do so.. hence the
> wordings..
>
The only thing a process ALLOWED to do after vfork() is exec(), but it is
ABLE to do whatever it wants to do. I am not ALLOWED to drive while drunk,
but I am ABLE to.


> ~Abu.
>
> -----Original Message-----
> From: owner-linux-mm@kvack.org [mailto:owner-linux-mm@kvack.org]On
> Behalf Of Gleb Natapov
> Sent: Wednesday, June 14, 2006 3:16 PM
> To: Abu M. Muttalib
> Cc: linux-mm@kvack.org
> Subject: Re: vfork implementation...
>
>
> On Wed, Jun 14, 2006 at 03:11:58PM +0530, Abu M. Muttalib wrote:
> > Hi,
> >
> > This mail is intended for Robert Love, I hope I can find him on the
list.
> >
> > Please refer to Pg 24 of chapter 2 of Linux Kernel Development.
> >
> > As mentioned in the description of vfork call, it is said that child is
> not
> > allowed to write to the address space, but in the following example its
> not
> > so. The child is able to write to the process address space. This
program
> > was tested with Linux Kernel 2.6.9. Why is it so?
> >
> Not allowed and not able are two different things.
>
> > fork.c
>
> --------------------------------------------------------------------------
> --
> > ---------------------------------------
> > #include <stdio.h>
> >
> > unsigned char *glob_var = NULL;
> >
> > void main()
> > {
> > 	int pid = -8,i;
> > 	pid = vfork();
> >
> > 	if(pid < 0)
> > 		printf("\n FORK ERROR \n");
> >
> > 	if(pid == 0)
> > 	{
> > 		unsigned char * local_var = NULL;
> > 		local_var = (unsigned char *)malloc(5);
> > 		strcpy(local_var,"ABCD");
> > 		glob_var = local_var;
> > 		printf("\nCHILD :Value of glob_var is  %X local_var is %X glob_var is
%c
> > \n",glob_var,local_var,*glob_var);
> > 		for(i=0;i<4;i++)
> > 		{
> > 			printf("\n CHAR is %c \n",glob_var[i]);
> > 		}
> > 		printf("\nCHILD1 :Value of glob_var is %X %c\n",glob_var,*(glob_var));
> > 	}
> >
> > 	if(pid > 0)
> > 	{
> > 		printf("\nParent : Value of glob_var is  %X
%c\n",glob_var,*(glob_var));
> > 		free(glob_var);
> > 		printf("\nParent : Value of glob_var is %X
%c\n",glob_var,*(glob_var));
> > 		exit(0);
> > 	}
> > }
>
> --------------------------------------------------------------------------
> --
> > ---------------------------------------
> >
> > Regards,
> > Abu.
> >
> > --
> > To unsubscribe, send a message with 'unsubscribe linux-mm' in
> > the body to majordomo@kvack.org.  For more info on Linux MM,
> > see: http://www.linux-mm.org/ .
> > Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
>
> --
> 			Gleb.
>
> --
> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> the body to majordomo@kvack.org.  For more info on Linux MM,
> see: http://www.linux-mm.org/ .
> Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

--
			Gleb.

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Re: vfork implementation...

[Gleb Natapov]

On Wed, Jun 14, 2006 at 03:47:07PM +0530, Abu M. Muttalib wrote:
> HI,
> 
> Should the wording be interpreted as "should not write"?
> 
Yes. After vfork() a child and a parent share an address space. Whatever a child
does to memory is visible in a parent.

> ~Abu.
> 
> -----Original Message-----
> From: Gleb Natapov [mailto:gleb@minantech.com]
> Sent: Wednesday, June 14, 2006 3:37 PM
> To: Abu M. Muttalib
> Cc: linux-mm@kvack.org
> Subject: Re: vfork implementation...
> 
> 
> On Wed, Jun 14, 2006 at 03:33:27PM +0530, Abu M. Muttalib wrote:
> > I meant, its NOT ALLOWED, still program is ABLE to do so.. hence the
> > wordings..
> >
> The only thing a process ALLOWED to do after vfork() is exec(), but it is
> ABLE to do whatever it wants to do. I am not ALLOWED to drive while drunk,
> but I am ABLE to.
> 
> 
> > ~Abu.
> >
> > -----Original Message-----
> > From: owner-linux-mm@kvack.org [mailto:owner-linux-mm@kvack.org]On
> > Behalf Of Gleb Natapov
> > Sent: Wednesday, June 14, 2006 3:16 PM
> > To: Abu M. Muttalib
> > Cc: linux-mm@kvack.org
> > Subject: Re: vfork implementation...
> >
> >
> > On Wed, Jun 14, 2006 at 03:11:58PM +0530, Abu M. Muttalib wrote:
> > > Hi,
> > >
> > > This mail is intended for Robert Love, I hope I can find him on the
> list.
> > >
> > > Please refer to Pg 24 of chapter 2 of Linux Kernel Development.
> > >
> > > As mentioned in the description of vfork call, it is said that child is
> > not
> > > allowed to write to the address space, but in the following example its
> > not
> > > so. The child is able to write to the process address space. This
> program
> > > was tested with Linux Kernel 2.6.9. Why is it so?
> > >
> > Not allowed and not able are two different things.
> >
> > > fork.c
> >
> > --------------------------------------------------------------------------
> > --
> > > ---------------------------------------
> > > #include <stdio.h>
> > >
> > > unsigned char *glob_var = NULL;
> > >
> > > void main()
> > > {
> > > 	int pid = -8,i;
> > > 	pid = vfork();
> > >
> > > 	if(pid < 0)
> > > 		printf("\n FORK ERROR \n");
> > >
> > > 	if(pid == 0)
> > > 	{
> > > 		unsigned char * local_var = NULL;
> > > 		local_var = (unsigned char *)malloc(5);
> > > 		strcpy(local_var,"ABCD");
> > > 		glob_var = local_var;
> > > 		printf("\nCHILD :Value of glob_var is  %X local_var is %X glob_var is
> %c
> > > \n",glob_var,local_var,*glob_var);
> > > 		for(i=0;i<4;i++)
> > > 		{
> > > 			printf("\n CHAR is %c \n",glob_var[i]);
> > > 		}
> > > 		printf("\nCHILD1 :Value of glob_var is %X %c\n",glob_var,*(glob_var));
> > > 	}
> > >
> > > 	if(pid > 0)
> > > 	{
> > > 		printf("\nParent : Value of glob_var is  %X
> %c\n",glob_var,*(glob_var));
> > > 		free(glob_var);
> > > 		printf("\nParent : Value of glob_var is %X
> %c\n",glob_var,*(glob_var));
> > > 		exit(0);
> > > 	}
> > > }
> >
> > --------------------------------------------------------------------------
> > --
> > > ---------------------------------------
> > >
> > > Regards,
> > > Abu.
> > >
> > > --
> > > To unsubscribe, send a message with 'unsubscribe linux-mm' in
> > > the body to majordomo@kvack.org.  For more info on Linux MM,
> > > see: http://www.linux-mm.org/ .
> > > Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
> >
> > --
> > 			Gleb.
> >
> > --
> > To unsubscribe, send a message with 'unsubscribe linux-mm' in
> > the body to majordomo@kvack.org.  For more info on Linux MM,
> > see: http://www.linux-mm.org/ .
> > Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
> 
> --
> 			Gleb.

--
			Gleb.

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Re: vfork implementation...

[Mel Gorman]

On Wed, 14 Jun 2006, Abu M. Muttalib wrote:

> This mail is intended for Robert Love, I hope I can find him on the list.
>
> Please refer to Pg 24 of chapter 2 of Linux Kernel Development.
>
> As mentioned in the description of vfork call, it is said that child is not
> allowed to write to the address space,

>From the vfork() manpage;

 	The vfork() function has the same
        effect as fork(), except that the behaviour is undefined if the process
        created  by  vfork()  either modifies any data other than a variable of
        type pid_t used to store the return value from vfork(), or returns from
        the  function  in which vfork() was called, or calls any other function
        before successfully calling _exit() or one  of  the  exec()  family  of
        functions.

In other words, the child created by vfork() may be *able* to change the 
address space, but it *should not* because the behaviour is undefined. 
IIRC, there is no guarantee that the parent will even run again until the 
child calls exit or exec so a child cannot depend on any behavior from the 
parent or it could deadlock.

Historically, the point of vfork() is to create a process that 
immediately called exec(). It does not create a copy of the parent address 
space which was an important optimisation later replaced by Copy-On-Write.

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab

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