Unbacked shared memory not included in ELF core dump

Unbacked shared memory not included in ELF core dump

[Jochen Suckfuell]

Hi!

Since 2.6.10, unbacked shared memory allocated via shmget is not
included in core dumps. The relevant patch has been done to binfmt_elf.c
after the discussion to "include all vmas with unbacked pages in ELF
core dumps", here:
http://www.ussg.iu.edu/hypermail/linux/kernel/0410.2/1890.html

The result was:

static int maydump(struct vm_area_struct *vma)
{
	/* Do not dump I/O mapped devices, shared memory, or special mappings */
	if (vma->vm_flags & (VM_IO | VM_SHARED | VM_RESERVED))
		return 0;
...

I consider this a bug, since we clearly lose unbacked shared memory in
the process.

bye
Jochen Suckfüll

2.6.11 bug: unbacked private shared memory segments missing in core dump

[Jochen Suckfuell]

Hello!

Since 2.6.10, unbacked private shared memory allocated via shmget is not
included in core dumps.

This is a simple example code demonstrating the bug:

#include <sys/shm.h>

int main(int argc, char ** argv)
{
	int size = 1000;
	int id = shmget(IPC_PRIVATE, size, (IPC_CREAT | 0660));
	if(id < 0) return(1);
	int *buffer = (int *)shmat(id, 0, 0);
	int i;
	for(i = 0; i < 1000; i++)
		buffer[i] = i;

	// now dump core
	*((unsigned long *)1) = 0;

	// The private shared memory is not included in the core dump,
	// although it's not backed and cannot be accessed any more in any
	// way.
	return 0;
}

This bug was introduced in 2.6.10 by a patch to binfmt_elf.c that
resulted in:

static int maydump(struct vm_area_struct *vma)
{
	/* Do not dump I/O mapped devices, shared memory, or special mappings */
	if (vma->vm_flags & (VM_IO | VM_SHARED | VM_RESERVED))
		return 0;
...

(See the thread at
http://www.ussg.iu.edu/hypermail/linux/kernel/0410.2/1890.html)

Excluding all pages with VM_SHARED set is also excluding the unbacked
private mapping and should be replaced by a more specific criterion.

Bye
Jochen

Re: 2.6.11 bug: unbacked private shared memory segments missing in core dump

[Andrew Morton]

Jochen Suckfuell <jo-lkml@suckfuell.net> wrote:
>
> Hello!
> 
> Since 2.6.10, unbacked private shared memory allocated via shmget is not
> included in core dumps.

Can you please confirm that 2.6.12-rc1 fixed this?

Thanks.

> This is a simple example code demonstrating the bug:
> 
> #include <sys/shm.h>
> 
> int main(int argc, char ** argv)
> {
> 	int size = 1000;
> 	int id = shmget(IPC_PRIVATE, size, (IPC_CREAT | 0660));
> 	if(id < 0) return(1);
> 	int *buffer = (int *)shmat(id, 0, 0);
> 	int i;
> 	for(i = 0; i < 1000; i++)
> 		buffer[i] = i;
> 
> 	// now dump core
> 	*((unsigned long *)1) = 0;
> 
> 	// The private shared memory is not included in the core dump,
> 	// although it's not backed and cannot be accessed any more in any
> 	// way.
> 	return 0;
> }
> 
> This bug was introduced in 2.6.10 by a patch to binfmt_elf.c that
> resulted in:
> 
> static int maydump(struct vm_area_struct *vma)
> {
> 	/* Do not dump I/O mapped devices, shared memory, or special mappings */
> 	if (vma->vm_flags & (VM_IO | VM_SHARED | VM_RESERVED))
> 		return 0;
> ...
> 
> (See the thread at
> http://www.ussg.iu.edu/hypermail/linux/kernel/0410.2/1890.html)
> 
> Excluding all pages with VM_SHARED set is also excluding the unbacked
> private mapping and should be replaced by a more specific criterion.
> 
> Bye
> Jochen
> 
> -
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Re: 2.6.11 bug: unbacked private shared memory segments missing in core dump

[Jochen Suckfuell]

On Mon, Mar 21, 2005 at 03:29:48PM -0800, Andrew Morton wrote:
> Jochen Suckfuell <jo-lkml@suckfuell.net> wrote:
> >
> > Hello!
> > 
> > Since 2.6.10, unbacked private shared memory allocated via shmget is not
> > included in core dumps.
> 
> Can you please confirm that 2.6.12-rc1 fixed this?

Yes, it's fixed. 

Thanks to everyone involved.


Bye
Jochen Suckfuell