Re: 2.6.33-rc8 breaks UML with Restrict initial stack space expansion to rlimit

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

> 
> 
> In message <20100214164023.GA2726@jm.kir.nu> you wrote:
> > It looks like the commit 803bf5ec259941936262d10ecc84511b76a20921
> > (fs/exec.c: restrict initial stack space expansion to rlimit) broke my
> > user mode Linux setup by somehow preventing system setup from running
> > properly (or killing some processes that try to mount things, etc.).
> > This commit turned up as the reason based on git bisect and reverting it
> > fixes my UML test setup (Ubuntu 9.10 on both host and in UML and AMD64
> > arch for both). I have no idea what exactly would be the main cause for
> > this issue, but this looks like a somewhat unfortunately timed
> > regression in 2.6.33-rc8.
> > 
> > The failed run shows like this (with current linux-2.6.git):
> > 
> > ...
> > EXT3-fs (ubda): mounted filesystem with writeback data mode
> > VFS: Mounted root (ext3 filesystem) readonly on device 98:0.
> > IRQ 3/console-write: IRQF_DISABLED is not guaranteed on shared IRQs
> > IRQ 2/console: IRQF_DISABLED is not guaranteed on shared IRQs
> > IRQ 10/winch: IRQF_DISABLED is not guaranteed on shared IRQs
> > IRQ 10/winch: IRQF_DISABLED is not guaranteed on shared IRQs
> > mountall: mount /sys/kernel/debug [218] killed by KILL signal
> > mountall: Filesystem could not be mounted: /sys/kernel/debug
> > mountall: mount /dev [219] killed by KILL signal
> > mountall: Filesystem could not be mounted: /dev
> > mountall: mount /tmp [220] killed by KILL signal
> > mountall: Filesystem could not be mounted: /tmp
> > mountall: mount /var/lock [222] killed by KILL signal
> > mountall: Filesystem could not be mounted: /var/lock
> > ...
> > 
> > 
> > With 803bf5ec reverted, UML comes up and the output looks like this:
> > 
> > ...
> > EXT3-fs (ubda): mounted filesystem with writeback data mode
> > VFS: Mounted root (ext3 filesystem) readonly on device 98:0.
> > IRQ 3/console-write: IRQF_DISABLED is not guaranteed on shared IRQs
> > IRQ 2/console: IRQF_DISABLED is not guaranteed on shared IRQs
> > IRQ 10/winch: IRQF_DISABLED is not guaranteed on shared IRQs
> > IRQ 10/winch: IRQF_DISABLED is not guaranteed on shared IRQs
> > init: procps main process (226) terminated with status 255
> > fsck from util-linux-ng 2.16
> > ...
> 
> Jouni,
> 
> I can reproduce this now.  
> 
> We got the logic wrong in one of the cleanups and hence we aren't
> actually changing the stack reservation ever, when we intended on
> allocating up to 20 new pages.  
> 
> The:
> 	rlim_stack = min(rlim_stack, stack_size);
> always chooses stack_size hence we end up not changing the stack at all.
> This seems to cause fatal problems on UML, but is obviously not what was
> intended for archs as well.  
> 
> The following works for me on PPC64 64k and 4k pages and UML on x86_64. 
> 
> Let me know if it fixes it for you also.
> 
> Mikey
> 
> 
> exec/fs: fix initial stack reservation
> 
> 803bf5ec259941936262d10ecc84511b76a20921 (fs/exec.c: restrict initial
> stack space expansion to rlimit) attempts to limit the initial stack to
> 20*PAGE_SIZE.  Unfortunately, in also attempting ensure the stack is not
> reduced in size, we ended up not changing the stack at all.  
> 
> This caused a regression in UML resulting in most guest processes to be
> killed. 
> 
> Signed-off-by: Michael Neuling <mikey@neuling.org>
> cc: <stable@kernel.org>
> 
> diff --git a/fs/exec.c b/fs/exec.c
> index e95c692..e0e7b3c 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -637,15 +637,16 @@ int setup_arg_pages(struct linux_binprm *bprm,
>  	 * will align it up.
>  	 */
>  	rlim_stack = rlimit(RLIMIT_STACK) & PAGE_MASK;
> -	rlim_stack = min(rlim_stack, stack_size);
>  #ifdef CONFIG_STACK_GROWSUP
>  	if (stack_size + stack_expand > rlim_stack)
> -		stack_base = vma->vm_start + rlim_stack;
> +		/*  Expand only to rlimit, making sure not to shrink it */
> +		stack_base = vma->vm_start + max(rlim_stack,stack_size);
>  	else
>  		stack_base = vma->vm_end + stack_expand;
>  #else
>  	if (stack_size + stack_expand > rlim_stack)
> -		stack_base = vma->vm_end - rlim_stack;
> +		/*  Expand only to rlimit, making sure not to shrink it */
> +		stack_base = vma->vm_end - max(rlim_stack,stack_size);
>  	else
>  		stack_base = vma->vm_start - stack_expand;
>  #endif

-	rlim_stack = min(rlim_stack, stack_size);
+	/*  Expand only to rlimit, making sure not to shrink it */
+	rlim_stack = max(rlim_stack, stack_size);

is better fix?