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From: Michael Neuling on 9 Feb 2010 01:20
When reserving stack space for a new process, make sure we're not
attempting to expand the stack by more than rlimit allows.

This fixes a bug caused by b6a2fea39318e43fee84fa7b0b90d68bed92d2ba "mm:
variable length argument support" and unmasked by
fc63cf237078c86214abcb2ee9926d8ad289da9b "exec: setup_arg_pages() fails
to return errors". This bug means when limiting the stack to less the
20*PAGE_SIZE (eg. 80K on 4K pages or 'ulimit -s 79') all processes will
be killed before they start. This is particularly bad with 64K pages,
where a ulimit below 1280K will kill every process.

Signed-off-by: Michael Neuling <mikey(a)neuling.org>
Cc: stable(a)kernel.org
---
Attempts to answer comments from Kosaki Motohiro.

Tested on PPC only, hence !CONFIG_STACK_GROWSUP. Someone should
probably ACK for an arch with CONFIG_STACK_GROWSUP.

As noted, stable needs the same patch, but 2.6.32 doesn't have the
rlimit() helper.

fs/exec.c | 21 ++++++++++++++++++---
1 file changed, 18 insertions(+), 3 deletions(-)

Index: linux-2.6-ozlabs/fs/exec.c
===================================================================
--- linux-2.6-ozlabs.orig/fs/exec.c
+++ linux-2.6-ozlabs/fs/exec.c
@@ -555,6 +555,7 @@ static int shift_arg_pages(struct vm_are
}

#define EXTRA_STACK_VM_PAGES 20 /* random */
+#define ALIGN_DOWN(addr,size) ((addr)&(~((size)-1)))

/*
* Finalizes the stack vm_area_struct. The flags and permissions are updated,
@@ -570,7 +571,7 @@ int setup_arg_pages(struct linux_binprm
struct vm_area_struct *vma = bprm->vma;
struct vm_area_struct *prev = NULL;
unsigned long vm_flags;
- unsigned long stack_base;
+ unsigned long stack_base, stack_expand, stack_expand_lim, stack_size;

#ifdef CONFIG_STACK_GROWSUP
/* Limit stack size to 1GB */
@@ -627,10 +628,24 @@ int setup_arg_pages(struct linux_binprm
goto out_unlock;
}

+ stack_expand = EXTRA_STACK_VM_PAGES * PAGE_SIZE;
+ stack_size = vma->vm_end - vma->vm_start;
+ if (rlimit(RLIMIT_STACK) < stack_size)
+ stack_expand_lim = 0; /* don't shrick the stack */
+ else
+ /*
+ * Align this down to a page boundary as expand_stack
+ * will align it up.
+ */
+ stack_expand_lim = ALIGN_DOWN(rlimit(RLIMIT_STACK) - stack_size,
+ PAGE_SIZE);
+ /* Initial stack must not cause stack overflow. */
+ if (stack_expand > stack_expand_lim)
+ stack_expand = stack_expand_lim;
#ifdef CONFIG_STACK_GROWSUP
- stack_base = vma->vm_end + EXTRA_STACK_VM_PAGES * PAGE_SIZE;
+ stack_base = vma->vm_end + stack_expand;
#else
- stack_base = vma->vm_start - EXTRA_STACK_VM_PAGES * PAGE_SIZE;
+ stack_base = vma->vm_start - stack_expand;
#endif
ret = expand_stack(vma, stack_base);
if (ret)
--
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