[PATCH v3] ipc/shm: kill the historical/wrong mm->start_stack check

[Oleg Nesterov <oleg@redhat.com>]

do_shmat() is the only user of ->start_stack (proc just reports its
value), and this check looks ugly and wrong.

The reason for this check is not clear at all, and it wrongly assumes
that the stack can only grow down.

But the main problem is that in general mm->start_stack has nothing
to do with stack_vma->vm_start. Not only the application can switch
to another stack and even unmap this area, setup_arg_pages() expands
the stack without updating mm->start_stack during exec(). This means
that in the likely case "addr > start_stack - size - PAGE_SIZE * 5"
is simply impossible after find_vma_intersection() == F, or the stack
can't grow anyway because of RLIMIT_STACK.

Many thanks to Hugh for his explanations.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 ipc/shm.c |    7 -------
 1 files changed, 0 insertions(+), 7 deletions(-)

diff --git a/ipc/shm.c b/ipc/shm.c
index 7fc9f9f..0145479 100644
--- a/ipc/shm.c
+++ b/ipc/shm.c
@@ -1172,13 +1172,6 @@ long do_shmat(int shmid, char __user *shmaddr, int shmflg, ulong *raddr,
 
 		if (find_vma_intersection(current->mm, addr, addr + size))
 			goto invalid;
-		/*
-		 * If shm segment goes below stack, make sure there is some
-		 * space left for the stack to grow (at least 4 pages).
-		 */
-		if (addr < current->mm->start_stack &&
-		    addr > current->mm->start_stack - size - PAGE_SIZE * 5)
-			goto invalid;
 	}
 
 	addr = do_mmap_pgoff(file, addr, size, prot, flags, 0, &populate);
-- 
1.5.5.1