On Tue, 26 Aug 2014, Oleg Nesterov wrote: > 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 <o...@redhat.com>
Acked-by: Hugh Dickins <hu...@google.com> But you're much too generous to me: I never even noticed how exec's expand_stack() comes *after* setting start_stack, so that the shmat() check is nowadays utterly irrelevant. Thank you for persisting and observing that and finally nailing down this coffin! > --- > 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
