[patch] bugfix for do_execve

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Hi Linus,

I found a SMP race in execve():
cpu1:
        void main() { execve(argv[0],argv,envp);}

cpu2:
        $ cd /proc/<pid>;while true;do cat cmdline>/dev/null;done

After a few seconds, execve on cpu1 will fail: there is a race between
concurrent access_process_vm and setup_arg_pages().

setup_arg_pages() must acquire the mmap semaphore, that solved the
problem.

Could you add this patch to the next kernel?
[tested with 2.3.99-pre8, i386, SMP]

<<<<<<<<<<<
--- 2.3/fs/exec.c Fri May 12 08:52:15 2000
+++ build-2.3/fs/exec.c Sat May 13 21:24:38 2000
@@ -288,6 +288,7 @@
         if (!mpnt)
                 return -ENOMEM;
         
+ down(&current->mm->mmap_sem);
         {
                 mpnt->vm_mm = current->mm;
                 mpnt->vm_start = PAGE_MASK & (unsigned long) bprm->p;
@@ -311,6 +312,7 @@
                 }
                 stack_base += PAGE_SIZE;
         }
+ up(&current->mm->mmap_sem);
         
         return 0;
 }
>>>>>>>>>>>

--
	Manfred
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