Again: Fix multithread coredump deadlock (patch Manfred Spraul)

Again: Fix multithread coredump deadlock (patch Manfred Spraul)

[Kirill Korotaev]

Hi!

There was a patch some time ago included in linux-2.4.17-pre6 which fixed mmap 
semaphore deadlock in do_coredump (double down_read() on mmap_sem).
This fix introduces down_write() on mmap_sem and uses get_user_pages() 
function to avoid do_page_fault().
The question is why down_write() is used in elf_core_dump() (instead of 
down_read())?

Kirill

Re: Again: Fix multithread coredump deadlock (patch Manfred Spraul)

[Manfred Spraul]

Kirill Korotaev wrote:

>Hi!
>
>There was a patch some time ago included in linux-2.4.17-pre6 which fixed mmap 
>semaphore deadlock in do_coredump (double down_read() on mmap_sem).
>This fix introduces down_write() on mmap_sem and uses get_user_pages() 
>function to avoid do_page_fault().
>The question is why down_write() is used in elf_core_dump() (instead of 
>down_read())?
>  
>
down_write is required to prevent expand_stack() from growing the stack 
- expand_stack is called by the page fault handler under 
down_read(&->mmap_sem) and changes vma->vm_start. A change of vm_start 
between writing the program headers and the actual segment dump would 
corrupt the coredump.

--
    Manfred