Re: [patch 2.6.13-rc4] fix get_user_pages bug

[Nick Piggin <nickpiggin@yahoo.com.au>]

Ingo Molnar wrote:

> 
> Hugh's posting said:
> 
>  "it's trying to avoid an endless loop of finding the pte not writable 
>   when ptrace is modifying a page which the user is currently protected 
>   against writing to (setting a breakpoint in readonly text, perhaps?)"
> 
> i'm wondering, why should that case generate an infinite fault? The 
> first write access should copy the shared-library page into a private 
> page and map it into the task's MM, writable. If this make-writable 

It will be mapped readonly.

> operation races with a read access then we return a minor fault and the 
> page is still readonly, but retrying the write should then break up the 
> COW protection and generate a writable page, and a subsequent 
> follow_page() success. If the page cannot be made writable, shouldnt the 
> vma flags reflect this fact by not having the VM_MAYWRITE flag, and 
> hence get_user_pages() should have returned with -EFAULT earlier?
> 

If it cannot be written to, then yes. If it can be written to
but is mapped readonly then you have the problem.

Aside, that brings up an interesting question - why should readonly
mappings of writeable files (with VM_MAYWRITE set) disallow ptrace
write access while readonly mappings of readonly files not? Or am I
horribly confused?

-- 
SUSE Labs, Novell Inc.

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