Re: Understanding Linux addr space, malloc, and heap

Arjan van de Ven wrote:

On Fri, 2005-10-21 at 16:37 +0100, Alex Bligh - linux-kernel wrote:

--On 21 October 2005 17:22 +0200 Arjan van de Ven <[email protected]> wrote:

Ok I meant in the "while adhering to the standard" :)

More precisely, as per the man page:

POSIX.1b says that mprotect can be used only on regions of memory
obtained from mmap(2).

But what is interesting (if anything) is this:

ERRORS
      EINVAL addr is not a valid pointer, or not a  multiple  of
      PAGESIZE.

So if he calls mprotect with memory allocated by malloc (which should
fail), why doesn't he get EINVAL? He says it returns 0 (meaning it
succeeded). Which it shouldn't (unless he is stupendously lucky in
malloc's allocation, in which case it should work).

it succeeds all right; it just does other things than you expect
perhaps ;)

your alignment code had a bug, so it would align potentially to the
wrong piece of memory

Actually, it should give a SIGSEGV for the first byte allocated, although I agree that it might not work for any other byte allocated in the case where malloc returns a pointer just at the end of a page.

I just tested the sample code and in fact I do get a SIGSEGV.

What kernel version / architecture are you testing this on?

--
Paulo Marques
Software Development Department - Grupo PIE, S.A.
Phone: +351 252 290600, Fax: +351 252 290601
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adversaries are insane.
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• References:
  • Re: Understanding Linux addr space, malloc, and heap
    • From: "Vincent W. Freeh" <[email protected]>
  • Re: Understanding Linux addr space, malloc, and heap
    • From: Arjan van de Ven <[email protected]>
  • Re: Understanding Linux addr space, malloc, and heap
    • From: "Vincent W. Freeh" <[email protected]>
  • Re: Understanding Linux addr space, malloc, and heap
    • From: Arjan van de Ven <[email protected]>
  • Re: Understanding Linux addr space, malloc, and heap
    • From: Alex Bligh - linux-kernel <[email protected]>
  • Re: Understanding Linux addr space, malloc, and heap
    • From: Arjan van de Ven <[email protected]>

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