Re: Understanding Linux addr space, malloc, and heap

[Paulo Marques <pmarques@grupopie.com>]

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 <arjan@infradead.org> 
>>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
Web: www.grupopie.com

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