Re: regarding mprotect() implementation in 2.6.26 kernel

[Michael Kerrisk]

On Mon, Aug 4, 2008 at 12:34 PM, Michael Kerrisk
<mtk.manpages@xxxxxxxxxxxxxx> wrote:
> Hallo Maxin,
>
> On Mon, Aug 4, 2008 at 12:19 PM, Maxin John <maxin.john@xxxxxxxxx> wrote:
>> Dear Jeremy,
>>
>>              Thank you very much for the information and I am sorry
>> for my delayed reply.
>>
>>        As per the patch created by Mr. hirofumi for the 2.5.26 kernel
>> which is described in
>> http://www.kernel.org/pub/linux/kernel/v2.5/ChangeLog-2.5.26 ,
>> and
>> http://linux.bkbits.net:8080/linux-2.6/?PAGE=gnupatch&REV=1.403.147.22
>> , the mprotect system call will set errno as ENOMEM instead of
>> EFAULT.
>>
>> But the latest man page(man-pages-3.05) of mprotect still contains
>> information regarding EFAULT.
>
> Yes, that text looks to be in error.
>
>> The SuSv3 specification of mprotect also
>> doesn't say anything about EFAULT in the mprotect() details. The
>> following patch removes the information regarding EFAULT from the
>> mprotect man page.
>
> Before I apply this...  Did you check what was the situation in 2.4 kernels?

So, after a quick search, it looks as though in kernels before 2.4.19,
the EFAULT error resulted instead of ENOMEM for this case.  Does that
sound right to you?

Cheers,

Michael

>> diff -Naur man-pages-3.05/man2/mprotect.2
>> man-pages-3.05_modified/man2/mprotect.2
>> --- man-pages-3.05/man2/mprotect.2      2008-07-23 19:42:13.000000000 +0530
>> +++ man-pages-3.05_modified/man2/mprotect.2     2008-08-04
>> 15:34:33.400869088 +0530
>> @@ -87,9 +87,6 @@
>>  to mark it
>>  .BR PROT_WRITE .
>>  .TP
>> -.B EFAULT
>> -The memory cannot be accessed.
>> -.TP
>>  .B EINVAL
>>  \fIaddr\fP is not a valid pointer,
>>  or not a multiple of the system page size.
>> ~
>>
>>  Please advise me if this information is irrelevant or wrong.
>>
>> Thanks and Regards,
>>
>> Maxin B. John
>> Bangalore, India.
>>
>>
>> On Thu, Jul 31, 2008 at 8:39 PM, Jeremy Fitzhardinge <jeremy@xxxxxxxx> wrote:
>>> Maxin John wrote:
>>>> Dear Christoph Hellwig,
>>>>
>>>> ( I guess you are the right person to ask this question ?)
>>>>
>>>> The POSIX.2 specification of mprotect() says:
>>>>
>>>> errorno should be set as ENOMEM if the addresses in the range [addr,
>>>> (addr + len)] are invalid for the address space of a process, or
>>>> specify one or more pages which are not mapped.
>>>>
>>>> However, in the mprotect implementation  (asmlinkage long
>>>> sys_mprotect(unsigned long start, size_t len, unsigned long prot)) in
>>>> linux/mm/mprotect.c file, if we call mprotect() with start as NULL and
>>>> len as 0, mprotect() returns 0 and it is not setting the errono.The
>>>> following code confirms this behaviour.
>>>>
>>>
>>> Address 0 is a valid process address.  And you've set the length to
>>> zero, so you technically haven't referred to any memory at all, so it
>>> doesn't matter what the address is.
>>>
>>>    J
>>>
>>
>
>
>
> --
> Michael Kerrisk
> Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
> man-pages online: http://www.kernel.org/doc/man-pages/online_pages.html
> Found a bug? http://www.kernel.org/doc/man-pages/reporting_bugs.html
>



-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
man-pages online: http://www.kernel.org/doc/man-pages/online_pages.html
Found a bug? http://www.kernel.org/doc/man-pages/reporting_bugs.html
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