linux-kernel - Re: [patch v2] madvise.2: Add MADV

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Message-ID: <CAKgNAkjGX5BanVCB=fauy9tYt+eAOb990LDOd73dvUp-bm03KA@mail.gmail.com>
Date:   Mon, 9 Oct 2017 21:11:24 +0200
From:   "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:     Rik van Riel <riel@...hat.com>
Cc:     Colm MacCárthaigh <colm@...costs.net>,
        linux-man <linux-man@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Linux API <linux-api@...r.kernel.org>, nilal@...hat.com,
        Florian Weimer <fweimer@...hat.com>,
        Mike Kravetz <mike.kravetz@...cle.com>
Subject: Re: [patch v2] madvise.2: Add MADV_WIPEONFORK documentation

Hi Rik,

Thanks for the blazingly fast response :-)

On 9 October 2017 at 21:08, Rik van Riel <riel@...hat.com> wrote:
> On Mon, 2017-10-09 at 21:06 +0200, Michael Kerrisk (man-pages) wrote:
>> Hi Rik,
>>
>> I have a follow-up question re wipe-on-fork. What are the semantics
>> for this setting with respect to fork() and exec()? That is, in the
>> child of a fork(), does the flag remain set for the specified address
>> range? (My quick read of the source suggests yes, but I have not
>> tested.) And, when we do an exec(), my assumption is that the flag is
>> cleared for the address range, but it would be good to have
>> confirmation.
>
> Indeed, on exec() the flag is cleared, because all
> memory regions get replaced on exec().

Thanks.

> The flag remains across a fork(), so if a child task
> were to fork, the memory would be empty of contents
> again in its child. This seems to most closely match
> the use case of discarding things like cryptographic
> secrets at fork time.

Thanks!

I'll add this info to the madvise(2) page.

Cheers,

Michael



-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/