Re: [patch v2] madvise.2: Add MADV_WIPEONFORK documentation
From: Michael Kerrisk (man-pages)
Date: Tue Sep 19 2017 - 15:08:07 EST
Hello Rik, (and Colm)
On 09/14/2017 09:05 PM, Rik van Riel wrote:
> v2: implement the improvements suggested by Colm, and add
> Colm's text to the fork.2 man page
> (Colm, I have added a signed-off-by in your name - is that ok?)
> Add MADV_WIPEONFORK and MADV_KEEPONFORK documentation to
> madvise.2. The new functionality was recently merged by
> Linus, and should be in the 4.14 kernel.
> While documenting what EINVAL means for MADV_WIPEONFORK,
> I realized that MADV_FREE has the same thing going on,
> so I documented EINVAL for both in the ERRORS section.
> This patch documents the following kernel commit:
> commit d2cd9ede6e193dd7d88b6d27399e96229a551b19
> Author: Rik van Riel <riel@xxxxxxxxxx>
> Date: Wed Sep 6 16:25:15 2017 -0700
> mm,fork: introduce MADV_WIPEONFORK
Thanks. I applied this, and tweaked the madvise.2 text a little, to
read as follows (please let me know if I messed anything up):
MADV_WIPEONFORK (since Linux 4.14)
Present the child process with zero-filled memory in this
range after a fork(2). This is useful in forking servers
in order to ensure that sensitive per-process data (for
example, PRNG seeds, cryptographic secrets, and so on) is
not handed to child processes.
The MADV_WIPEONFORK operation can be applied only to priâ
vate anonymous pages (see mmap(2)).
> Signed-off-by: Rik van Riel <riel@xxxxxxxxxx>
> Signed-off-by: Colm MacCÃrthaigh <colm@xxxxxxxxxxxx>
> diff --git a/man2/fork.2 b/man2/fork.2
> index b5af58ca08c0..b11e750e3876 100644
> --- a/man2/fork.2
> +++ b/man2/fork.2
> @@ -140,6 +140,12 @@ Memory mappings that have been marked with the
> flag are not inherited across a
> .BR fork ().
> .IP *
> +Memory in mappings that have been marked with the
> +.BR madvise (2)
> +.B MADV_WIPEONFORK
> +flag is zeroed in the child after a
> +.BR fork ().
> +.IP *
> The termination signal of the child is always
> .B SIGCHLD
> diff --git a/man2/madvise.2 b/man2/madvise.2
> index dfb31b63dba3..bb0ac469c509 100644
> --- a/man2/madvise.2
> +++ b/man2/madvise.2
> @@ -31,6 +31,9 @@
> .\" 2010-06-19, Andi Kleen, Add documentation of MADV_SOFT_OFFLINE.
> .\" 2011-09-18, Doug Goldstein <cardoe@xxxxxxxxxx>
> .\" Document MADV_HUGEPAGE and MADV_NOHUGEPAGE
> +.\" 2017-09-14, Rik van Riel <riel@xxxxxxxxxx>
> +.\" Document MADV_WIPEONFORK and MADV_KEEPONFORK
> +.\" commit d2cd9ede6e193dd7d88b6d27399e96229a551b19
> .TH MADVISE 2 2017-07-13 "Linux" "Linux Programmer's Manual"
> .SH NAME
> @@ -405,6 +408,22 @@ can be applied only to private anonymous pages (see
> .BR mmap (2)).
> On a swapless system, freeing pages in a given range happens instantly,
> regardless of memory pressure.
> +.BR MADV_WIPEONFORK " (since Linux 4.14)"
> +Present the child process with zero-filled memory in this range after a
> +.BR fork (2).
> +This is useful for per-process data in forking servers that should be
> +re-initialized in the child process after a fork, for example PRNG seeds,
> +cryptographic secrets, etc.
> +.B MADV_WIPEONFORK
> +operation can only be applied to private anonymous pages (see
> +.BR mmap (2)).
> +.BR MADV_KEEPONFORK " (since Linux 4.14)"
> +Undo the effect of an earlier
> +.BR MADV_WIPEONFORK .
> .SH RETURN VALUE
> On success,
> .BR madvise ()
> @@ -457,6 +476,18 @@ or
> but the kernel was not configured with
> .BR CONFIG_KSM .
> +.B EINVAL
> +.I advice
> +.BR MADV_FREE
> +.BR MADV_WIPEONFORK
> +but the specified address range includes file, Huge TLB,
> +.BR MAP_SHARED ,
> +.BR VM_PFNMAP
> .B EIO
> .BR MADV_WILLNEED )
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