Re: [patch v2] madvise.2: Add MADV_WIPEONFORK documentation
From: Michael Kerrisk (man-pages) <hidden>
Date: 2017-09-19 19:08:04
Also in:
linux-man, linux-mm, lkml
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 [off-list ref]
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 pri‐
vate anonymous pages (see mmap(2)).
Thanks,
Michael
quoted hunk ↗ jump to hunk
Signed-off-by: Rik van Riel <redacted> Signed-off-by: Colm MacCárthaigh <colm-ZXBCfW2eEe/k1uMJSBkQmQ@public.gmane.org>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 (seediff --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-VPKZcK2rSRzQT0dZR+AlfA@public.gmane.org> .\" Document MADV_HUGEPAGE and MADV_NOHUGEPAGE +.\" 2017-09-14, Rik van Riel <riel-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> +.\" 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. +.TP +.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. +.IP +The +.B MADV_WIPEONFORK +operation can only be applied to private anonymous pages (see +.BR mmap (2)). +.TP +.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 . .TP +.B EINVAL +.I advice +is +.BR MADV_FREE +or +.BR MADV_WIPEONFORK +but the specified address range includes file, Huge TLB, +.BR MAP_SHARED , +or +.BR VM_PFNMAP +ranges. +.TP .B EIO (for .BR MADV_WILLNEED )
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