Re: [Xen-devel] [PATCH XEN v8 24/29] tools/libs/call: linux: touch newly allocated pages after madvise lockdown

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Tue, Jan 19, 2016 at 03:54:54PM +0100, Roger Pau Monné wrote:
> El 19/01/16 a les 14.24, Wei Liu ha escrit:
> > On Fri, Jan 15, 2016 at 01:23:03PM +0000, Ian Campbell wrote:
> >> This avoids a potential issue with a fork after allocation but before
> >> madvise.
> >>
> >> Signed-off-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
> >> ---
> >> v7: New, replacing "tools/libs/call: linux: avoid forking between mmap
> >>     and madvise".
> >> ---
> >>  tools/libs/call/linux.c | 14 +++++++++++++-
> >>  1 file changed, 13 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/tools/libs/call/linux.c b/tools/libs/call/linux.c
> >> index 3641e41..651f380 100644
> >> --- a/tools/libs/call/linux.c
> >> +++ b/tools/libs/call/linux.c
> > 
> > I didn't notice you only handled this for Linux until now.
> > 
> > I think FreeBSD and NetBSD need similar treatment, too? But then current
> > BSD* code doesn't even support DONTFORK in madvise.
> > 
> > Adding Roger for more input.
> 
> Hm, right, thanks for noticing this. I don't think FreeBSD needs a
> similar treatment (pre-faulting), because mlock will remove any CoW when
> making the pages wired.
> 
> Also, AFAICT we don't need to call madvise or minherit(2) because
> mlock(2) already takes care of preventing the memory region from being
> copied to the child on fork:
> 
> "Locked mappings are not inherited by the child process after a
> fork(2)." [0]
> 
> So I think we are safe on the FreeBSD side.
> 

But what if the process forks between mmap and mlock? I think that
warrants touching the area like we do for Linux here.

Wei.

> Roger.
> 
> [0] https://www.freebsd.org/cgi/man.cgi?query=mlock
> 

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