Re: [Qemu-devel] [PATCH] libseccomp: add cacheflush to whitelist

[Eduardo Otubo]

On Thu, Sep 24, 2015 at 03=50=04PM +0200, Andrew Jones wrote:
> On Thu, Sep 24, 2015 at 11:31:19AM +0200, Eduardo Otubo wrote:
> > On Wed, Jul 01, 2015 at 09=12=33AM -0400, Andrew Jones wrote:
> > > cacheflush is an arm-specific syscall that qemu built for arm
> > > uses. Add it to the whitelist.
> > > 
> > > Signed-off-by: Andrew Jones <address@hidden>
> > > 
> > > ---
> > > 
> > > I'm not sure about the priority selection. Maybe cacheflush gets
> > > used frequently enough that it deserves a higher one?
> > 
> > The frequency is measured using strace and comparing the frequency they
> > appear among other syscalls. Can you run this analysis and double check
> > if the lowest priority is still accurate?
> 
> Hi Eduardo,
> 
> Short answer: The lowest priority is definitely correct.
> 
> Long answer:
> 
> I ran strace while installing a new guest, of 3.6 million syscalls,
> only 5 were cacheflush. Of course the syscalls used (and their frequency)
> is host-type, qemu machine-type, config (qemu command line), and guest
> workload specific. So, ideally, qemu machine-types would register their
> own whitelists, possibly modified by host-type. For example, I ran the
> mach-virt machine-type on both a midway and a mustang. In both cases it
> was a basic guest config and an install-type workload. For the mustang,
> over 55% of the syscalls were ioctl, but, for the midway, ioctls were
> 16% and 43% were clock_gettime. I generated a most-used-first list for
> each. Neither list really matched up well with seccomp_whitelist (except
> for futex).
> 
> Besides allowing machine types to help set priorities, it may also be
> nice if both compile-time and run-time configs could further reduce the
> whitelist. For example, mlockall is only necessary if '-realtime mlock=on'
> is passed on the command line.
> 
> Thanks,
> drew
> 
> > 
> > Thanks for the patch.
> > 
> > > 
> > > This patch isn't really necessary yet due to ae6e8ef11e6c: "Revert
> > > seccomp tests that allow it to be used on non-x86 architectures",
> > > which we can't revert until libseccomp has released a fix for
> > > arm-specific syscall symbol naming, but when linking to a patched
> > > libseccomp and reverting ae6e8ef11e6c, then this patch allows
> > > guests to boot with '-sandbox on'.
> > > 
> > > Signed-off-by: Andrew Jones <address@hidden>
> > > ---
> > >  qemu-seccomp.c | 3 ++-
> > >  1 file changed, 2 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/qemu-seccomp.c b/qemu-seccomp.c
> > > index f9de0d3390feb..33644a4e3c3d3 100644
> > > --- a/qemu-seccomp.c
> > > +++ b/qemu-seccomp.c
> > > @@ -237,7 +237,8 @@ static const struct QemuSeccompSyscall 
> > > seccomp_whitelist[] = {
> > >      { SCMP_SYS(fadvise64), 240 },
> > >      { SCMP_SYS(inotify_init1), 240 },
> > >      { SCMP_SYS(inotify_add_watch), 240 },
> > > -    { SCMP_SYS(mbind), 240 }
> > > +    { SCMP_SYS(mbind), 240 },
> > > +    { SCMP_SYS(cacheflush), 240 },
> > >  };
> > >  
> > >  int seccomp_start(void)
> > > -- 
> > > 2.1.0
> > > 

Acked-by: Eduardo Otubo <address@hidden>

-- 
Eduardo Otubo
ProfitBricks GmbH

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