Re: [PATCH v2] ipc,shm: disable shmmax and shmall by default

[Davidlohr Bueso]

On Fri, 2014-04-18 at 07:28 +0200, Michael Kerrisk (man-pages) wrote:
> Hello Davidlohr,
> 
> On Fri, Apr 18, 2014 at 12:31 AM, Davidlohr Bueso <davidlohr@xxxxxx> wrote:
> > On Thu, 2014-04-17 at 22:23 +0200, Michael Kerrisk (man-pages) wrote:
> >> Hi Manfred!
> >>
> >> On Thu, Apr 17, 2014 at 6:22 PM, Manfred Spraul
> >> <manfred@xxxxxxxxxxxxxxxx> wrote:
> >> > Hi Michael,
> >> >
> >> >
> >> > On 04/17/2014 12:53 PM, Michael Kerrisk wrote:
> >> >>
> >> >> On Sat, Apr 12, 2014 at 5:22 AM, Davidlohr Bueso <davidlohr@xxxxxx> wrote:
> 
> [...]
> 
> >> >> Of the two proposed approaches (the other being
> >> >> marc.info/?l=linux-kernel&m=139730332306185), this looks preferable to
> >> >> me, since it allows strange users to maintain historical behavior
> >> >> (i.e., the ability to set a limit) if they really want it, so:
> >> >>
> >> >> Acked-by: Michael Kerrisk <mtk.manpages@xxxxxxxxx>
> >> >>
> >> >> One or two comments below, that you might consider for your v3 patch.
> >> >
> >> > I don't understand what you mean.
> >>
> >> As noted in the other mail, you don't understand, because I was being
> >> dense (and misled a little by the commit message).
> >>
> >> > After a
> >> >     # echo 33554432 > /proc/sys/kernel/shmmax
> >> >     # echo 2097152 > /proc/sys/kernel/shmmax
> >> >
> >> > both patches behave exactly identical.
> >>
> >> Yes.
> >>
> >> > There are only two differences:
> >> > - Davidlohr's patch handles
> >> >     # echo <really huge number that doesn't fit into 64-bit> >
> >> > /proc/sys/kernel/shmmax
> >> >    With my patch, shmmax would end up as 0 and all allocations fail.
> >> >
> >> > - My patch handles the case if some startup code/installer checks
> >> >    shmmax and complains if it is below the requirement of the application.
> >>
> >> Thanks for that clarification. I withdraw my Ack.
> >
> > :(
> >
> >> In fact, maybe I
> >> even like your approach a little more, because of that last point.
> >
> > And it is a fair point. However, this is my counter argument: if users
> > are checking shmmax then they sure better be checking shmmin as well! So
> > if my patch causes shmctl(,IPC_INFO,) to return shminfo.shmmax = 0 and a
> > user only checks this value and breaks the application, then *he's*
> > doing it wrong. Checking shmmin is just as important...  0 value is
> > *bogus*,
> 
> That counter-argument sounds bogus. On all systems that I know/knew
> of, SHMIN always defaulted to 1. (Stevens APUE 1e documents this as
> the typical default even as far back as 1992.) Furthermore, the limit
> was always 1 on Linux, and as far as I know it has always been
> immutable. I very much doubt any sysadmin ever changed SHMMIN (why
> would they?), even on those systems where it was possible (and both
> SHMMIN and SHMMAX seem to have been obsolete on Solaris for some time
> now), or that any application ever checked the limit.

I'm not talking about *changing* SHMMIN, but checking for the value...
anything less than 1 is of course complete crap. And that's not the
kernel's fault.


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