Re: [PATCH] Expose SHM_HUGETLB in shmctl(id, IPC_STAT, ...)

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Date:	Fri, 11 Nov 2005 23:38:24 -0800
To: "Michael Kerrisk" <michael.kerrisk@gmx.net>

"Michael Kerrisk" <michael.kerrisk@gmx.net> wrote:
>
> > Bear in mind that the sort of apps we're talking about here are
> > dubiously-written monsters with long and costly upgrade cycles and we tend
> > to not get any reports until many many months after we made a kernel
> > change. It's very costly all round and we need to be cautious.
>
> Andrew,
>
> I am late to this discussion, but for what it's worth, a
> portable application really must use checks of the like
> (perm.mode & 0777 = 0666), because many implementations
> define additional read-only flags for perm.mode:
>
> Tru64 5.1
> #define SHM_LOCKED 01000 /* segment locked in memory */
> #define SHM_REMOVED 02000 /* already removed */
>
> Linux
> #define SHM_DEST 01000 /* segment will be destroyed on last detach */
> #define SHM_LOCKED 02000 /* segment will not be swapped */
>
> HP-UX 11
> # define SHM_CLEAR 01000 /* clear segment on next attach */
> # define SHM_DEST 02000 /* destroy segment when # attached = 0 */
> # define SHM_NOSWAP 010000 /* region for shared memory is memory locked */
> /* (or should be when the region is allocated) */
>
> AIX 5.1
> #define SHM_DEST 02000 /* destroy segment when # attached = 0 */
>
> So the chances are probably good that portable applications
> wouldn't break with Arun's proposal.

The chances of breakage I agree are very low. But non-zero. I'd still
like us to find a way which is completely safe.

> Of course applications
> that were written just for Linux, and don't take care, might
> also be at risk, but I think the risk is probably low.
> A check of the form:
>
> if (mode == 0666|SHM_LOCKED)
>
> instead of:
>
> if (mode & SHM_LOCKED)
>
> is very obtuse.

Yes, but

        if ((mode & ~(SHM_LOCKED|SHM_REMOVED)) == 0666)

is pretty perverse, but more likely. Stranger things have
been seen ;)

> This might not change your point of view (there is a theoretical risk
> after all), but I thought it worth mentioning.

Thanks.
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• Previous message: Michael Kerrisk: "Re: [PATCH] Expose SHM_HUGETLB in shmctl(id, IPC_STAT, ...)"
• In reply to: Michael Kerrisk: "Re: [PATCH] Expose SHM_HUGETLB in shmctl(id, IPC_STAT, ...)"
• Next in thread: William Lee Irwin III: "Re: [PATCH] Expose SHM_HUGETLB in shmctl(id, IPC_STAT, ...)"
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