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SubjectRe: [PATCH] fix adjtimex() freq return error
On Tue, 19 Aug 2008 19:29:06 -0700 john stultz <johnstul@us.ibm.com> wrote:

> Martin Ziegler reported a bug in adjtimex(), where odd positive
> frequencies are reduced by 1 while negative values are correct.
>
> See http://bugzilla.kernel.org/show_bug.cgi?id=11370 for details
>
> The issue was introduced in the following commit that increased
> time_freq to a 64bit value and increased its resolution:
>
> http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=074b3b87941c99bc0ce35385b5817924b1ed0c23;hp=eea83d896e318bda54be2d2770d2c5d6668d11db
>
> The problem is how we convert from our internal high res value back to
> userland. Instead of dividing, we shift, then mult, then shift again,
> causing the slight error seen in the bug.
>
> I don't think adjtimex() is a terribly hot path, so this patch simply
> uses div_s64() to convert back to the external resolution and removes
> the unused shift defines to resolve the issue.
>
> Signed-off-by: John Stultz <johnstul@us.ibm.com>
>
> Index: 2.6-git/kernel/time/ntp.c
> ===================================================================
> --- 2.6-git.orig/kernel/time/ntp.c
> +++ 2.6-git/kernel/time/ntp.c
> @@ -402,9 +402,7 @@ int do_adjtimex(struct timex *txc)
> if (!(time_status & STA_NANO))
> txc->offset /= NSEC_PER_USEC;
> }
> - txc->freq = shift_right((s32)(time_freq >> PPM_SCALE_INV_SHIFT) *
> - (s64)PPM_SCALE_INV,
> - NTP_SCALE_SHIFT);
> + txc->freq = div_s64(time_freq, PPM_SCALE);
> txc->maxerror = time_maxerror;
> txc->esterror = time_esterror;
> txc->status = time_status;
> Index: 2.6-git/include/linux/timex.h
> ===================================================================
> --- 2.6-git.orig/include/linux/timex.h
> +++ 2.6-git/include/linux/timex.h
> @@ -82,9 +82,6 @@
> */
> #define SHIFT_USEC 16 /* frequency offset scale (shift) */
> #define PPM_SCALE (NSEC_PER_USEC << (NTP_SCALE_SHIFT - SHIFT_USEC))
> -#define PPM_SCALE_INV_SHIFT 20
> -#define PPM_SCALE_INV ((1ll << (PPM_SCALE_INV_SHIFT + NTP_SCALE_SHIFT)) / \
> - PPM_SCALE + 1)
>
> #define MAXPHASE 500000000l /* max phase error (ns) */
> #define MAXFREQ 500000 /* max frequency error (ns/s) */
>

Thanks, John. And Martin for the perfect bug report.

Based on Martin's observation that the bug has been there since 2.6.21
I marked this as needed in 2.6.25.x and in 2.6.26.x. However the patch
doesn't apply at all to 2.6.25, so someone(tm) will have a bit of
rework to do if we want to fix 2.6.25.x.

This fix caused Roman's
ntp-fix-adj_offset_ss_read-bug-and-do_adjtimex-cleanup.patch (which I
have queued for when Thomas wakes up) to toss a reject, but fixing that
seemed pretty simple. I think. The scripts/kconfig/conf segfaults are
being a bit of a PITA at present.



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