Re: [PATCH v4] virtio-9p: fix build on !CONFIG_UTIMENSAT

[Philipp Hahn <hahn@univention.de>]

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Hello,

Am Donnerstag 18 November 2010 01:41:39 schrieb Hidetoshi Seto:
> This patch introduce a fallback mechanism for old systems that do not
> support utimensat().  This fix build failure with following warnings:

> +#ifdef CONFIG_UTIMENSAT
> +    return utimensat(dirfd, path, times, flags);
> +#else
> +    /* Fallback: use utimes() instead of utimensat() */

Since we also had a problem with utimestat() some time ago with Samba
<http://lists.samba.org/archive/samba-technical/2010-November/074613.html>
I'd like to comment on that:

Your have to be careful about compile-time-detection and runtime-detection: If 
you later run your utimestat()-enabled binary on an older kernel not 
supporting that syscall, you get -1 as the return-value and errno=ENOSYS. So 
even if you detected utimesatat() during compile-time, please always provide 
a fallback for run-time.
This is less important for people compiling there own version of kvm, but is 
essential for Linux distributions, since people often run newer kvm versions 
on older kernels.

Sincerely
Philipp Hahn
-- 
Philipp Hahn           Open Source Software Engineer      hahn@univention.de   
Univention GmbH        Linux for Your Business        fon: +49 421 22 232- 0
Mary-Somerville-Str.1  28359 Bremen                   fax: +49 421 22 232-99
                                                    http://www.univention.de

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