linux-kernel - Re: [PATCH 1/3] lib/vsprintf: add snprintf

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Date:   Sat, 15 Jun 2019 07:08:41 -0400
From:   Jeff Layton <jlayton@...nel.org>
To:     Joe Perches <joe@...ches.com>, "Yan, Zheng" <ukernel@...il.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        ceph-devel <ceph-devel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ilya Dryomov <idryomov@...il.com>, Zheng Yan <zyan@...hat.com>,
        Sage Weil <sage@...hat.com>, agruenba@...hat.com
Subject: Re: [PATCH 1/3] lib/vsprintf: add snprintf_noterm

On Fri, 2019-06-14 at 19:58 -0700, Joe Perches wrote:
> On Sat, 2019-06-15 at 10:41 +0800, Yan, Zheng wrote:
> > On Fri, Jun 14, 2019 at 9:48 PM Jeff Layton <jlayton@...nel.org> wrote:
> > > The getxattr interface returns a length after filling out the value
> > > buffer, and the convention with xattrs is to not NULL terminate string
> > > data.
> > > 
> > > CephFS implements some virtual xattrs by using snprintf to fill the
> > > buffer, but that always NULL terminates the string. If userland sends
> > > down a buffer that is just the right length to hold the text without
> > > termination then we end up truncating the value.
> > > 
> > > Factor the formatting piece of vsnprintf into a separate helper
> > > function, and have vsnprintf call that and then do the NULL termination
> > > afterward. Then add a snprintf_noterm function that calls the new helper
> > > to populate the string but skips the termination.
> 
> Is this function really necessary enough to add
> the additional stack use to the generic case?
> 

The only alternative I saw was to allocate an extra buffer in the
callers, call snprintf to populate that and then copy the result into
the destination buffer sans termination. I really would like to avoid
that here.

Does breaking this code out into a helper add any significant stack
usage? I didn't see it that way, but I am quite concerned about not
slowing down the generic vsnprintf routine.

> Why not add have this function call vsnprintf
> and then terminate the string separately?
> 

I don't quite follow what you're suggesting here. vsnprintf is what does
the termination today, and we need a function that doesn't do that.

-- 
Jeff Layton <jlayton@...nel.org>