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Message-ID: <2024030803-bulk-grader-3ad1@gregkh>
Date: Fri, 8 Mar 2024 09:01:24 +0000
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: Alexey Dobriyan <adobriyan@...il.com>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sysfs: make sysfs_emit() return ssize_t
On Fri, Mar 08, 2024 at 09:26:01AM +0300, Alexey Dobriyan wrote:
> On Thu, Mar 07, 2024 at 10:04:41PM +0000, Greg Kroah-Hartman wrote:
> > On Mon, Feb 05, 2024 at 01:11:36PM +0300, Alexey Dobriyan wrote:
> > > sysfs_emit() is most often found in functions returning ssize_t
> > > not int:
> > >
> > > static ssize_t oops_count_show(...)
> > > {
> > > return sysfs_emit(page, ...);
> > > }
> > >
> > > This pattern results in sign-extension instruction between
> > > sysfs_emit() return value (int) and caller return value (which is
> > > ssize_t).
> >
> > Is that a problem?
>
> Small problem, but, yes.
>
> If sysfs_emit() returns "int", then every user compiles to
>
> call sysfs_emit
> movsx rax, eax
> ret
>
> Given than sysfs_emit() is the official way to print in sysfs,
> there are lots of users and there will be more users in the future
> as it grows.
>
> This trailing "movsx" instruction is duplicated every time.
>
> If sysfs_emit() returns ssize_t then it is folded into sysfs_emit() and
> appears in the code _once_ saving bytes.
>
> Ultimately, all this confusion and mismatches come from snprintf()
> accepting "size_t" but returning "int" (despite working on very large
> strings!) which sysfs_emit() copied.
True, then why not fix up the base function here, vscnprintf() and
vsnprintf() and then propagate the fixes outward?
thanks,
greg k-h
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