linux-kernel - Re: [PATCH] sysctl: move local variable in proc_do_large

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Message-ID: <20201113141702.GI4332@42.do-not-panic.com>
Date:   Fri, 13 Nov 2020 14:17:02 +0000
From:   Luis Chamberlain <mcgrof@...nel.org>
To:     Tom Rix <trix@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Lukas Bulwahn <lukas.bulwahn@...il.com>,
        Kees Cook <keescook@...omium.org>,
        Iurii Zaikin <yzaikin@...gle.com>,
        linux-fsdevel@...r.kernel.org,
        Nathan Chancellor <natechancellor@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        clang-built-linux@...glegroups.com,
        kernel-janitors@...r.kernel.org, linux-safety@...ts.elisa.tech,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sysctl: move local variable in proc_do_large_bitmap() to
 proper scope

On Mon, Nov 09, 2020 at 05:26:37PM -0800, Tom Rix wrote:
> 
> On 11/8/20 11:11 PM, Lukas Bulwahn wrote:
> > make clang-analyzer caught my attention with:
> >
> >   kernel/sysctl.c:1511:4: warning: Value stored to 'first' is never read \
> >   [clang-analyzer-deadcode.DeadStores]
> >                           first = 0;
> >                           ^
> >
> > Commit 9f977fb7ae9d ("sysctl: add proc_do_large_bitmap") introduced
> > proc_do_large_bitmap(), where the variable first is only effectively used
> > when write is false; when write is true, the variable first is only used in
> > a dead assignment.
> >
> > So, simply remove this dead assignment and put the variable in local scope.
> >
> > As compilers will detect this unneeded assignment and optimize this anyway,
> > the resulting object code is identical before and after this change.
> >
> > No functional change. No change to object code.
> >
> > Signed-off-by: Lukas Bulwahn <lukas.bulwahn@...il.com>
> > ---
> > applies cleanly on v5.10-rc3 and next-20201106
> >
> > Luis, Kees, Iurii, please pick this minor non-urgent clean-up patch.
> >
> >  kernel/sysctl.c | 3 +--
> >  1 file changed, 1 insertion(+), 2 deletions(-)
> >
> > diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> > index ce75c67572b9..cc274a431d91 100644
> > --- a/kernel/sysctl.c
> > +++ b/kernel/sysctl.c
> > @@ -1423,7 +1423,6 @@ int proc_do_large_bitmap(struct ctl_table *table, int write,
> >  			 void *buffer, size_t *lenp, loff_t *ppos)
> >  {
> >  	int err = 0;
> > -	bool first = 1;
> >  	size_t left = *lenp;
> >  	unsigned long bitmap_len = table->maxlen;
> >  	unsigned long *bitmap = *(unsigned long **) table->data;
> > @@ -1508,12 +1507,12 @@ int proc_do_large_bitmap(struct ctl_table *table, int write,
> >  			}
> >  
> >  			bitmap_set(tmp_bitmap, val_a, val_b - val_a + 1);
> > -			first = 0;
> >  			proc_skip_char(&p, &left, '\n');
> >  		}
> >  		left += skipped;
> >  	} else {
> >  		unsigned long bit_a, bit_b = 0;
> > +		bool first = 1;
> 
> This looks fine, but while you are here how about setting, to match the type
> 
> first = true
> 
> And then only clearing first once
> 
> if (!first)                                                                            
>   proc_put_char(&buffer, &left, ',');
> 
> else
> 
>   first = false
> 
> Instead of at every loop iteraction

Thanks for your patch Lukas!

Agreed, please resend with that change as requested by Tom. And also
please add Andrew Morton <akpm@...ux-foundation.org> to your To address.

  Luis