Re: [PATCH] sysctl: drop the pointer dance in proc_put_char()

[Joel Granados]

On Wed, Sep 16, 2026 at 01:47:00PM +0100, David Laight wrote:
> On Wed, 16 Sep 2026 13:46:40 +0200
> Joel Granados <joel.granados@xxxxxxxxxx> wrote:
> 
> > On Sat, Aug 22, 2026 at 06:42:44PM +0000, Bradley Morgan wrote:
> > > proc_put_char() still drags around a char **buffer alias, writing
> > > the char through it, advancing it, then copying it back into the
> > > slot it was loaded from. That only made sense when the buffer was
> > > __user and the char went through put_user() (which could fail).
> > > 
> > > Since commit 32927393dc1c ("sysctl: pass kernel pointers to  
> > > ->proc_handler") the buffer is just a kernel pointer, so the alias  
> > > is dead weight. proc_put_long() and the skip helpers already advance
> > > *buf directly, so do the same here. No functional change.
> > > 
> > > Signed-off-by: Bradley Morgan <include@xxxxxxxxx>
> > > ---
> > >  kernel/sysctl.c | 7 ++-----
> > >  1 file changed, 2 insertions(+), 5 deletions(-)
> > > 
> > > diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> > > index f7b7598..2b92b30 100644
> > > --- a/kernel/sysctl.c
> > > +++ b/kernel/sysctl.c
> > > @@ -350,12 +350,9 @@ static void proc_put_long(void **buf, size_t *size, unsigned long val, bool neg)
> > >  static void proc_put_char(void **buf, size_t *size, char c)
> > >  {
> > >  	if (*size) {
> > > -		char **buffer = (char **)buf;
> > > -		**buffer = c;
> > > -
> > > +		*(char *)*buf = c;
> > >  		(*size)--;
> > > -		(*buffer)++;
> > > -		*buf = *buffer;
> > > +		(*buf)++;
> > >  	}
> > >  }
> > >  
> > > -- 
> > > 2.47.3
> > >   
> > 
> > I was thinking more along these lines https://lore.kernel.org/all/20260916-lklm-sysctl-void-vs-char-v1-1-c3f8b2eb8b1e@xxxxxxxxxx/
> 
> I think I'd decrement *size before writing to buf.
> Should ensure the compiler never reloads *size.
> Could also be worth an __always_inline - likely to make the code smaller.

This seems sensible. I'll update my the one in [1]

[1] https://lore.kernel.org/all/20260916-lklm-sysctl-void-vs-char-v1-1-c3f8b2eb8b1e@xxxxxxxxxx/

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