linux-kernel - Re: [PATCH 08/12] printk: Replace strncmp with str_has

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Date:   Thu, 01 Aug 2019 10:51:22 -0700
From:   Joe Perches <joe@...ches.com>
To:     Chuhong Yuan <hslester96@...il.com>
Cc:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 08/12] printk: Replace strncmp with str_has_prefix

On Thu, 2019-08-01 at 23:23 +0800, Chuhong Yuan wrote:
> Joe Perches <joe@...ches.com> 于2019年7月30日周二 上午8:16写道：
> > On Mon, 2019-07-29 at 23:15 +0800, Chuhong Yuan wrote:
> > > strncmp(str, const, len) is error-prone.
> > > We had better use newly introduced
> > > str_has_prefix() instead of it.
> > []
> > > diff --git a/kernel/printk/braille.c b/kernel/printk/braille.c
> > []
> > > @@ -11,10 +11,10 @@
> > > 
> > >  int _braille_console_setup(char **str, char **brl_options)
> > >  {
> > > -     if (!strncmp(*str, "brl,", 4)) {
> > > +     if (str_has_prefix(*str, "brl,")) {
> > >               *brl_options = "";
> > >               *str += 4;
> > > -     } else if (!strncmp(*str, "brl=", 4)) {
> > > +     } else if (str_has_prefix(*str, "brl=")) {
> > >               *brl_options = *str + 4;
> > 
> > Better to get rid of the += 4 uses too by storing the result
> > of str_has_prefix and using that as the addend.
> > 
> > Perhaps
> >         size_t len;
> > 
> >         if ((len = str_has_prefix(*str, "brl,"))) {
> >                 *brl_options = "";
> >                 *str += len;
> >         } else if ((len = str_has_prefix(*str, "brl="))) {
> >                 etc...
> > 
> 
> I find that checkpatch.pl forbids assignment in if condition.
> So this seems to be infeasible...

So the code could be rewritten like below:
(though it's trivial as-is)
---
 kernel/printk/braille.c | 30 ++++++++++++++++++++----------
 1 file changed, 20 insertions(+), 10 deletions(-)

diff --git a/kernel/printk/braille.c b/kernel/printk/braille.c
index 1d21ebacfdb8..46dd9fcc7525 100644
--- a/kernel/printk/braille.c
+++ b/kernel/printk/braille.c
@@ -11,19 +11,29 @@
 
 int _braille_console_setup(char **str, char **brl_options)
 {
-	if (!strncmp(*str, "brl,", 4)) {
+	size_t len;
+
+	len = str_has_prefix(*str, "brl,");
+	if (len) {
 		*brl_options = "";
-		*str += 4;
-	} else if (!strncmp(*str, "brl=", 4)) {
-		*brl_options = *str + 4;
-		*str = strchr(*brl_options, ',');
-		if (!*str) {
-			pr_err("need port name after brl=\n");
-			return -EINVAL;
-		}
-		*((*str)++) = 0;
+		*str += len;
+		return 0;
+	}
+
+	len = str_has_prefix(*str, "brl=");
+	if (!len)
+		return 0;
+
+	*brl_options = *str + len;
+
+	*str = strchr(*brl_options, ',');
+	if (!*str) {
+		pr_err("need port name after brl=\n");
+		return -EINVAL;
 	}
 
+	*((*str)++) = 0;
+
 	return 0;
 }