linux-kernel - Re: [PATCH v4] string.h: Add str_has

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Message-ID: <20181222072404.27a7e9a3@vmware.local.home>
Date:   Sat, 22 Dec 2018 07:24:04 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Namhyung Kim <namhyung@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Joe Perches <joe@...ches.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Tom Zanussi <zanussi@...nel.org>,
        Andreas Schwab <schwab@...ux-m68k.org>, kernel-team@....com
Subject: Re: [PATCH v4] string.h: Add str_has_prefix() helper function

On Sat, 22 Dec 2018 18:33:46 +0900
Namhyung Kim <namhyung@...nel.org> wrote:

> > +static __always_inline size_t str_has_prefix(const char *str, const char *prefix)
> > +{
> > +	size_t len = strlen(prefix);
> > +	return strncmp(str, prefix, len) == 0 ? len : 0;  
> 
> As it already knows the length (and it needs to use it for return
> value), isn't it (slightly) better using memcmp() instead?

No, because we don't know the length of str.


	[ str = "h\0[bad memory]" ]


	str_has_prefix(str, "TEST THIS BIG STRING AT FRONT")


If we use memcmp(), then we are testing way after str has ended, and
that can cause a memory fault.

-- Steve