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

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Date:   Sat, 22 Dec 2018 23:24:11 +0900
From:   Namhyung Kim <namhyung@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.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, Dec 22, 2018 at 07:24:04AM -0500, Steven Rostedt wrote:
> 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.

I don't know what's the bad memory causing memory fault but anyway
memcpy() should stop at the NUL character first as it's different, no?

Thanks,
Namhyung