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

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Date:   Sat, 22 Dec 2018 11:16:30 -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 10:12:44 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:

> On Sat, 22 Dec 2018 23:24:11 +0900
> Namhyung Kim <namhyung@...nel.org> wrote:
> 
> > > No, because we don't know the length of str.
> > > 
> > > 
> > > 	[ str = "h\0[bad memory]" ]


> > 
> > I don't know what's the bad memory causing memory fault but anyway

What I meant by that is if a string is allocated at a end of a page,
and the next page is marked as not present. A read into that page will
cause a page fault, and since memcmp() does not stop at the '\0' it
will read into that not-present memory and trigger a fault, and that
read wont be in the exception table, and it will then BUG.

> > memcpy() should stop at the NUL character first as it's different, no?  
> 
> No, that's the difference between memcpy() and strncpy(), memcpy()
> doesn't care about nul characters. It's copying memory not strings.

I think we both meant s/cpy/cmp/ ;-)

-- Steve