linux-kernel - Re: [PATCH v2 1/5] kprobes: convert kprobe_lookup

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Date:   Wed, 19 Apr 2017 16:37:13 +0530
From:   "'Naveen N. Rao'" <naveen.n.rao@...ux.vnet.ibm.com>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Ingo Molnar <mingo@...nel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>
Subject: Re: [PATCH v2 1/5] kprobes: convert kprobe_lookup_name() to a
 function

On 2017/04/19 08:48AM, David Laight wrote:
> From: Naveen N. Rao
> > Sent: 19 April 2017 09:09
> > To: David Laight; Michael Ellerman
> > Cc: linux-kernel@...r.kernel.org; linuxppc-dev@...ts.ozlabs.org; Masami Hiramatsu; Ingo Molnar
> > Subject: RE: [PATCH v2 1/5] kprobes: convert kprobe_lookup_name() to a function
> > 
> > Excerpts from David Laight's message of April 18, 2017 18:22:
> > > From: Naveen N. Rao
> > >> Sent: 12 April 2017 11:58
> > > ...
> > >> +kprobe_opcode_t *kprobe_lookup_name(const char *name)
> > >> +{
> > > ...
> > >> +	char dot_name[MODULE_NAME_LEN + 1 + KSYM_NAME_LEN];
> > >> +	const char *modsym;
> > >> +	bool dot_appended = false;
> > >> +	if ((modsym = strchr(name, ':')) != NULL) {
> > >> +		modsym++;
> > >> +		if (*modsym != '\0' && *modsym != '.') {
> > >> +			/* Convert to <module:.symbol> */
> > >> +			strncpy(dot_name, name, modsym - name);
> > >> +			dot_name[modsym - name] = '.';
> > >> +			dot_name[modsym - name + 1] = '\0';
> > >> +			strncat(dot_name, modsym,
> > >> +				sizeof(dot_name) - (modsym - name) - 2);
> > >> +			dot_appended = true;
> > >
> > > If the ':' is 'a way down' name[] then although the strncpy() won't
> > > overrun dot_name[] the rest of the code can.
> > 
> > Nice catch, thanks David!
> > We need to be validating the length of 'name'. I'll put out a patch for
> > that.
> 
> Silent truncation is almost certainly wrong here.

Indeed. This will be handled by the earlier validation to ensure that 
the module name as well as the symbol name are within the expected 
lengths.

> 
> > As an aside, I'm not sure I follow what you mean when you say that the
> > strncpy() won't overrun dot_name[]. If we have a name[] longer than
> > sizeof(dot_name) with the ':' after that, the strncpy() can also overrun
> > dot_name[].
> 
> Yes, that should just be a memcpy(), as should the strncat().
> 
> Using strncpy() where the length is other than the size of the target buffer
> should be banned. Not that it ever does what people expect.
> strncat() is even worse.

Sure, but with the proper validation, I still think string functions are 
convenient and useful here. I agree about your view on strncat(), so 
I'll switch to strlcat() instead.

Thanks,
Naveen