linux-hardening - Re: [PATCH v2] checkpatch: Warn about 0-length and 1-element arrays

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Message-ID: <202305261859.310EB2972A@keescook>
Date:   Fri, 26 May 2023 19:01:02 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Joe Perches <joe@...ches.com>
Cc:     Andy Whitcroft <apw@...onical.com>,
        Dwaipayan Ray <dwaipayanray1@...il.com>,
        Lukas Bulwahn <lukas.bulwahn@...il.com>,
        "Gustavo A . R . Silva" <gustavoars@...nel.org>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2] checkpatch: Warn about 0-length and 1-element arrays

On Fri, May 26, 2023 at 06:38:27PM -0700, Joe Perches wrote:
> On Fri, 2023-05-26 at 10:39 -0700, Kees Cook wrote:
> > Fake flexible arrays have been deprecated since last millennium. Proper
> > C99 flexible arrays must be used throughout the kernel so
> > CONFIG_FORTIFY_SOURCE and CONFIG_UBSAN_BOUNDS can provide proper array
> > bounds checking.
> []
> > diff --git a/scripts/checkpatch.pl b/scripts/checkpatch.pl
> > []
> > @@ -7430,6 +7430,21 @@ sub process {
> >  			}
> >  		}
> >  
> > +# check for array definition/declarations that should use flexible arrays instead
> > +		if ($sline =~ /^[\+ ]\s*}\s*;\s*$/ &&
> > +		    $prevline =~ /^\+\s*(?:(?:struct|union|enum)\s+$Ident|\}|$Type)\s*$Ident\s*\[\s*(0|1)\s*\]\s*;\s*$/) {
> 
> I think this is overly complicated and not necessary
> 
> 		    $prevline =~ /^\+\s*$Type\s*$Ident\s*\[\s*(0|1)\s*\]\s*;\s*$/) {
> 
> should work no?
> 
> ($Type already includes this from @typeList):
> 	qr{struct\s+$Ident},
> 	qr{union\s+$Ident},
> 	qr{enum\s+$Ident},

Hm, I didn't when I originally tried it. I will double-check.

> 
> > +			if ($1 == '0') {
> > +				if (WARN("ZERO_LENGTH_ARRAY",
> > +					 "Use C99 flexible arrays instead of zero-length arrays - see https://github.com/KSPP/linux/issues/78\n" . $hereprev) &&
> > +				    $fix) {
> > +					$fixed[$fixlinenr - 1] =~ s/\[0\]/[]/g;
> 
> And this $fix doesn't work if the line is struct foo bar[ 0 ];
> and the use of /g is odd.

Thanks!

> Because the message is a WARN and not an ERR, please use
> "Prefer/over" and not "Use/instead of"

I will change to ERR, then. We must not use [0]-sized arrays ever.

> 
> 					$fixed[$fixlinenr - 1] =~ s/\[\s*0\s*\]/[]/;
> > +				}
> > +			} else {
> > +				WARN("ONE_ELEMENT_ARRAY",
> > +				     "Use C99 flexible arrays instead of one-element arrays - see https://github.com/KSPP/linux/issues/79\n" . $hereprev);
> > +			}
> 
> And this could have a $fix change too
> 
> 				if (WARN("ONE_ELEMENT_ARRAY",
> 					 ...) &&
> 				    $fix) {
> 					$fixed[$fixlinenr - 1] =~ s/\[\s*1\s*\]/[]/;

Almost never is it possible to only fix the struct when it's a 1-element
array (due to the impact of the size changes that be present in other
code), so I left it out.

I'll send a v3.

-Kees

-- 
Kees Cook