linux-kernel - Re: [PATCH] net: seg6: initialize induction variable to first valid array index

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Message-Id: <20220804025245.b61ebbfd313a49927aa61f9a@uniroma2.it>
Date:   Thu, 4 Aug 2022 02:52:45 +0200
From:   Andrea Mayer <andrea.mayer@...roma2.it>
To:     Nick Desaulniers <ndesaulniers@...gle.com>,
        David Ahern <dsahern@...nel.org>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        "David S. Miller" <davem@...emloft.net>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Tom Rix <trix@...hat.com>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
        Andrea Mayer <andrea.mayer@...roma2.it>
Subject: Re: [PATCH] net: seg6: initialize induction variable to first valid
 array index

Hi Nick,
please see below, thanks.

> [ cc Andrea ]
@David Ahern: thanks David!

> On 8/2/22 10:12 AM, Nick Desaulniers wrote:
> > Fixes the following warnings observed when building
> > CONFIG_IPV6_SEG6_LWTUNNEL=y with clang:
> > 
> >   net/ipv6/seg6_local.o: warning: objtool: seg6_local_fill_encap() falls
> >   through to next function seg6_local_get_encap_size()
> >   net/ipv6/seg6_local.o: warning: objtool: seg6_local_cmp_encap() falls
> >   through to next function input_action_end()
> > 
> > LLVM can fully unroll loops in seg6_local_get_encap_size() and
> > seg6_local_cmp_encap(). One issue in those loops is that the induction
> > variable is initialized to 0. The loop iterates over members of
> > seg6_action_params, a global array of struct seg6_action_param calling
> > their put() function pointer members.  seg6_action_param uses an array
> > initializer to initialize SEG6_LOCAL_SRH and later elements, which is
> > the third enumeration of an anonymous union.
> > 
> > The guard `if (attrs & SEG6_F_ATTR(i))` may prevent this from being
> > called at runtime, but it would still be UB for
> > `seg6_action_params[0]->put` to be called; the unrolled loop will make
> > the initial iterations unreachable, which LLVM will later rotate to
> > fallthrough to the next function.
> > 
> > Make this more obvious that this cannot happen to the compiler by
> > initializing the loop induction variable to the minimum valid index that
> > seg6_action_params is initialized to.

By looking at the patch, it looks that your changes seem reasonable.
The first two SEG6_LOCAL_{UNSPEC,ACTION} attributes are quite "special" in the
sense that they do not have the seg6_action_param callbacks set.
The SEG6_LOCAL_UNSPEC attribute is practically not used; while
SEG6_LOCAL_ACTION is handled out of the several loops.

Hence, I think we can skip these two attributes (in the loops) by setting the
induction variable to the value you proposed, i.e. SEG6_LOCAL_SRH.

Ciao,
Andrea

> > 
> > Reported-by: Thomas Gleixner <tglx@...utronix.de>
> > Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
> > ---
> >  net/ipv6/seg6_local.c | 10 +++++-----
> >  1 file changed, 5 insertions(+), 5 deletions(-)
> > 
> > diff --git a/net/ipv6/seg6_local.c b/net/ipv6/seg6_local.c
> > index 2cd4a8d3b30a..b7de5e46fdd8 100644
> > --- a/net/ipv6/seg6_local.c
> > +++ b/net/ipv6/seg6_local.c
> > @@ -1614,7 +1614,7 @@ static void __destroy_attrs(unsigned long parsed_attrs, int max_parsed,
> >  	 * callback. If the callback is not available, then we skip to the next
> >  	 * attribute; otherwise, we call the destroy() callback.
> >  	 */
> > -	for (i = 0; i < max_parsed; ++i) {
> > +	for (i = SEG6_LOCAL_SRH; i < max_parsed; ++i) {
> >  		if (!(parsed_attrs & SEG6_F_ATTR(i)))
> >  			continue;
> >  
> > @@ -1643,7 +1643,7 @@ static int parse_nla_optional_attrs(struct nlattr **attrs,
> >  	struct seg6_action_param *param;
> >  	int err, i;
> >  
> > -	for (i = 0; i < SEG6_LOCAL_MAX + 1; ++i) {
> > +	for (i = SEG6_LOCAL_SRH; i < SEG6_LOCAL_MAX + 1; ++i) {
> >  		if (!(desc->optattrs & SEG6_F_ATTR(i)) || !attrs[i])
> >  			continue;
> >  
> > @@ -1742,7 +1742,7 @@ static int parse_nla_action(struct nlattr **attrs, struct seg6_local_lwt *slwt)
> >  	}
> >  
> >  	/* parse the required attributes */
> > -	for (i = 0; i < SEG6_LOCAL_MAX + 1; i++) {
> > +	for (i = SEG6_LOCAL_SRH; i < SEG6_LOCAL_MAX + 1; i++) {
> >  		if (desc->attrs & SEG6_F_ATTR(i)) {
> >  			if (!attrs[i])
> >  				return -EINVAL;
> > @@ -1847,7 +1847,7 @@ static int seg6_local_fill_encap(struct sk_buff *skb,
> >  
> >  	attrs = slwt->desc->attrs | slwt->parsed_optattrs;
> >  
> > -	for (i = 0; i < SEG6_LOCAL_MAX + 1; i++) {
> > +	for (i = SEG6_LOCAL_SRH; i < SEG6_LOCAL_MAX + 1; i++) {
> >  		if (attrs & SEG6_F_ATTR(i)) {
> >  			param = &seg6_action_params[i];
> >  			err = param->put(skb, slwt);
> > @@ -1927,7 +1927,7 @@ static int seg6_local_cmp_encap(struct lwtunnel_state *a,
> >  	if (attrs_a != attrs_b)
> >  		return 1;
> >  
> > -	for (i = 0; i < SEG6_LOCAL_MAX + 1; i++) {
> > +	for (i = SEG6_LOCAL_SRH; i < SEG6_LOCAL_MAX + 1; i++) {
> >  		if (attrs_a & SEG6_F_ATTR(i)) {
> >  			param = &seg6_action_params[i];
> >  			if (param->cmp(slwt_a, slwt_b))
>