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SubjectRe: [PATCH] net: seg6: initialize induction variable to first valid array index
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[ cc Andrea ]

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.
>
> Reported-by: Thomas Gleixner <tglx@linutronix.de>
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.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))

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