Messages in this thread Patch in this message | | | Date | Thu, 4 Aug 2022 02:52:45 +0200 | From | Andrea Mayer <> | Subject | Re: [PATCH] net: seg6: initialize induction variable to first valid array index |
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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@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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