Re: [PATCH 19/64] ip: Use struct_group() for memcpy() regions

Greg Kroah-Hartman
2021-07-28T06:19:48Z

On Wed, Jul 28, 2021 at 01:14:33AM -0500, Gustavo A. R. Silva wrote:
>
>
> On 7/28/21 00:55, Greg Kroah-Hartman wrote:
> > On Tue, Jul 27, 2021 at 01:58:10PM -0700, Kees Cook wrote:
> >> In preparation for FORTIFY_SOURCE performing compile-time and run-time
> >> field bounds checking for memcpy(), memmove(), and memset(), avoid
> >> intentionally writing across neighboring fields.
> >>
> >> Use struct_group() in struct flowi4, struct ipv4hdr, and struct ipv6hdr
> >> around members saddr and daddr, so they can be referenced together. This
> >> will allow memcpy() and sizeof() to more easily reason about sizes,
> >> improve readability, and avoid future warnings about writing beyond the
> >> end of saddr.
> >>
> >> "pahole" shows no size nor member offset changes to struct flowi4.
> >> "objdump -d" shows no meaningful object code changes (i.e. only source
> >> line number induced differences.)
> >>
> >> Note that since this is a UAPI header, struct_group() has been open
> >> coded.
> >>
> >> Signed-off-by: Kees Cook <keescook@chromium.org>
> >> ---
> >> include/net/flow.h | 6 ++++--
> >> include/uapi/linux/if_ether.h | 12 ++++++++++--
> >> include/uapi/linux/ip.h | 12 ++++++++++--
> >> include/uapi/linux/ipv6.h | 12 ++++++++++--
> >> net/core/flow_dissector.c | 10 ++++++----
> >> net/ipv4/ip_output.c | 6 ++----
> >> 6 files changed, 42 insertions(+), 16 deletions(-)
> >>
> >> diff --git a/include/net/flow.h b/include/net/flow.h
> >> index 6f5e70240071..f1a3b6c8eae2 100644
> >> --- a/include/net/flow.h
> >> +++ b/include/net/flow.h
> >> @@ -81,8 +81,10 @@ struct flowi4 {
> >> #define flowi4_multipath_hash __fl_common.flowic_multipath_hash
> >>
> >> /* (saddr,daddr) must be grouped, same order as in IP header */
> >> - __be32 saddr;
> >> - __be32 daddr;
> >> + struct_group(addrs,
> >> + __be32 saddr;
> >> + __be32 daddr;
> >> + );
> >>
> >> union flowi_uli uli;
> >> #define fl4_sport uli.ports.sport
> >> diff --git a/include/uapi/linux/if_ether.h b/include/uapi/linux/if_ether.h
> >> index a0b637911d3c..8f5667b2ea92 100644
> >> --- a/include/uapi/linux/if_ether.h
> >> +++ b/include/uapi/linux/if_ether.h
> >> @@ -163,8 +163,16 @@
> >>
> >> #if __UAPI_DEF_ETHHDR
> >> struct ethhdr {
> >> - unsigned char h_dest[ETH_ALEN]; /* destination eth addr */
> >> - unsigned char h_source[ETH_ALEN]; /* source ether addr */
> >> + union {
> >> + struct {
> >> + unsigned char h_dest[ETH_ALEN]; /* destination eth addr */
> >> + unsigned char h_source[ETH_ALEN]; /* source ether addr */
> >> + };
> >> + struct {
> >> + unsigned char h_dest[ETH_ALEN]; /* destination eth addr */
> >> + unsigned char h_source[ETH_ALEN]; /* source ether addr */
> >> + } addrs;
> >
> > A union of the same fields in the same structure in the same way?
> >
> > Ah, because struct_group() can not be used here? Still feels odd to see
> > in a userspace-visible header.
> >
> >> + };
> >> __be16 h_proto; /* packet type ID field */
> >> } __attribute__((packed));
> >> #endif
> >> diff --git a/include/uapi/linux/ip.h b/include/uapi/linux/ip.h
> >> index e42d13b55cf3..33647a37e56b 100644
> >> --- a/include/uapi/linux/ip.h
> >> +++ b/include/uapi/linux/ip.h
> >> @@ -100,8 +100,16 @@ struct iphdr {
> >> __u8 ttl;
> >> __u8 protocol;
> >> __sum16 check;
> >> - __be32 saddr;
> >> - __be32 daddr;
> >> + union {
> >> + struct {
> >> + __be32 saddr;
> >> + __be32 daddr;
> >> + } addrs;
> >> + struct {
> >> + __be32 saddr;
> >> + __be32 daddr;
> >> + };
> >
> > Same here (except you named the first struct addrs, not the second,
> > unlike above).
> >
> >
> >> + };
> >> /*The options start here. */
> >> };
> >>
> >> diff --git a/include/uapi/linux/ipv6.h b/include/uapi/linux/ipv6.h
> >> index b243a53fa985..1c26d32e733b 100644
> >> --- a/include/uapi/linux/ipv6.h
> >> +++ b/include/uapi/linux/ipv6.h
> >> @@ -130,8 +130,16 @@ struct ipv6hdr {
> >> __u8 nexthdr;
> >> __u8 hop_limit;
> >>
> >> - struct in6_addr saddr;
> >> - struct in6_addr daddr;
> >> + union {
> >> + struct {
> >> + struct in6_addr saddr;
> >> + struct in6_addr daddr;
> >> + } addrs;
> >> + struct {
> >> + struct in6_addr saddr;
> >> + struct in6_addr daddr;
> >> + };
> >
> > addrs first? Consistancy is key :)
>
> I think addrs should be second. In general, I think all newly added
> non-anonymous structures should be second.

Why not use a local version of the macro like was done in the DRM header
file, to make it always work the same and more obvious what is
happening? If I were a userspace developer and saw the above, I would
think that the kernel developers have lost it :)

thanks,

greg k-h