netdev - Re: ether_addr_equal_64bits breakage with gcc-12

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Date:   Fri, 11 Feb 2022 16:35:41 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Marc Kleine-Budde <mkl@...gutronix.de>,
        Kees Cook <keescook@...omium.org>
Cc:     Eric Dumazet <edumazet@...gle.com>, netdev@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: ether_addr_equal_64bits breakage with gcc-12

On Fri, 11 Feb 2022 15:12:13 +0100 Marc Kleine-Budde wrote:
> Hello,
> 
> the current arm-linux-gnueabihf-gcc 12 snapshot in Debian breaks (at
> least with CONFIG_WERROR=y):
> 
> |   CC      net/core/dev.o
> | net/core/dev.c: In function ‘bpf_prog_run_generic_xdp’:
> | net/core/dev.c:4618:21: warning: ‘ether_addr_equal_64bits’ reading 8 bytes from a region of size 6 [-Wstringop-overread]
> |  4618 |         orig_host = ether_addr_equal_64bits(eth->h_dest, skb->dev->dev_addr);
> |       |                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> | net/core/dev.c:4618:21: note: referencing argument 1 of type ‘const u8[8]’ {aka ‘const unsigned char[8]’}
> | net/core/dev.c:4618:21: note: referencing argument 2 of type ‘const u8[8]’ {aka ‘const unsigned char[8]’}
> | In file included from net/core/dev.c:91:
> | include/linux/etherdevice.h:375:20: note: in a call to function ‘ether_addr_equal_64bits’
> |   375 | static inline bool ether_addr_equal_64bits(const u8 addr1[6+2],
> |       |                    ^~~~~~~~~~~~~~~~~~~~~~~
> | net/core/dev.c:4619:22: warning: ‘is_multicast_ether_addr_64bits’ reading 8 bytes from a region of size 6 [-Wstringop-overread]
> |  4619 |         orig_bcast = is_multicast_ether_addr_64bits(eth->h_dest);
> |       |                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> | net/core/dev.c:4619:22: note: referencing argument 1 of type ‘const u8[8]’ {aka ‘const unsigned char[8]’}
> | include/linux/etherdevice.h:137:20: note: in a call to function ‘is_multicast_ether_addr_64bits’
> |   137 | static inline bool is_multicast_ether_addr_64bits(const u8 addr[6+2])
> |       |                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> | net/core/dev.c:4646:27: warning: ‘ether_addr_equal_64bits’ reading 8 bytes from a region of size 6 [-Wstringop-overread]
> |  4646 |             (orig_host != ether_addr_equal_64bits(eth->h_dest,
> |       |                           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> |  4647 |                                                   skb->dev->dev_addr)) ||
> |       |                                                   ~~~~~~~~~~~~~~~~~~~
> | net/core/dev.c:4646:27: note: referencing argument 1 of type ‘const u8[8]’ {aka ‘const unsigned char[8]’}
> | net/core/dev.c:4646:27: note: referencing argument 2 of type ‘const u8[8]’ {aka ‘const unsigned char[8]’}
> | include/linux/etherdevice.h:375:20: note: in a call to function ‘ether_addr_equal_64bits’
> |   375 | static inline bool ether_addr_equal_64bits(const u8 addr1[6+2],
> |       |                    ^~~~~~~~~~~~~~~~~~~~~~~
> | net/core/dev.c:4648:28: warning: ‘is_multicast_ether_addr_64bits’ reading 8 bytes from a region of size 6 [-Wstringop-overread]
> |  4648 |             (orig_bcast != is_multicast_ether_addr_64bits(eth->h_dest))) {
> |       |                            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> | net/core/dev.c:4648:28: note: referencing argument 1 of type ‘const u8[8]’ {aka ‘const unsigned char[8]’}
> | include/linux/etherdevice.h:137:20: note: in a call to function ‘is_multicast_ether_addr_64bits’
> |   137 | static inline bool is_multicast_ether_addr_64bits(const u8 addr[6+2])
> |       |                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 
> | arm-linux-gnueabihf-gcc -v
> | Using built-in specs.
> | COLLECT_GCC=/usr/bin/arm-linux-gnueabihf-gcc
> | COLLECT_LTO_WRAPPER=/usr/lib/gcc-cross/arm-linux-gnueabihf/12/lto-wrapper
> | Target: arm-linux-gnueabihf
> | Configured with: ../src/configure -v --with-pkgversion='Debian 12-20220126-1' --with-bugurl=file:///usr/share/doc/gcc-12/README.Bugs --enable-languages=c,ada,c++,go,d,fortran,objc,obj-c++,m2 --prefix=/usr --with-gcc-major-version-only --program-suffix=-12 --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-gnu-unique-object --disable-libitm --disable-libquadmath --disable-libquadmath-support --enable-plugin --enable-default-pie --with-system-zlib --enable-libphobos-checking=release --without-target-system-zlib --enable-multiarch --disable-sjlj-exceptions --with-arch=armv7-a+fp --with-float=hard --with-mode=thumb --disable-werror --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=arm-linux-gnueabihf --program-prefix=arm-linux-gnueabihf- --includedir=/usr/arm-linux-gnueabihf/include
> | Thread model: posix
> | Supported LTO compression algorithms: zlib zstd
> | gcc version 12.0.1 20220126 (experimental) [master r12-6872-gf3e6ef7d873] (Debian 12-20220126-1)

Maybe Kees will have as suggestion - Kees, are there any best practices
for dealing with such issues? For the reference we do a oversized load
from a structure (read 8B of a 6B array):

struct ethhdr {
	unsigned char	h_dest[6];
	unsigned char	h_source[6];
	__be16		h_proto;
} __attribute__((packed));

But then discard the irrelevant bytes:

#if defined(CONFIG_HAVE_EFFICIENT...
	u64 fold = (*(const u64 *)addr1) ^ (*(const u64 *)addr2);
#ifdef __BIG_ENDIAN
	return (fold >> 16) == 0;
#else ...


The structure is uAPI, for added fun.