On Wed, Feb 21, 2024 at 05:39:55PM +0100, Daniel Borkmann wrote:
On 2/20/24 7:54 PM, Kees Cook wrote:
Replace deprecated 0-length array in struct bpf_lpm_trie_key with flexible array. Found with GCC 13:
../kernel/bpf/lpm_trie.c:207:51: warning: array subscript i is outside array bounds of 'const __u8[0]' {aka 'const unsigned char[]'} [-Warray-bounds=] 207 | *(__be16 *)&key->data[i]); | ^~~~~~~~~~~~~ ../include/uapi/linux/swab.h:102:54: note: in definition of macro '__swab16' 102 | #define __swab16(x) (__u16)__builtin_bswap16((__u16)(x)) | ^ ../include/linux/byteorder/generic.h:97:21: note: in expansion of macro '__be16_to_cpu' 97 | #define be16_to_cpu __be16_to_cpu | ^~~~~~~~~~~~~ ../kernel/bpf/lpm_trie.c:206:28: note: in expansion of macro 'be16_to_cpu' 206 | u16 diff = be16_to_cpu(*(__be16 *)&node->data[i] ^ | ^~~~~~~~~~~ In file included from ../include/linux/bpf.h:7: ../include/uapi/linux/bpf.h:82:17: note: while referencing 'data' 82 | __u8 data[0]; /* Arbitrary size */ | ^~~~
And found at run-time under CONFIG_FORTIFY_SOURCE:
UBSAN: array-index-out-of-bounds in kernel/bpf/lpm_trie.c:218:49 index 0 is out of range for type '__u8 [*]'
Changing struct bpf_lpm_trie_key is difficult since has been used by userspace. For example, in Cilium:
struct egress_gw_policy_key { struct bpf_lpm_trie_key lpm_key; __u32 saddr; __u32 daddr; };
While direct references to the "data" member haven't been found, there are static initializers what include the final member. For example, the "{}" here:
struct egress_gw_policy_key in_key = { .lpm_key = { 32 + 24, {} }, .saddr = CLIENT_IP, .daddr = EXTERNAL_SVC_IP & 0Xffffff, };To avoid the build time and run time warnings seen with a 0-sized trailing array for struct bpf_lpm_trie_key, introduce a new struct that correctly uses a flexible array for the trailing bytes, struct bpf_lpm_trie_key_u8. As part of this, include the "header" portion (which is just the "prefixlen" member), so it can be used by anything building a bpf_lpr_trie_key that has trailing members that aren't a u8 flexible array (like the self-test[1]), which is named struct bpf_lpm_trie_key_hdr.
Adjust the kernel code to use struct bpf_lpm_trie_key_u8 through-out, and for the selftest to use struct bpf_lpm_trie_key_hdr. Add a comment to the UAPI header directing folks to the two new options.
Link: https://lore.kernel.org/all/202206281009.4332AA33@keescook/ [1] Reported-by: Mark Rutland mark.rutland@arm.com Closes: https://paste.debian.net/hidden/ca500597/ Acked-by: Gustavo A. R. Silva gustavoars@kernel.org Signed-off-by: Kees Cook keescook@chromium.org
[...]
The build in BPF CI is still broken, did you try to build selftests?
I did! I didn't have this error. :(
https://github.com/kernel-patches/bpf/actions/runs/7978647641
Thanks for the pointer. It took a bit of digging, but I found this: https://github.com/libbpf/ci/blob/main/build-selftests/build_selftests.sh which is much more involved than just "make -C tools/testing/selftests/bpf"
[...] GEN-SKEL [test_progs] linked_funcs.skel.h LINK-BPF [test_progs] test_usdt.bpf.o GEN-SKEL [test_progs-no_alu32] profiler1.skel.h GEN-SKEL [test_progs] test_usdt.skel.h In file included from /tmp/work/bpf/bpf/tools/include/uapi/linux/bpf.h:11, from test_cpp.cpp:4: /tmp/work/bpf/bpf/tools/include/uapi/linux/bpf.h:92:17: error: ‘struct bpf_lpm_trie_key_u8::<unnamed union>::bpf_lpm_trie_key_hdr’ invalid; an anonymous union may only have public non-static data members [-fpermissive] 92 | __struct_group(bpf_lpm_trie_key_hdr, hdr, /* no attrs */, | ^~~~~~~~~~~~~~~~~~~~ /tmp/work/bpf/bpf/tools/include/uapi/linux/stddef.h:29:10: note: in definition of macro ‘__struct_group’ 29 | struct TAG { MEMBERS } ATTRS NAME; \ | ^~~
I'll see if I can figure out where this is coming from. This sounds like something is being built with an unexpectedly strict option. (The above report seems weird -- this isn't coming from -fpermissive, and is actually an _error_ not a warning, which is the opposite of what -fpermissive is supposed to do.) Also the mention of "public" is scary here... that implies a C++ compiler is involved? Maybe that's why my builds didn't catch this?
make: *** [Makefile:703: /tmp/work/bpf/bpf/tools/testing/selftests/bpf/test_cpp] Error 1
Ah yes, cpp. Fun. I will try to reproduce this failure.