Re: [PATCH v4] bpf: Replace bpf_lpm_trie_key 0-length array with flexible array

[Daniel Borkmann]

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@xxxxxxx>
> Closes: https://paste.debian.net/hidden/ca500597/
> Acked-by: Gustavo A. R. Silva <gustavoars@xxxxxxxxxx>
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
[...]

The build in BPF CI is still broken, did you try to build selftests?

  https://github.com/kernel-patches/bpf/actions/runs/7978647641

  [...]
    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; \
        |          ^~~
    BINARY   bench
  make: *** [Makefile:703: /tmp/work/bpf/bpf/tools/testing/selftests/bpf/test_cpp] Error 1
  make: *** Waiting for unfinished jobs....
  make: Leaving directory '/tmp/work/bpf/bpf/tools/testing/selftests/bpf'
  Error: Process completed with exit code 2.

> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 754e68ca8744..31e9bdd4641e 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -8,6 +8,7 @@
>   #ifndef _UAPI__LINUX_BPF_H__
>   #define _UAPI__LINUX_BPF_H__
>   
> +#include <linux/stddef.h>
>   #include <linux/types.h>
>   #include <linux/bpf_common.h>
>   
> @@ -77,12 +78,24 @@ struct bpf_insn {
>   	__s32	imm;		/* signed immediate constant */
>   };
>   
> -/* Key of an a BPF_MAP_TYPE_LPM_TRIE entry */
> +/* Deprecated: use struct bpf_lpm_trie_key_u8 (when the "data" member is needed for
> + * byte access) or struct bpf_lpm_trie_key_hdr (when using an alternative type for
> + * the trailing flexible array member) instead.
> + */
>   struct bpf_lpm_trie_key {
>   	__u32	prefixlen;	/* up to 32 for AF_INET, 128 for AF_INET6 */
>   	__u8	data[0];	/* Arbitrary size */
>   };
>   
> +/* Key of an a BPF_MAP_TYPE_LPM_TRIE entry, with trailing byte array. */
> +struct bpf_lpm_trie_key_u8 {
> +	__struct_group(bpf_lpm_trie_key_hdr, hdr, /* no attrs */,
> +		/* up to 32 for AF_INET, 128 for AF_INET6 */
> +		__u32	prefixlen;
> +	);
> +	__u8	data[];		/* Arbitrary size */
> +};
> +
>   struct bpf_cgroup_storage_key {
>   	__u64	cgroup_inode_id;	/* cgroup inode id */
>   	__u32	attach_type;		/* program attach type (enum bpf_attach_type) */