[PATCH bpf-next] bpf: pack struct bpf_fib_lookup

From: Anton Protopopov <aspsk@isovalent.com>
To: Alexei Starovoitov <ast@kernel.org>,
	Andrii Nakryiko <andrii@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Jiri Olsa <jolsa@kernel.org>,
	Martin KaFai Lau <martin.lau@linux.dev>,
	Stanislav Fomichev <sdf@google.com>,
	bpf@vger.kernel.org
Cc: Anton Protopopov <aspsk@isovalent.com>,
	Alexander Lobakin <aleksander.lobakin@intel.com>,
	Arnd Bergmann <arnd@arndb.de>,
	Naresh Kamboju <naresh.kamboju@linaro.org>
Subject: [PATCH bpf-next] bpf: pack struct bpf_fib_lookup
Date: Wed,  3 Apr 2024 14:33:03 +0200	[thread overview]
Message-ID: <20240403123303.1452184-1-aspsk@isovalent.com> (raw)

The struct bpf_fib_lookup is supposed to be of size 64. A recent commit
59b418c7063d ("bpf: Add a check for struct bpf_fib_lookup size") added
a static assertion to check this property so that future changes to the
structure will not accidentally break this assumption.

As it immediately turned out, on some 32-bit arm systems, when AEABI=n,
the total size of the structure was equal to 68, see [1]. This happened
because the bpf_fib_lookup structure contains a union of two 16-bit
fields:

    union {
            __u16 tot_len;
            __u16 mtu_result;
    };

which was supposed to compile to a 16-bit-aligned 16-bit field. On the
aforementioned setups it was instead both aligned and padded to 32-bits.

Declare this inner union as __attribute__((packed, aligned(2))) such
that it always is of size 2 and is aligned to 16 bits.

  [1] https://lore.kernel.org/all/CA+G9fYtsoP51f-oP_Sp5MOq-Ffv8La2RztNpwvE6+R1VtFiLrw@mail.gmail.com/#t

Signed-off-by: Anton Protopopov <aspsk@isovalent.com>
---
 include/uapi/linux/bpf.h       | 2 +-
 tools/include/uapi/linux/bpf.h | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 79c548276b6b..6fe9f11c8abe 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -7157,7 +7157,7 @@ struct bpf_fib_lookup {
 
 		/* output: MTU value */
 		__u16	mtu_result;
-	};
+	} __attribute__((packed, aligned(2)));
 	/* input: L3 device index for lookup
 	 * output: device index from FIB lookup
 	 */
diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index 79c548276b6b..6fe9f11c8abe 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -7157,7 +7157,7 @@ struct bpf_fib_lookup {
 
 		/* output: MTU value */
 		__u16	mtu_result;
-	};
+	} __attribute__((packed, aligned(2)));
 	/* input: L3 device index for lookup
 	 * output: device index from FIB lookup
 	 */
-- 
2.34.1

