[PATCH] net: ipv6: fix ARM64 alignment fault in fib_multipath_hash_from_keys()

[Yung Chih Su <yuuchihsu@gmail.com>]

struct sysctl_fib_multipath_hash_seed contains two u32 fields (user_seed
and mp_seed), making it an 8-byte structure with a 4-byte alignment requirement.

In fib_multipath_hash_from_keys(), the code evaluates the entire struct
atomically via READ_ONCE():

mp_seed = READ_ONCE(net->ipv4.sysctl_fib_multipath_hash_seed).mp_seed;
While this silently works on GCC by falling back to unaligned regular loads
(e.g., LDR/LDUR) which the ARM64 kernel tolerates, it causes a fatal kernel
panic when compiled with Clang and LTO enabled.

Commit e35123d83ee3 ("arm64: lto: Strengthen READ_ONCE() to acquire when
CONFIG_LTO=y") strengthens READ_ONCE() to use Load-Acquire instructions
(ldar / ldapr) to prevent compiler reordering bugs under Clang LTO.

Since the macro evaluates the full 8-byte struct, Clang emits a 64-bit
ldar instruction. ARM64 architecture strictly requires ldar to be
naturally aligned. Executing a 64-bit ldar on a 4-byte aligned address
(e.g., ending in 0xEC) triggers a strict Alignment Fault (FSC = 0x21).

Fix this by moving the READ_ONCE() directly to the specific u32 member:

mp_seed = READ_ONCE(net->ipv4.sysctl_fib_multipath_hash_seed.mp_seed);
This instructs the compiler to emit a 32-bit load (ldar Wn or ldr Wn),
which perfectly satisfies the 4-byte alignment requirement and resolves
the crash.

Fixes: [4ee2a8cace3fb9a34aea6a56426f89d26dd514f3] ("net: ipv4: Add a sysctl to set multipath hash seed")
Signed-off-by: Yung Chih Su <yuuchihsu@gmail.com>
---
 include/net/ip_fib.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/net/ip_fib.h b/include/net/ip_fib.h
index b4495c38e0a0..318593743b6e 100644
--- a/include/net/ip_fib.h
+++ b/include/net/ip_fib.h
@@ -559,7 +559,7 @@ static inline u32 fib_multipath_hash_from_keys(const struct net *net,
 	siphash_aligned_key_t hash_key;
 	u32 mp_seed;
 
-	mp_seed = READ_ONCE(net->ipv4.sysctl_fib_multipath_hash_seed).mp_seed;
+	mp_seed = READ_ONCE(net->ipv4.sysctl_fib_multipath_hash_seed.mp_seed);
 	fib_multipath_hash_construct_key(&hash_key, mp_seed);
 
 	return flow_hash_from_keys_seed(keys, &hash_key);
-- 
2.43.0