Re: [PATCH v2] net: ipv4: fix ARM64 alignment fault in multipath hash seed

Hi Eric,

Got it! Thank you so much for the clarification and all your help with
the review.

Best regards,
Yung Chih.

Eric Dumazet [off-list ref] 於 2026年3月2日週一 下午2:12寫道：

On Mon, Mar 2, 2026 at 7:03 AM Yung Chih Su [off-list ref] wrote:

quoted

`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 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, thus executing it on a 4-byte
aligned address triggers a strict Alignment Fault (FSC = 0x21).

Fix the read side by moving the `READ_ONCE()` directly to the `u32`
member, which emits a safe 32-bit `ldar Wn`.

Furthermore, Eric Dumazet pointed out that `WRITE_ONCE()` on the entire
struct in `proc_fib_multipath_hash_set_seed()` is also flawed. Analysis
shows that Clang splits this 8-byte write into two separate 32-bit
`str` instructions. While this avoids an alignment fault, it destroys
atomicity and exposes a tear-write vulnerability. Fix this by
explicitly splitting the write into two 32-bit `WRITE_ONCE()`
operations.

Finally, add the missing `READ_ONCE()` when reading `user_seed` in
`proc_fib_multipath_hash_seed()` to ensure proper pairing and
concurrency safety.

Fixes: 4ee2a8cace3f ("net: ipv4: Add a sysctl to set multipath hash seed")
Suggested-by: Eric Dumazet <edumazet@google.com>

Nit: Use of Suggested-by: implies I made the original suggestion for
this patch (before the V1)
I simply gave a feedback on your V1. The idea of the patch came from you :)

So next time, do not add a "Suggested-by:", as the expected step is
that I add a Reviewed-by tag when I am happy with a new version,
if I see it in time before it is merged.

No need for a V3, this is simply a reminder for your next patches.

quoted

Signed-off-by: Yung Chih Su <redacted>
---

Reviewed-by: Eric Dumazet <edumazet@google.com>

Thanks !