[Bug report] hash_name() may cross page boundary and trigger sleep in RCU context

[Zizhi Wo]

We're running into the following issue on an ARM32 platform with the linux
5.10 kernel:

[<c0300b78>] (__dabt_svc) from [<c0529cb8>] (link_path_walk.part.7+0x108/0x45c)
[<c0529cb8>] (link_path_walk.part.7) from [<c052a948>] (path_openat+0xc4/0x10ec)
[<c052a948>] (path_openat) from [<c052cf90>] (do_filp_open+0x9c/0x114)
[<c052cf90>] (do_filp_open) from [<c0511e4c>] (do_sys_openat2+0x418/0x528)
[<c0511e4c>] (do_sys_openat2) from [<c0513d98>] (do_sys_open+0x88/0xe4)
[<c0513d98>] (do_sys_open) from [<c03000c0>] (ret_fast_syscall+0x0/0x58)
...
[<c0315e34>] (unwind_backtrace) from [<c030f2b0>] (show_stack+0x20/0x24)
[<c030f2b0>] (show_stack) from [<c14239f4>] (dump_stack+0xd8/0xf8)
[<c14239f4>] (dump_stack) from [<c038d188>] (___might_sleep+0x19c/0x1e4)
[<c038d188>] (___might_sleep) from [<c031b6fc>] (do_page_fault+0x2f8/0x51c)
[<c031b6fc>] (do_page_fault) from [<c031bb44>] (do_DataAbort+0x90/0x118)
[<c031bb44>] (do_DataAbort) from [<c0300b78>] (__dabt_svc+0x58/0x80)
...

During the execution of hash_name()->load_unaligned_zeropad(), a potential
memory access beyond the PAGE boundary may occur. For example, when the
filename length is near the PAGE_SIZE boundary. This triggers a page fault,
which leads to a call to do_page_fault()->mmap_read_trylock(). If we can't
acquire the lock, we have to fall back to the mmap_read_lock() path, which
calls might_sleep(). This breaks RCU semantics because path lookup occurs
under an RCU read-side critical section. In linux-mainline, arm/arm64
do_page_fault() still has this problem:

lock_mm_and_find_vma->get_mmap_lock_carefully->mmap_read_lock_killable.

And before commit bfcfaa77bdf0 ("vfs: use 'unsigned long' accesses for
dcache name comparison and hashing"), hash_name accessed the name byte by
byte.

To prevent load_unaligned_zeropad() from accessing beyond the valid memory
region, we would need to intercept such cases beforehand? But doing so
would require replicating the internal logic of load_unaligned_zeropad(),
including handling endianness and constructing the correct value manually.
Given that load_unaligned_zeropad() is used in many places across the
kernel, we currently haven't found a good solution to address this cleanly.

What would be the recommended way to handle this situation? Would
appreciate any feedback and guidance from the community. Thanks!