Re: [PATCH v3 0/4] s390: Enable Rust support and add required arch glue

[Miguel Ojeda]

On Tue, May 26, 2026 at 6:13 PM Heiko Carstens <hca@xxxxxxxxxxxxx> wrote:
>
> An Ack from you would be appreciated, even though you made it already
> clear that this series looks ok.

I did a quick `LLVM=1` test and booted it in QEMU. That worked and I
could see the KUnit tests, though we get the:

  warning: unstable feature specified for `-Ctarget-feature`: `backchain`

But I assume you are OK with that for now (and if you are, then
avoiding a custom target is worth it, I would say).

I also found a couple bits: in a KASAN build, I got [1] -- Cc'ing
Boqun in case he is interested:

  [    5.323670] ok 5 rust_allocator
  [    5.324134]     KTAP version 1
  [    5.324194]     # Subtest: rust_atomics
  [    5.324251]     # speed: normal
  [    5.324367]     1..8
  [    5.332184]     # atomic_basic_tests.speed: normal
  [    5.332356]     ok 1 atomic_basic_tests
  [    5.340797]     # atomic_acquire_release_tests.speed: normal
  [    5.341054]     ok 2 atomic_acquire_release_tests
  [    5.344127]
==================================================================
  [    5.344584] BUG: KASAN: stack-out-of-bounds in
rust_helper_atomic_i8_xchg+0xb2/0xc0
  [    5.345283] Read of size 4 at addr 001bff7ffdbefcf0 by task
kunit_try_catch/142
  [    5.345493]
  [    5.346499] CPU: 2 UID: 0 PID: 142 Comm: kunit_try_catch Tainted:
G                 N  7.1.0-rc4-00009-g60c6b4b89923 #10 PREEMPTLAZY
  [    5.347065] Tainted: [N]=TEST
  [    5.347124] Hardware name: QEMU 8561 QEMU (KVM/Linux)
  [    5.347323] Call Trace:
  [    5.347408]  [<001bfffffcb9c6b0>] dump_stack_lvl+0x80/0xa0
  [    5.347663]  [<001bfffffcb90846>] print_report+0xd6/0x310
  [    5.347784]  [<001bfffffd596e08>] kasan_report+0xe8/0x110
  [    5.347895]  [<001bfffffde5a042>] rust_helper_atomic_i8_xchg+0xb2/0xc0
  [    5.347983]  [<001bfffffde917fe>]
_RNvNtNtNtNtCseBLeUu9QniM_6kernel4sync6atomic9predefine5tests36kunit_rust_wrapper_atomic_xchg_tests+0x5be/0x4f90
  [    5.348088]  [<001bfffffdf59e50>] kunit_try_run_case+0x150/0x3d0
  [    5.348194]  [<001bfffffdf60500>]
kunit_generic_run_threadfn_adapter+0x90/0x130
  [    5.348257]  [<001bfffffccf9fde>] kthread+0x33e/0x410
  [    5.348309]  [<001bfffffcba122e>] __ret_from_fork+0x9e/0x3c0
  [    5.348357]  [<001bfffffec96faa>] ret_from_fork+0xa/0x30
  [    5.348531]
  [    5.349184]
  [    5.349261] The buggy address belongs to a vmalloc virtual mapping
  [    5.349620] The buggy address belongs to the physical page:
  [    5.350145] page: refcount:1 mapcount:0 mapping:0000000000000000
index:0x0 pfn:0x791f
  [    5.350563] flags: 0x1ffff00000000000(node=0|zone=0|lastcpupid=0x1ffff)
  [    5.351563] raw: 1ffff00000000000 0000000000000000
001b913b001e47c8 0000000000000000
  [    5.351694] raw: 0000000000000000 0000000000000000
ffffffff00000001 0000000000000000
  [    5.351808] page dumped because: kasan: bad access detected
  [    5.351912]
  [    5.351989] Memory state around the buggy address:
  [    5.352240]  001bff7ffdbefb80: f2 f2 f8 f8 f8 f8 f8 f8 f2 f2 f2
f2 f8 f2 f2 f2
  [    5.352338]  001bff7ffdbefc00: f8 f2 f2 f2 f8 f8 f2 f2 f8 f2 f2
f2 f8 f8 f8 f8
  [    5.352428] >001bff7ffdbefc80: f8 f8 f2 f2 f2 f2 f8 f2 f2 f2 f8
f2 f2 f2 01 f3
  [    5.352553]                                                              ^
  [    5.352762]  001bff7ffdbefd00: 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00
  [    5.352862]  001bff7ffdbefd80: 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00
  [    5.352976]
==================================================================

I also tried a GCC one without KASAN/KUnit, and got:

  error[E0425]: cannot find function `memchr` in crate `bindings`
       --> rust/kernel/str.rs:857:38
        |
    857 |         let ptr = unsafe {
bindings::memchr(buf.as_ptr().cast(), 0, f.bytes_written() - 1) };
        |                                      ^^^^^^
        |
       ::: rust/bindings/bindings_generated.rs:20572:5
        |
  20572 |     pub fn memcmp(s1: *const ffi::c_void, s2: *const
ffi::c_void, n: usize) -> ffi::c_int;
        |     --------------------------------------------------------------------------------------
similarly named function `memcmp` defined here
        |

So we likely want a Rust helper here in `rust/helpers/`.

Up to you which configs you want to support initially and how clean
you want everything to be, of course, so:

Acked-by: Miguel Ojeda <ojeda@xxxxxxxxxx>

I hope this helps!

Cheers,
Miguel