Re: Re: [PATCH] riscv: fix KUnit test_kprobes crash when building with Clang

[Nathan Chancellor <nathan@kernel.org>]

Hi Paul and Jiakai,

On Tue, Nov 18, 2025 at 01:21:50PM -0700, Paul Walmsley wrote:
> Not all of it, but some edited summary is appropriate.  I've queued the 
> following revision for v6.18-rc fixes, under the theory that it would be 
> good to get the test working for Clang users while the LLVM folks 
> investigate.  Please let us know ASAP if you think anything in it should 
> be changed.  I plan to send a PR out later this week.

I spent some time today trying to minimize a small set of configurations
that triggers this issue on top of defconfig since that occasionally
reveals a path to head down for triage and I landed on:

  $ cat arch/riscv/configs/repro.config
  CONFIG_KPROBES=y
  CONFIG_KUNIT=y
  CONFIG_RELOCATABLE=y
  CONFIG_RISCV_KPROBES_KUNIT=y

  $ make -skj"$(nproc)" ARCH=riscv LLVM=1 clean defconfig repro.config Image

  $ curl -LSs https://github.com/ClangBuiltLinux/boot-utils/releases/download/20241120-044434/riscv-rootfs.cpio.zst | zstd -d >rootfs.cpio

  $ qemu-system-riscv64 \
      -display none \
      -nodefaults \
      -bios default \
      -M virt \
      -append earlycon \
      -kernel arch/riscv/boot/Image \
      -initrd rootfs.cpio \
      -m 512m \
      -serial mon:stdio
  ...
  [    0.000000] Booting Linux on hartid 0
  [    0.000000] Linux version 6.18.0-rc6-dirty (nathan@ax162) (ClangBuiltLinux clang version 21.1.5 (https://github.com/llvm/llvm-project.git 8e2cd28cd4ba46613a46467b0c91b1cabead26cd), ClangBuiltLinux LLD 21.1.5 (https://github.com/llvm/llvm-project.git 8e2cd28cd4ba46613a46467b0c91b1cabead26cd)) #1 SMP Tue Nov 18 17:58:23 MST 2025
  ...
  [    1.041311] KTAP version 1
  [    1.041433] 1..1
  [    1.042111]     KTAP version 1
  [    1.042310]     # Subtest: kprobes_riscv
  [    1.042610]     # module: kprobes_riscv_kunit
  [    1.042732]     1..1
  [    1.047104] ------------[ cut here ]------------
  [    1.047293] WARNING: CPU: 0 PID: 45 at mm/page_alloc.c:5159 __alloc_frozen_pages_noprof+0x160/0x228
  [    1.047602] Modules linked in:
  [    1.047851] CPU: 0 UID: 0 PID: 45 Comm: kunit_try_catch Tainted: G                 N  6.18.0-rc6-dirty #1 NONE
  [    1.047933] Tainted: [N]=TEST
  [    1.047948] Hardware name: riscv-virtio,qemu (DT)
  [    1.048024] epc : __alloc_frozen_pages_noprof+0x160/0x228
  [    1.048057]  ra : ___kmalloc_large_node+0x52/0x1a0
  [    1.048073] epc : ffffffff8023e2e8 ra : ffffffff80247c26 sp : ff200000001f3c10
  [    1.048086]  gp : ffffffff81a25900 tp : ff600000023924c0 t0 : 00000000264163d0
  [    1.048097]  t1 : 00000000264163d0 t2 : 00000000000aae60 s0 : ff200000001f3c90
  [    1.048108]  s1 : 000000000a83ca20 a0 : 0000000000000001 a1 : ffffffff819e71ba
  [    1.048119]  a2 : 0000000000000000 a3 : 0000000000000000 a4 : 000000003b9aca00
  [    1.048130]  a5 : 0000000000016698 a6 : ff600000021480d0 a7 : 00000000000aae60
  [    1.048140]  s2 : 0000000000000010 s3 : ffffffff8024857a s4 : 0000000000000dc0
  [    1.048151]  s5 : ffffffff80018e20 s6 : ff2000000000bb40 s7 : 0000000000000000
  [    1.048161]  s8 : 0000000000000000 s9 : 0000000000166e7c s10: 0000000000000000
  [    1.048171]  s11: 0000000000000000 t3 : 0000000000000000 t4 : 0000000000000002
  [    1.048181]  t5 : ff60000002392580 t6 : 0000000000000001
  [    1.048191] status: 0000000200000120 badaddr: ffffffff8023e2e8 cause: 0000000000000003
  [    1.048281] [<ffffffff8023e2e8>] __alloc_frozen_pages_noprof+0x160/0x228
  [    1.048362] [<ffffffff80247c26>] ___kmalloc_large_node+0x52/0x1a0
  [    1.048374] [<ffffffff80247dd8>] __kmalloc_large_node_noprof+0x1c/0x114
  [    1.048384] [<ffffffff8024857a>] __kmalloc_noprof+0x252/0x450
  [    1.048393] [<ffffffff80018ae6>] test_kprobe_riscv+0x5a/0x210
  [    1.048406] [<ffffffff8050d6a6>] kunit_try_run_case+0x5a/0x15c
  [    1.048417] [<ffffffff8050f69a>] kunit_generic_run_threadfn_adapter+0x1a/0x34
  [    1.048439] [<ffffffff8004fa88>] kthread+0x178/0x1a0
  [    1.048450] [<ffffffff8001311a>] ret_from_fork_kernel+0xe/0x48
  [    1.048464] [<ffffffff80b500f2>] ret_from_fork_kernel_asm+0x16/0x1c
  [    1.048513] ---[ end trace 0000000000000000 ]---
  [    1.056832]     # test_kprobe_riscv: EXPECTATION FAILED at arch/riscv/kernel/tests/kprobes/test-kprobes.c:24
  [    1.056832]     Expected kp to be true, but is false
  [    1.058370]     not ok 1 test_kprobe_riscv
  [    1.058484] not ok 1 kprobes_riscv
  ...

This appears to happen back to LLVM 15 so I am not sure it is a recent
recent regression on the toolchain side.

Without CONFIG_RELOCATABLE enabled, the test passes:

  [    1.049384] KTAP version 1
  [    1.049656] 1..1
  [    1.050246]     KTAP version 1
  [    1.050447]     # Subtest: kprobes_riscv
  [    1.050760]     # module: kprobes_riscv_kunit
  [    1.050867]     1..1
  [    1.061790]     ok 1 test_kprobe_riscv
  [    1.061929] ok 1 kprobes_riscv

Interestingly, if I enable CONFIG_RISCV_KPROBES_KUNIT as a module, I get
a failure when trying to load it in QEMU:

  ~ # modprobe kprobes_riscv_kunit
  [   42.214642] kprobes_riscv_kunit: The unexpected relocation type 'R_RISCV_ALIGN' from PC = 00000000abb84ce7
  [   42.222778] kprobes_riscv_kunit: The unexpected relocation type 'R_RISCV_ALIGN' from PC = 00000000abb84ce7
  modprobe: can't load module kprobes_riscv_kunit (kernel/arch/riscv/kernel/tests/kprobes/kprobes_riscv_kunit.ko): Invalid argument

which is probably unrelated but I figured it was worth mentioning, as it
does not allow me to see if this has something to do with the flags
added to LDFLAGS_vmlinux.

Maybe this is useful for the upstream report and further triage.

Cheers,
Nathan