[PATCH v3 bpf-next 0/3] riscv: bpf: Add BPF private stack support

[Chen Pei]

This series adds private-stack support to the RV64 BPF JIT.

The verifier rejects a struct_ops program that requests a private stack
with -EACCES when the JIT does not report
bpf_jit_supports_private_stack(), and sched_ext requests one for its
dispatch callback, so no sched_ext scheduler can load on riscv64 today.

Allocation, the guard value and the lifetime handling follow arm64,
x86-64 and powerpc64; the 16 byte two-copy guard layout follows arm64
and powerpc64, since x86-64 uses an 8 byte guard with a single copy.
What is specific to riscv64 is where the pointers end up:

 - Every BPF stack access goes through BPF_REG_FP (S5), so pointing S5
   at the top of the per-CPU area and leaving SP on the kernel stack is
   all the JIT has to do. S5 is callee-saved and each subprog derives
   its own, so bpf2bpf calls need no extra save and restore.
 - The outgoing stack argument area stays in the kernel frame: callers
   write it relative to SP while callees read it relative to RV_REG_FP.
   Unlike x86-64, no register here is shared with the private-stack
   frame pointer, so no verifier-side exclusion is needed and stack
   arguments keep working together with a private stack. arm64 already
   does the same, and patch 3 tests that combination.
 - The per-CPU lookup is emitted after the tail call entry point. A
   program using a private stack can still be entered as a tail call
   target, at RV_TAILCALL_OFFSET, so it has to set up its own S5 there;
   and since that offset is a constant added to the target's bpf_func,
   everything in front of it must be the same size in every program.

The base address is materialized with absolute moves, as on arm64,
x86-64 and powerpc64, which reach the per-CPU offset through gs:, TPIDR
or r13; riscv64 has no such register, so the address of __per_cpu_offset
is loaded too. Patch 1 has the detail, including why emit_imm() is a
choice there and why the prologue length stays fixed across passes.

RV32 does not declare support and keeps the existing behaviour; the
prologue signature is unchanged, so bpf_jit_comp32.c needs no edit.

Changes since v2:
 - Patch 1: PRIV_STACK_GUARD_VAL moved to bpf_jit.h.
 - Patch 3: The test redone as two asm programs in
   progs/verifier_stack_arg.c.

Changes since v1:
 - Patch 1 commit message: guard layout credited to arm64 and powerpc64,
   value alone to x86-64; __per_cpu_offset described as a choice.
 - Patch 3: test_private_stack_runtime() renamed to
   serial_test_private_stack_runtime(), and the stack argument skip
   prints a reason.

Testing
=======
QEMU virt rv64, an 8 vCPU SMP kernel and a 1 vCPU UP kernel, both built
on the base commit below, source tree clean at the head of this series,
selftests built from these patches. There are no private-stack related
failures, and the private-stack suites logged no BUG, WARNING, unknown
opcode or guard corruption. On UP, kfunc_call/subprog,
kfunc_call/subprog_lskel and scx_simple are not run: all three fail on
the unpatched base as well.

 - verifier_private_stack, struct_ops_private_stack.
 - tailcalls, stack_arg, subprogs, fentry_fexit, kfunc_call,
   percpu_alloc, async_stack_depth, arena_atomics, arena_ldsx.
 - verifier_stack_arg, stack_arg_precision.
 - may_goto_priv_stack, may_goto_far, verifier_may_goto_1,
   verifier_may_goto_2.
 - scx_simple.
 - sched_ext runner sweep on SMP: 40 cases, 39 pass. enable_cmask fails
   because its ops.enable() takes a struct scx_cmask __arena pointer and
   riscv64 does not implement bpf_jit_supports_arena_args().
 - patch 3's test again on x86-64 and arm64, so all three
   architectures it is gated on have run it.

Chen Pei (3):
  riscv: bpf: Add BPF private stack support
  selftests/bpf: Enable private stack tests for riscv64
  selftests/bpf: Test a private stack together with stack arguments

 arch/riscv/net/bpf_jit.h                      |  2 +
 arch/riscv/net/bpf_jit_comp64.c               | 42 +++++++++-
 arch/riscv/net/bpf_jit_core.c                 | 82 +++++++++++++++++--
 .../bpf/prog_tests/struct_ops_private_stack.c |  3 +-
 .../bpf/progs/verifier_private_stack.c        | 41 +++++++++-
 .../selftests/bpf/progs/verifier_stack_arg.c  | 49 +++++++++++
 6 files changed, 205 insertions(+), 14 deletions(-)


base-commit: a3c8813409f13779fec225468aba7d4067a66468
-- 
2.50.1