[PATCH bpf v3 0/2] bpf, arm64: fix the exception callback's frame pointer

[Donggeun Yoo <donggeunyoo.kernel@gmail.com>]

The arm64 JIT does not set BPF_REG_FP in the prologue of an exception
callback, so the callback runs with whatever x25 held when bpf_throw()
was called. A callback that materializes the register, for instance to
pass the address of a local variable to a helper, then works on the
frame of the subprogram that threw.

Patch 1 sets ctx->fp_used on that path, the same fix commit b114fcee766d
("bpf, arm64: Fix fp initialization for exception boundary") made for
the exception boundary. Patch 2 adds a selftest that reaches the case.

Tested on aarch64 under QEMU with vmtest.sh, on the base below. Without
patch 1 the new test panics the kernel, because the address handed to
the helper lands on the helper's own saved return address:

  pc : 0x1234
  lr : 0x1234
  Call trace:
   0x1234 (P)
   bpf_test_run+0x188/0x3e0
   bpf_prog_test_run_skb+0x47c/0x998
   __sys_bpf+0xbdc/0xdd8
  Kernel panic - not syncing: Oops: Fatal exception in interrupt

0x1234 is the value the callback reads, so the helper wrote it over its
own return address. With patch 1 applied the whole group passes:

  #117/11  exceptions/exception_throw_subprog_stack_cb:OK
  #117     exceptions:OK
  Summary: 1/118 PASSED, 0 SKIPPED, 0/0 FAILED

Not tested on other architectures.

v1: https://lore.kernel.org/bpf/20260904070210.4163193-1-donggeunyoo.kernel@gmail.com/
v2: https://lore.kernel.org/bpf/20260907054235.473103-1-donggeunyoo.kernel@gmail.com/

v2 -> v3:
 - patch 2: use the standard multi-line comment style
 - patch 1: no change, added the Acked-by

Nothing compiled changed, so the numbers above are still v2's.

v1 -> v2:
 - rebase onto bpf/master; CI could not apply v1
 - spell the three new declarations u64 rather than __u64, to match the
   rest of progs/exceptions.c
 - no change to patch 1

Donggeun Yoo (2):
  bpf, arm64: set up the frame pointer for the exception callback
  selftests/bpf: cover the exception callback using its own BPF stack

 arch/arm64/net/bpf_jit_comp.c                 |  2 ++
 .../selftests/bpf/prog_tests/exceptions.c     |  1 +
 .../testing/selftests/bpf/progs/exceptions.c  | 30 +++++++++++++++++++
 3 files changed, 33 insertions(+)


base-commit: df2908090cda368b01ff43709f51890076c56157
-- 
2.53.0