Re: [PATCH bpf-next v2 2/3] bpf, arm64: JIT support for private stack

[Yonghong Song <yonghong.song@linux.dev>]

On 7/24/25 5:02 AM, Puranjay Mohan wrote:
> The private stack is allocated in bpf_int_jit_compile() with 16-byte
> alignment. It includes additional guard regions to detect stack
> overflows and underflows at runtime.
>
> Memory layout:
>
>                +------------------------------------------------------+
>                |                                                      |
>                |  16 bytes padding (overflow guard - stack top)       |
>                |  [ detects writes beyond top of stack ]              |
>       BPF FP ->+------------------------------------------------------+
>                |                                                      |
>                |  BPF private stack (sized by verifier)               |
>                |  [ 16-byte aligned ]                                 |
>                |                                                      |
> BPF PRIV SP ->+------------------------------------------------------+
>                |                                                      |
>                |  16 bytes padding (underflow guard - stack bottom)   |
>                |  [ detects accesses before start of stack ]          |
>                |                                                      |
>                +------------------------------------------------------+
>
> On detection of an overflow or underflow, the kernel emits messages
> like:
>      BPF private stack overflow/underflow detected for prog <prog_name>
>
> After commit bd737fcb6485 ("bpf, arm64: Get rid of fpb"), Jited BPF
> programs use the stack in two ways:
> 1. Via the BPF frame pointer (top of stack), using negative offsets.
> 2. Via the stack pointer (bottom of stack), using positive offsets in
>     LDR/STR instructions.
>
> When a private stack is used, ARM64 callee-saved register x27 replaces
> the stack pointer. The BPF frame pointer usage remains unchanged; but it
> now points to the top of the private stack.
>
> Relevant tests (Enabled in following patch):
>
>   #415/1   struct_ops_private_stack/private_stack:OK
>   #415/2   struct_ops_private_stack/private_stack_fail:OK
>   #415/3   struct_ops_private_stack/private_stack_recur:OK
>   #415     struct_ops_private_stack:OK
>   #549/1   verifier_private_stack/Private stack, single prog:OK
>   #549/2   verifier_private_stack/Private stack, subtree > MAX_BPF_STACK:OK
>   #549/3   verifier_private_stack/No private stack:OK
>   #549/4   verifier_private_stack/Private stack, callback:OK
>   #549/5   verifier_private_stack/Private stack, exception in main prog:OK
>   #549/6   verifier_private_stack/Private stack, exception in subprog:OK
>   #549/7   verifier_private_stack/Private stack, async callback, not nested:OK
>   #549/8   verifier_private_stack/Private stack, async callback, potential nesting:OK
>   #549     verifier_private_stack:OK
>   Summary: 2/11 PASSED, 0 SKIPPED, 0 FAILED
>
> Signed-off-by: Puranjay Mohan <puranjay@kernel.org>

Acked-by: Yonghong Song <yonghong.song@linux.dev>