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6c17a882d3
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 bd737fcb64 ("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>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/bpf/20250724120257.7299-3-puranjay@kernel.org