powerpc64/bpf: remove BPF redzone protection in trampoline stack

powerpc64/bpf: remove BPF redzone protection in trampoline stack

Since bpf2bpf tailcall support is enabled for 64-bit powerpc with
kernel commit 2ed2d8f6fb38 ("powerpc64/bpf: Support tailcalls with
subprogs"), 'tailcalls/tailcall_bpf2bpf_hierarchy_fexit' BPF selftest
is triggering "corrupted stack end detected inside scheduler" with the
config option CONFIG_SCHED_STACK_END_CHECK enabled. While reviewing
the stack layout for BPF trampoline, observed that the dummy frame is
trying to protect the redzone of BPF program. This is because tail
call info and NVRs save area are in redzone at the time of tailcall
as the current BPF program stack frame is teared down before the
tailcall. But saving this redzone in the dummy frame of trampoline
is unnecessary because of the follow reasons:

  1) Firstly, trampoline can be attached to BPF entry/main program
     or subprog. But prologue part of the BPF entry/main program,
     where the trampoline attachpoint is, is skipped during tailcall.
     So, protecting the redzone does not arise when the trampoline is
     not even triggered in this scenario.
  2) In case of subprog, the caller's stackframe is already setup
     and the subprog's stackframe is yet to be setup. So, nothing
     on the redzone to be protected.

Also, using dummy frame in BPF trampoline, wastes critically scarce
kernel stack space, especially in tailcall sequence, for marginal
benefit in stack unwinding. So, drop setting up the dummy frame.
Instead, save return address in bpf trampoline frame and use it as
appropriate. Pruning this unnecessary stack usage mitigates the
likelihood of stack overflow in scenarios where bpf2bpf tailcalls
and fexit programs are mixed.

Reported-by: Saket Kumar Bhaskar <skb99@linux.ibm.com>
Fixes: 2ed2d8f6fb38 ("powerpc64/bpf: Support tailcalls with subprogs")
Tested-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
Signed-off-by: Hari Bathini <hbathini@linux.ibm.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20260303181031.390073-5-hbathini@linux.ibm.com