s390/bpf: Fix clobbering the caller's backchain in the trampoline

One of the first things that s390x kernel functions do is storing the
the caller's frame address (backchain) on stack. This makes unwinding
possible. The backchain is always stored at frame offset 152, which is
inside the 160-byte stack area, that the functions allocate for their
callees. The callees must preserve the backchain; the remaining 152
bytes they may use as they please.

Currently the trampoline uses all 160 bytes, clobbering the backchain.
This causes kernel panics when using __builtin_return_address() in
functions called by the trampoline.

Fix by reducing the usage of the caller-reserved stack area by 8 bytes
in the trampoline.

Fixes: 528eb2cb87bc ("s390/bpf: Implement arch_prepare_bpf_trampoline()")
Reported-by: Song Liu <[e-mail]>
Signed-off-by: Ilya Leoshkevich <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

arch/s390/net/bpf_jit_comp.c

···
 	tjit->run_ctx_off = alloc_stack(tjit,
 					sizeof(struct bpf_tramp_run_ctx));
 	tjit->tccnt_off = alloc_stack(tjit, sizeof(u64));
-	/* The caller has already reserved STACK_FRAME_OVERHEAD bytes. */
-	tjit->stack_size -= STACK_FRAME_OVERHEAD;
+	/*
+	 * In accordance with the s390x ABI, the caller has allocated
+	 * STACK_FRAME_OVERHEAD bytes for us. 8 of them contain the caller's
+	 * backchain, and the rest we can use.
+	 */
+	tjit->stack_size -= STACK_FRAME_OVERHEAD - sizeof(u64);
 	tjit->orig_stack_args_off = tjit->stack_size + STACK_FRAME_OVERHEAD;
 
 	/* aghi %r15,-stack_size */