s390/bpf: Fix unwinding past the trampoline

When functions called by the trampoline panic, the backtrace that is
printed stops at the trampoline, because the trampoline does not store
its caller's frame address (backchain) on stack; it also stores the
return address at a wrong location.

Store both the same way as is already done for the regular eBPF programs.

Fixes: 528eb2cb87bc ("s390/bpf: Implement arch_prepare_bpf_trampoline()")
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

···
 				 * func_addr's original caller
 				 */
 	int stack_size;		/* Trampoline stack size */
+	int backchain_off;	/* Offset of backchain */
 	int stack_args_off;	/* Offset of stack arguments for calling
 				 * func_addr, has to be at the top
 				 */
···
 				 * for __bpf_prog_enter() return value and
 				 * func_addr respectively
 				 */
-	int r14_off;		/* Offset of saved %r14 */
 	int run_ctx_off;	/* Offset of struct bpf_tramp_run_ctx */
 	int tccnt_off;		/* Offset of saved tailcall counter */
+	int r14_off;		/* Offset of saved %r14, has to be at the
+				 * bottom */
 	int do_fexit;		/* do_fexit: label */
 };
 
···
 	 * Calculate the stack layout.
 	 */
 
-	/* Reserve STACK_FRAME_OVERHEAD bytes for the callees. */
+	/*
+	 * Allocate STACK_FRAME_OVERHEAD bytes for the callees. As the s390x
+	 * ABI requires, put our backchain at the end of the allocated memory.
+	 */
 	tjit->stack_size = STACK_FRAME_OVERHEAD;
+	tjit->backchain_off = tjit->stack_size - sizeof(u64);
 	tjit->stack_args_off = alloc_stack(tjit, nr_stack_args * sizeof(u64));
 	tjit->reg_args_off = alloc_stack(tjit, nr_reg_args * sizeof(u64));
 	tjit->ip_off = alloc_stack(tjit, sizeof(u64));
···
 	tjit->bpf_args_off = alloc_stack(tjit, nr_bpf_args * sizeof(u64));
 	tjit->retval_off = alloc_stack(tjit, sizeof(u64));
 	tjit->r7_r8_off = alloc_stack(tjit, 2 * sizeof(u64));
-	tjit->r14_off = alloc_stack(tjit, sizeof(u64));
 	tjit->run_ctx_off = alloc_stack(tjit,
 					sizeof(struct bpf_tramp_run_ctx));
 	tjit->tccnt_off = alloc_stack(tjit, sizeof(u64));
+	tjit->r14_off = alloc_stack(tjit, sizeof(u64) * 2);
 	/*
 	 * In accordance with the s390x ABI, the caller has allocated
 	 * STACK_FRAME_OVERHEAD bytes for us. 8 of them contain the caller's
···
 	tjit->stack_size -= STACK_FRAME_OVERHEAD - sizeof(u64);
 	tjit->orig_stack_args_off = tjit->stack_size + STACK_FRAME_OVERHEAD;
 
+	/* lgr %r1,%r15 */
+	EMIT4(0xb9040000, REG_1, REG_15);
 	/* aghi %r15,-stack_size */
 	EMIT4_IMM(0xa70b0000, REG_15, -tjit->stack_size);
+	/* stg %r1,backchain_off(%r15) */
+	EMIT6_DISP_LH(0xe3000000, 0x0024, REG_1, REG_0, REG_15,
+		      tjit->backchain_off);
 	/* mvc tccnt_off(4,%r15),stack_size+STK_OFF_TCCNT(%r15) */
 	_EMIT6(0xd203f000 | tjit->tccnt_off,
 	       0xf000 | (tjit->stack_size + STK_OFF_TCCNT));