s390/Kconfig: Define non-zero ILLEGAL_POINTER_VALUE

Define CONFIG_ILLEGAL_POINTER_VALUE to the eye-catching non-zero value
of 0xdead000000000000, consistent with other architectures. Assert at
compile-time that the poison pointers that include/linux/poison.h
defines based on this illegal pointer are beyond the largest useful
virtual addresses. Also, assert at compile-time that the range of poison
pointers per include/linux/poison.h (currently a range of less than
0x10000 addresses) does not overlap with the range used for address
handles for s390's non-MIO PCI instructions.

This enables s390 to track the DMA mappings by the network stack's
page_pool that was introduced with [0]. Other functional changes are not
intended.

Other archictectures have introduced this for various other reasons with
commit 5c17847 ("riscv: define ILLEGAL_POINTER_VALUE for 64bit")
commit f6853eb ("powerpc/64: Define ILLEGAL_POINTER_VALUE for 64-bit")
commit bf0c4e0 ("arm64: kconfig: Move LIST_POISON to a safe value")
commit a29815a ("core, x86: make LIST_POISON less deadly")

[0] https://lore.kernel.org/all/20250409-page-pool-track-dma-v9-0-6a9ef2e0cba8@redhat.com/

Reviewed-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Gerd Bayer <gbayer@linux.ibm.com>
Acked-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

Author: Gerd Bayer

Documentation/arch/s390/mm.rst

@@ -109,3 +109,7 @@ Virtual memory layout
 		     |	 KASAN shadow	| KASAN untracked
 		     |			|
 		     +------------------+ ASCE limit
+		     |			|
+		     | CONFIG_ILLEGAL_POINTER_VALUE causes memory access fault
+		     |			|
+		     +------------------+

arch/s390/Kconfig

@@ -719,6 +719,10 @@ config ARCH_SPARSEMEM_ENABLE
 config ARCH_SPARSEMEM_DEFAULT
 	def_bool y
 
+config ILLEGAL_POINTER_VALUE
+	hex
+	default 0xdead000000000000
+
 config MAX_PHYSMEM_BITS
 	int "Maximum size of supported physical memory in bits (42-53)"
 	range 42 53

arch/s390/boot/startup.c

@@ -336,6 +336,7 @@ static unsigned long setup_kernel_memory_layout(unsigned long kernel_size)
 	BUILD_BUG_ON(!IS_ALIGNED(TEXT_OFFSET, THREAD_SIZE));
 	BUILD_BUG_ON(!IS_ALIGNED(__NO_KASLR_START_KERNEL, THREAD_SIZE));
 	BUILD_BUG_ON(__NO_KASLR_END_KERNEL > _REGION1_SIZE);
+	BUILD_BUG_ON(CONFIG_ILLEGAL_POINTER_VALUE < _REGION1_SIZE);
 	vsize = get_vmem_size(ident_map_size, vmemmap_size, vmalloc_size, _REGION3_SIZE);
 	boot_debug("vmem size estimated: 0x%016lx\n", vsize);
 	if (IS_ENABLED(CONFIG_KASAN) || __NO_KASLR_END_KERNEL > _REGION2_SIZE ||

arch/s390/include/asm/pci_io.h

@@ -18,6 +18,7 @@
 #define ZPCI_IOMAP_SHIFT		48
 #define ZPCI_IOMAP_ADDR_SHIFT		62
 #define ZPCI_IOMAP_ADDR_BASE		(1UL << ZPCI_IOMAP_ADDR_SHIFT)
+#define ZPCI_IOMAP_ADDR_MAX		((1UL << (ZPCI_IOMAP_ADDR_SHIFT + 1)) - 1)
 #define ZPCI_IOMAP_ADDR_OFF_MASK	((1UL << ZPCI_IOMAP_SHIFT) - 1)
 #define ZPCI_IOMAP_MAX_ENTRIES							\
 	(1UL << (ZPCI_IOMAP_ADDR_SHIFT - ZPCI_IOMAP_SHIFT))

arch/s390/pci/pci.c

@@ -1065,6 +1065,8 @@ static int zpci_mem_init(void)
 {
 	BUILD_BUG_ON(!is_power_of_2(__alignof__(struct zpci_fmb)) ||
 		     __alignof__(struct zpci_fmb) < sizeof(struct zpci_fmb));
+	BUILD_BUG_ON((CONFIG_ILLEGAL_POINTER_VALUE + 0x10000 > ZPCI_IOMAP_ADDR_BASE) &&
+		     (CONFIG_ILLEGAL_POINTER_VALUE <= ZPCI_IOMAP_ADDR_MAX));
 
 	zdev_fmb_cache = kmem_cache_create("PCI_FMB_cache", sizeof(struct zpci_fmb),
 					   __alignof__(struct zpci_fmb), 0, NULL);