selftests/mm: two fixes for the pfnmap test

When unregistering the signal handler, we have to pass SIG_DFL, and
blindly reading from PFN 0 and PFN 1 seems to be problematic on !x86
systems.  In particularly, on arm64 tx2 machines where noting resides at
these physical memory locations, we can generate RAS errors.

Let's fix it by scanning /proc/iomem for actual "System RAM".

Link: https://lkml.kernel.org/r/20250528195244.1182810-1-david@redhat.com
Fixes: 2616b370323a ("selftests/mm: add simple VM_PFNMAP tests based on mmap'ing /dev/mem")
Signed-off-by: David Hildenbrand <david@redhat.com>
Reported-by: Ryan Roberts <ryan.roberts@arm.com>
Closes: https://lore.kernel.org/all/232960c2-81db-47ca-a337-38c4bce5f997@arm.com/T/#u
Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>
Tested-by: Aishwarya TCV <aishwarya.tcv@arm.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 57 insertions, 4 deletions

diff --git a/tools/testing/selftests/mm/pfnmap.c b/tools/testing/selftests/mm/pfnmap.c
index 8a9d19b6020c..866ac023baf5 100644
--- a/tools/testing/selftests/mm/pfnmap.c
+++ b/tools/testing/selftests/mm/pfnmap.c
@@ -12,6 +12,8 @@
 #include <stdint.h>
 #include <unistd.h>
 #include <errno.h>
+#include <stdio.h>
+#include <ctype.h>
 #include <fcntl.h>
 #include <signal.h>
 #include <setjmp.h>
@@ -43,14 +45,62 @@ static int test_read_access(char *addr, size_t size, size_t pagesize)
 			/* Force a read that the compiler cannot optimize out. */
 			*((volatile char *)(addr + offs));
 	}
-	if (signal(SIGSEGV, signal_handler) == SIG_ERR)
+	if (signal(SIGSEGV, SIG_DFL) == SIG_ERR)
 		return -EINVAL;
 
 	return ret;
 }
 
+static int find_ram_target(off_t *phys_addr,
+		unsigned long long pagesize)
+{
+	unsigned long long start, end;
+	char line[80], *end_ptr;
+	FILE *file;
+
+	/* Search /proc/iomem for the first suitable "System RAM" range. */
+	file = fopen("/proc/iomem", "r");
+	if (!file)
+		return -errno;
+
+	while (fgets(line, sizeof(line), file)) {
+		/* Ignore any child nodes. */
+		if (!isalnum(line[0]))
+			continue;
+
+		if (!strstr(line, "System RAM\n"))
+			continue;
+
+		start = strtoull(line, &end_ptr, 16);
+		/* Skip over the "-" */
+		end_ptr++;
+		/* Make end "exclusive". */
+		end = strtoull(end_ptr, NULL, 16) + 1;
+
+		/* Actual addresses are not exported */
+		if (!start && !end)
+			break;
+
+		/* We need full pages. */
+		start = (start + pagesize - 1) & ~(pagesize - 1);
+		end &= ~(pagesize - 1);
+
+		if (start != (off_t)start)
+			break;
+
+		/* We need two pages. */
+		if (end > start + 2 * pagesize) {
+			fclose(file);
+			*phys_addr = start;
+			return 0;
+		}
+	}
+	return -ENOENT;
+}
+
 FIXTURE(pfnmap)
 {
+	off_t phys_addr;
 	size_t pagesize;
 	int dev_mem_fd;
 	char *addr1;
@@ -63,14 +113,17 @@ FIXTURE_SETUP(pfnmap)
 {
 	self->pagesize = getpagesize();
 
+	/* We'll require two physical pages throughout our tests ... */
+	if (find_ram_target(&self->phys_addr, self->pagesize))
+		SKIP(return, "Cannot find ram target in '/proc/iomem'\n");
+
 	self->dev_mem_fd = open("/dev/mem", O_RDONLY);
 	if (self->dev_mem_fd < 0)
 		SKIP(return, "Cannot open '/dev/mem'\n");
 
-	/* We'll require the first two pages throughout our tests ... */
 	self->size1 = self->pagesize * 2;
 	self->addr1 = mmap(NULL, self->size1, PROT_READ, MAP_SHARED,
-			   self->dev_mem_fd, 0);
+			   self->dev_mem_fd, self->phys_addr);
 	if (self->addr1 == MAP_FAILED)
 		SKIP(return, "Cannot mmap '/dev/mem'\n");
 
@@ -129,7 +182,7 @@ TEST_F(pfnmap, munmap_split)
 	 */
 	self->size2 = self->pagesize;
 	self->addr2 = mmap(NULL, self->pagesize, PROT_READ, MAP_SHARED,
-			   self->dev_mem_fd, 0);
+			   self->dev_mem_fd, self->phys_addr);
 	ASSERT_NE(self->addr2, MAP_FAILED);
 }