[PATCH v1] selftests/mm: two fixes for the pfnmap test

[David Hildenbrand <david@redhat.com>]

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".

Reported-by: Ryan Roberts <ryan.roberts@arm.com>
Closes: https://lore.kernel.org/all/232960c2-81db-47ca-a337-38c4bce5f997@arm.com/T/#u
Fixes: 2616b370323a ("selftests/mm: add simple VM_PFNMAP tests based on mmap'ing /dev/mem")
Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>
Tested-by: Aishwarya TCV <aishwarya.tcv@arm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
---
 tools/testing/selftests/mm/pfnmap.c | 61 +++++++++++++++++++++++++++--
 1 file changed, 57 insertions(+), 4 deletions(-)

diff --git a/tools/testing/selftests/mm/pfnmap.c b/tools/testing/selftests/mm/pfnmap.c
index 8a9d19b6020c7..866ac023baf5c 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);
 }
 
-- 
2.49.0