syscalls/madvise: Handle zero page poisoning

Treat failures for an un-populated MAP_PRIVATE mapping as configuration failures because we are trying to do something which is not necessarily expected to work. I have included some documentation for what is happening with the zero page in this instance and what is likely to happen in the future.

Also add a test case for MAP_PRIVATE with MAP_POPULATE, which is well defined, and make the test slightly more verbose to help identify which variant is running when there is an error.

Signed-off-by Richard Palethorpe <[email protected]>

---
 testcases/kernel/syscalls/madvise/madvise07.c | 38 ++++++++++++++++++++++-----
 1 file changed, 31 insertions(+), 7 deletions(-)

testcases/kernel/syscalls/madvise/madvise07.c +9 -0

index 2f8c42efc..2fa553a07 100644
@@ -23,9 +23,18 @@
  *	      mark memory with MADV_HWPOISON inside child process,
  *	      access memory,
  *	      if SIGBUS is delivered to child the test passes else it fails
+ *
+ * When MAP_PRIVATE is set (without MAP_POPULATE) madvise() may error with
+ * EBUSY on the first attempt and succeed on the second, but without poisoning
+ * any memory. A private mapping is only populated with pages once it is
+ * accessed and poisoning an unmapped VM range is essentially undefined
+ * behaviour. However madvise() itself causes the address to be mapped to the
+ * zero page. If/when the zero page can be poisoned then the test may pass
+ * without any error. For now we just consider it a configuration failure.
  */
 
 #include <stdlib.h>
+#include <stdio.h>
 #include <sys/wait.h>
 #include <fcntl.h>
 #include <unistd.h>

testcases/kernel/syscalls/madvise/madvise07.c +19 -6

 static int maptypes[] = {
 	MAP_PRIVATE,
+	MAP_PRIVATE | MAP_POPULATE,
 	MAP_SHARED
 };
 
 static void run_child(int maptype)
 {
-	const size_t msize = 4096;
+	const size_t msize = getpagesize();
 	void *mem = NULL;
+	int first_attempt = 1;
 
+	tst_res(TINFO,
+		"mmap(..., MAP_ANONYMOUS | %s, ...)", MAPTYPE(maptype));
 	mem = SAFE_MMAP(NULL,
 			msize,
 			PROT_READ | PROT_WRITE,

@@ -54,22 +69,31 @@ static void run_child(int maptype)
 			-1,
 			0);
 
+do_madvise:
 	tst_res(TINFO, "madvise(%p, %zu, MADV_HWPOISON)", mem, msize);
 	if (madvise(mem, msize, MADV_HWPOISON) == -1) {
 		if (errno == EINVAL)
 			tst_res(TCONF | TERRNO,
 				"CONFIG_MEMORY_FAILURE probably not set in kconfig");
-		else
+		else if (errno == EBUSY && maptype == MAP_PRIVATE) {
+			tst_res(TCONF,
+				"Madvise failed with EBUSY");
+			if (first_attempt--)
+				goto do_madvise;
+		} else
 			tst_res(TFAIL | TERRNO, "Could not poison memory");
 		exit(0);
 	}
 
 	*((char *)mem) = 'd';
 
-	tst_res(TFAIL,
-		"Did not receive SIGBUS after accessing %s memory marked "
-		"with MADV_HWPOISON",
-		MAPTYPE(maptype));
+	if (maptype == MAP_PRIVATE)
+		tst_res(TCONF,
+			"Zero page poisoning is probably not implemented");
+	else
+		tst_res(TFAIL,
+			"Did not receive SIGBUS after accessing %s memory marked"
+			" with MADV_HWPOISON", MAPTYPE(maptype));
 }
 
 static void run(unsigned int n)

Signature · 4 lines

2.12.0

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