mm: slub: Print the broken data before restoring them

commit ed5ec2e
Author: Hyesoo Yu <[email protected]>
Date:   Wed Feb 26 17:12:00 2025 +0900

    mm: slub: Print the broken data before restoring them

    Previously, the restore occurred after printing the object in slub.
    After commit 47d911b ("slab: make check_object() more consistent"),
    the bytes are printed after the restore. This information about the bytes
    before the restore is highly valuable for debugging purpose.
    For instance, in a event of cache issue, it displays byte patterns
    by breaking them down into 64-bytes units. Without this information,
    we can only speculate on how it was broken. Hence the corrupted regions
    should be printed prior to the restoration process. However if an object
    breaks in multiple places, the same log may be output multiple times.
    Therefore the slub log is reported only once to prevent redundant printing,
    by sending a parameter indicating whether an error has occurred previously.

    Signed-off-by: Hyesoo Yu <[email protected]>
    Reviewed-by: Harry Yoo <[email protected]>
    Signed-off-by: Vlastimil Babka <[email protected]>

JIRA: https://issues.redhat.com/browse/RHEL-77742
Signed-off-by: Nico Pache <[email protected]>

Author: PacheNico

mm/slub.c

@@ -1181,8 +1181,8 @@ static void restore_bytes(struct kmem_cache *s, char *message, u8 data,
 
 static pad_check_attributes int
 check_bytes_and_report(struct kmem_cache *s, struct slab *slab,
-		       u8 *object, char *what,
-		       u8 *start, unsigned int value, unsigned int bytes)
+		       u8 *object, char *what, u8 *start, unsigned int value,
+		       unsigned int bytes, bool slab_obj_print)
 {
 	u8 *fault;
 	u8 *end;
@@ -1201,10 +1201,11 @@ check_bytes_and_report(struct kmem_cache *s, struct slab *slab,
 	if (slab_add_kunit_errors())
 		goto skip_bug_print;
 
-	slab_bug(s, "%s overwritten", what);
-	pr_err("0x%p-0x%p @offset=%tu. First byte 0x%x instead of 0x%x\n",
-					fault, end - 1, fault - addr,
-					fault[0], value);
+	pr_err("[%s overwritten] 0x%p-0x%p @offset=%tu. First byte 0x%x instead of 0x%x\n",
+	       what, fault, end - 1, fault - addr, fault[0], value);
+
+	if (slab_obj_print)
+		object_err(s, slab, object, "Object corrupt");
 
 skip_bug_print:
 	restore_bytes(s, what, value, fault, end);
@@ -1268,7 +1269,7 @@ static int check_pad_bytes(struct kmem_cache *s, struct slab *slab, u8 *p)
 		return 1;
 
 	return check_bytes_and_report(s, slab, p, "Object padding",
-			p + off, POISON_INUSE, size_from_object(s) - off);
+			p + off, POISON_INUSE, size_from_object(s) - off, true);
 }
 
 /* Check the pad bytes at the end of a slab page */
@@ -1318,11 +1319,11 @@ static int check_object(struct kmem_cache *s, struct slab *slab,
 
 	if (s->flags & SLAB_RED_ZONE) {
 		if (!check_bytes_and_report(s, slab, object, "Left Redzone",
-			object - s->red_left_pad, val, s->red_left_pad))
+			object - s->red_left_pad, val, s->red_left_pad, ret))
 			ret = 0;
 
 		if (!check_bytes_and_report(s, slab, object, "Right Redzone",
-			endobject, val, s->inuse - s->object_size))
+			endobject, val, s->inuse - s->object_size, ret))
 			ret = 0;
 
 		if (slub_debug_orig_size(s) && val == SLUB_RED_ACTIVE) {
@@ -1331,15 +1332,15 @@ static int check_object(struct kmem_cache *s, struct slab *slab,
 			if (s->object_size > orig_size  &&
 				!check_bytes_and_report(s, slab, object,
 					"kmalloc Redzone", p + orig_size,
-					val, s->object_size - orig_size)) {
+					val, s->object_size - orig_size, ret)) {
 				ret = 0;
 			}
 		}
 	} else {
 		if ((s->flags & SLAB_POISON) && s->object_size < s->inuse) {
 			if (!check_bytes_and_report(s, slab, p, "Alignment padding",
 				endobject, POISON_INUSE,
-				s->inuse - s->object_size))
+				s->inuse - s->object_size, ret))
 				ret = 0;
 		}
 	}
@@ -1355,11 +1356,11 @@ static int check_object(struct kmem_cache *s, struct slab *slab,
 			if (kasan_meta_size < s->object_size - 1 &&
 			    !check_bytes_and_report(s, slab, p, "Poison",
 					p + kasan_meta_size, POISON_FREE,
-					s->object_size - kasan_meta_size - 1))
+					s->object_size - kasan_meta_size - 1, ret))
 				ret = 0;
 			if (kasan_meta_size < s->object_size &&
 			    !check_bytes_and_report(s, slab, p, "End Poison",
-					p + s->object_size - 1, POISON_END, 1))
+					p + s->object_size - 1, POISON_END, 1, ret))
 				ret = 0;
 		}
 		/*
@@ -1385,11 +1386,6 @@ static int check_object(struct kmem_cache *s, struct slab *slab,
 		ret = 0;
 	}
 
-	if (!ret && !slab_in_kunit_test()) {
-		print_trailer(s, slab, object);
-		add_taint(TAINT_BAD_PAGE, LOCKDEP_NOW_UNRELIABLE);
-	}
-
 	return ret;
 }