[PATCH RFC 5/9] kmsan: report trie-backed stack depot traces

[Caleb Kan <calebkan1106@gmail.com>]

From: Caleb Kan <ckan@cloudflare.com>

KMSAN stores ordinary origin stacks and synthetic alloca and chain origins
as persistent stack depot records. Once trie storage is enabled, these
handles can be trie-backed, while kmsan_print_origin() still relies on the
hash-only stack_depot_fetch() API.

Use one KMSAN_STACK_DEPTH array to materialize each origin and chained
stack in turn. Preserve the chain's head and next-origin handles before
reusing the array for the chained stack. The array covers both the regular
save limit and the smaller synthetic records.

lib/stackdepot.c is uninstrumented, so stack_depot_fetch_into() unpoisons
the successfully copied range before returning it to KMSAN. Remove the
now-redundant explicit unpoisoning of chained entries. Origin depth and
use-after-free metadata remain in the handle's extra bits and are
unchanged.

Update test_stackdepot_roundtrip() to use caller-owned storage while
retaining its frame-count and kmsan_check_memory() checks. This verifies
that the copy-out API returns initialized entries to instrumented callers.

Signed-off-by: Caleb Kan <ckan@cloudflare.com>
---
 mm/kmsan/kmsan_test.c |  4 ++--
 mm/kmsan/report.c     | 17 +++++++----------
 2 files changed, 9 insertions(+), 12 deletions(-)

diff --git a/mm/kmsan/kmsan_test.c b/mm/kmsan/kmsan_test.c
index 31f47cc4dab4..7c04e4b21873 100644
--- a/mm/kmsan/kmsan_test.c
+++ b/mm/kmsan/kmsan_test.c
@@ -669,7 +669,7 @@ static void test_long_origin_chain(struct kunit *test)
  */
 static void test_stackdepot_roundtrip(struct kunit *test)
 {
-	unsigned long src_entries[16], *dst_entries;
+	unsigned long src_entries[16], dst_entries[16];
 	unsigned int src_nentries, dst_nentries;
 	EXPECTATION_NO_REPORT(expect);
 	depot_stack_handle_t handle;
@@ -680,7 +680,7 @@ static void test_stackdepot_roundtrip(struct kunit *test)
 		stack_trace_save(src_entries, ARRAY_SIZE(src_entries), 1);
 	handle = stack_depot_save(src_entries, src_nentries, GFP_KERNEL);
 	stack_depot_print(handle);
-	dst_nentries = stack_depot_fetch(handle, &dst_entries);
+	dst_nentries = stack_depot_fetch_into(handle, dst_entries, ARRAY_SIZE(dst_entries));
 	KUNIT_EXPECT_TRUE(test, src_nentries == dst_nentries);
 
 	kmsan_check_memory((void *)dst_entries,
diff --git a/mm/kmsan/report.c b/mm/kmsan/report.c
index d6853ce08954..c20c24cffde5 100644
--- a/mm/kmsan/report.c
+++ b/mm/kmsan/report.c
@@ -85,7 +85,7 @@ static char *pretty_descr(char *descr)
 
 void kmsan_print_origin(depot_stack_handle_t origin)
 {
-	unsigned long *entries = NULL, *chained_entries = NULL;
+	unsigned long entries[KMSAN_STACK_DEPTH];
 	unsigned int nr_entries, chained_nr_entries, skipnr;
 	void *pc1 = NULL, *pc2 = NULL;
 	depot_stack_handle_t head;
@@ -97,7 +97,8 @@ void kmsan_print_origin(depot_stack_handle_t origin)
 		return;
 
 	while (true) {
-		nr_entries = stack_depot_fetch(origin, &entries);
+		nr_entries =
+			stack_depot_fetch_into(origin, entries, ARRAY_SIZE(entries));
 		depth = kmsan_depth_from_eb(stack_depot_get_extra_bits(origin));
 		magic = nr_entries ? entries[0] : 0;
 		if ((nr_entries == 4) && (magic == KMSAN_ALLOCA_MAGIC_ORIGIN)) {
@@ -123,14 +124,10 @@ void kmsan_print_origin(depot_stack_handle_t origin)
 			origin = entries[2];
 			pr_err("Uninit was stored to memory at:\n");
 			chained_nr_entries =
-				stack_depot_fetch(head, &chained_entries);
-			kmsan_internal_unpoison_memory(
-				chained_entries,
-				chained_nr_entries * sizeof(*chained_entries),
-				/*checked*/ false);
-			skipnr = get_stack_skipnr(chained_entries,
-						  chained_nr_entries);
-			stack_trace_print(chained_entries + skipnr,
+				stack_depot_fetch_into(head, entries,
+						       ARRAY_SIZE(entries));
+			skipnr = get_stack_skipnr(entries, chained_nr_entries);
+			stack_trace_print(entries + skipnr,
 					  chained_nr_entries - skipnr, 0);
 			pr_err("\n");
 			continue;

-- 
Git-155)