[PATCH RFC v2 05/11] 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
in stack depot and retains the resulting persistent handles. Once trie
storage is enabled, these handles can refer to trie-backed entries, 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.

Pass the scratch array into a common origin-printing helper. Keep local
storage for standalone kmsan_print_origin() calls, but let kmsan_report()
reuse its existing stack_entries array after printing the report stack.
With x86-64 Clang 19, the regular nested report path uses 752 bytes, below
its 768-byte size before this conversion.

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     | 28 ++++++++++++++++------------
 2 files changed, 18 insertions(+), 14 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..0770658ba932 100644
--- a/mm/kmsan/report.c
+++ b/mm/kmsan/report.c
@@ -83,9 +83,9 @@ static char *pretty_descr(char *descr)
 	return report_local_descr;
 }
 
-void kmsan_print_origin(depot_stack_handle_t origin)
+static void kmsan_print_origin_with_buf(depot_stack_handle_t origin,
+					unsigned long *entries)
 {
-	unsigned long *entries = NULL, *chained_entries = NULL;
 	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, KMSAN_STACK_DEPTH);
 		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,
+						       KMSAN_STACK_DEPTH);
+			skipnr = get_stack_skipnr(entries, chained_nr_entries);
+			stack_trace_print(entries + skipnr,
 					  chained_nr_entries - skipnr, 0);
 			pr_err("\n");
 			continue;
@@ -147,6 +144,13 @@ void kmsan_print_origin(depot_stack_handle_t origin)
 	}
 }
 
+void kmsan_print_origin(depot_stack_handle_t origin)
+{
+	unsigned long entries[KMSAN_STACK_DEPTH];
+
+	kmsan_print_origin_with_buf(origin, entries);
+}
+
 void kmsan_report(depot_stack_handle_t origin, void *address, int size,
 		  int off_first, int off_last, const void __user *user_addr,
 		  enum kmsan_bug_reason reason)
@@ -193,7 +197,7 @@ void kmsan_report(depot_stack_handle_t origin, void *address, int size,
 			  0);
 	pr_err("\n");
 
-	kmsan_print_origin(origin);
+	kmsan_print_origin_with_buf(origin, stack_entries);
 
 	if (size) {
 		pr_err("\n");

-- 
Git-155)