Merge "Allow the kernel to upgrade ASYNC mode processes to SYNC mode." into sc-dev

2 files changed, 11 insertions, 2 deletions

diff --git a/libc/bionic/heap_tagging.cpp b/libc/bionic/heap_tagging.cpp
index ffbabb9a0..41aa20507 100644
--- a/libc/bionic/heap_tagging.cpp
+++ b/libc/bionic/heap_tagging.cpp
@@ -139,7 +139,12 @@ bool SetHeapTaggingLevel(HeapTaggingLevel tag_level) {
       }
 
       if (tag_level == M_HEAP_TAGGING_LEVEL_ASYNC) {
-        set_tcf_on_all_threads(PR_MTE_TCF_ASYNC);
+        // When entering ASYNC mode, specify that we want to allow upgrading to SYNC by OR'ing in
+        // the SYNC flag. But if the kernel doesn't support specifying multiple TCF modes, fall back
+        // to specifying a single mode.
+        if (!set_tcf_on_all_threads(PR_MTE_TCF_ASYNC | PR_MTE_TCF_SYNC)) {
+          set_tcf_on_all_threads(PR_MTE_TCF_ASYNC);
+        }
 #if defined(USE_SCUDO)
         scudo_malloc_set_track_allocation_stacks(0);
 #endif
diff --git a/libc/bionic/libc_init_static.cpp b/libc/bionic/libc_init_static.cpp
index 069ebb0ab..3a8513f98 100644
--- a/libc/bionic/libc_init_static.cpp
+++ b/libc/bionic/libc_init_static.cpp
@@ -311,7 +311,11 @@ __attribute__((no_sanitize("hwaddress", "memtag"))) void __libc_init_mte(const v
     unsigned long prctl_arg = PR_TAGGED_ADDR_ENABLE | PR_MTE_TAG_SET_NONZERO;
     prctl_arg |= (level == M_HEAP_TAGGING_LEVEL_SYNC) ? PR_MTE_TCF_SYNC : PR_MTE_TCF_ASYNC;
 
-    if (prctl(PR_SET_TAGGED_ADDR_CTRL, prctl_arg, 0, 0, 0) == 0) {
+    // When entering ASYNC mode, specify that we want to allow upgrading to SYNC by OR'ing in the
+    // SYNC flag. But if the kernel doesn't support specifying multiple TCF modes, fall back to
+    // specifying a single mode.
+    if (prctl(PR_SET_TAGGED_ADDR_CTRL, prctl_arg | PR_MTE_TCF_SYNC, 0, 0, 0) == 0 ||
+        prctl(PR_SET_TAGGED_ADDR_CTRL, prctl_arg, 0, 0, 0) == 0) {
       __libc_shared_globals()->initial_heap_tagging_level = level;
       return;
     }