[msan] Support prctl PR_GET_NAME call

Per the man page, PR_GET_NAME stores a null terminated string
into the input `char name[16]`.

This also adds prctl support in ASAN to detect freed memory being passed
to `prctl(PR_GET_NAME, ...)`:

Author: ccotter

compiler-rt/lib/sanitizer_common/sanitizer_common_interceptors.inc

@@ -1251,6 +1251,7 @@ INTERCEPTOR(int, prctl, int option, unsigned long arg2, unsigned long arg3,
   void *ctx;
   COMMON_INTERCEPTOR_ENTER(ctx, prctl, option, arg2, arg3, arg4, arg5);
   static const int PR_SET_NAME = 15;
+  static const int PR_GET_NAME = 16;
   static const int PR_SET_VMA = 0x53564d41;
   static const int PR_SCHED_CORE = 62;
   static const int PR_SCHED_CORE_GET = 0;
@@ -1264,7 +1265,18 @@ INTERCEPTOR(int, prctl, int option, unsigned long arg2, unsigned long arg3,
     internal_strncpy(buff, (char *)arg2, 15);
     buff[15] = 0;
     COMMON_INTERCEPTOR_SET_THREAD_NAME(ctx, buff);
-  } else if (res != -1 && option == PR_SCHED_CORE && arg2 == PR_SCHED_CORE_GET) {
+  } else if (res != 1 && option == PR_GET_NAME) {
+    unsigned long null_index = 0;
+    char *name = (char *)arg2;
+    while (name[null_index] && null_index < 16) {
+      ++null_index;
+    }
+    if (null_index > 15) {
+      null_index = 15;
+    }
+    COMMON_INTERCEPTOR_WRITE_RANGE(ctx, (u64 *)arg2, null_index + 1);
+  } else if (res != -1 && option == PR_SCHED_CORE &&
+             arg2 == PR_SCHED_CORE_GET) {
     COMMON_INTERCEPTOR_WRITE_RANGE(ctx, (u64*)(arg5), sizeof(u64));
   }
   return res;

compiler-rt/test/msan/prctl.cpp

@@ -0,0 +1,24 @@
+// RUN: %clangxx_msan -O0 %s -o %t && not %run %t 2>&1 | FileCheck %s
+// REQUIRES: linux
+
+#include <linux/prctl.h>
+#include <sys/prctl.h>
+
+int main(void) {
+  prctl(PR_SET_NAME, "tname");
+  char name[16];
+  prctl(PR_GET_NAME, name);
+
+  if (name[0] == 'A') {
+    return 0;
+  }
+  if (name[5] != '\0') {
+    return 0;
+  }
+  if (name[6] != '\0') {
+    return 0;
+  }
+  // CHECK: SUMMARY: MemorySanitizer: use-of-uninitialized-value {{.*prctl.cpp}}:[[@LINE-3]]
+
+  return 0;
+}