[compiler-rt] [msan] Support prctl PR_GET_NAME call (PR #98951)

[Vitaly Buka via llvm-commits]

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@@ -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 (null_index < 16 && name[null_index]) {
+      ++null_index;
+    }
+    if (null_index > 15) {
+      null_index = 15;
+    }
+    COMMON_INTERCEPTOR_WRITE_RANGE(ctx, (u64 *)arg2, null_index + 1);
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vitalybuka wrote:

`null_index` -> `internal_strnlen`

but actually just:
`null_index` -> `internal_strlen`

Because on success it must be null-terminated string anyway.

https://github.com/llvm/llvm-project/pull/98951