[lldb] Fix SIGSEGV in GetPtraceScope() in Procfs.cpp (llvm#142224)

# Symptom

We have seen SIGSEGV like this:
```
* thread #1, name = 'lldb-server', stop reason = SIGSEGV
    frame #0: 0x00007f39e529c993 libc.so.6`__pthread_kill_internal(signo=11, threadid=<unavailable>) at pthread_kill.c:46:37
    ...
  * frame #5: 0x000056027c94fe48 lldb-server`lldb_private::process_linux::GetPtraceScope() + 72
    frame #6: 0x000056027c92f94f lldb-server`lldb_private::process_linux::NativeProcessLinux::Attach(int) + 1087
    ...
```
See [full stack trace](https://pastebin.com/X0d6QhYj).

This happens on Linux where LLDB doesn't have access to
`/proc/sys/kernel/yama/ptrace_scope`.

A similar error (an unchecked `Error`) can be reproduced by running the
newly added unit test without the fix. See the "Test" section below.


# Root cause

`GetPtraceScope()`
([code](https://github.com/llvm/llvm-project/blob/328f40f408c218f25695ea42c844e43bef38660b/lldb/source/Plugins/Process/Linux/Procfs.cpp#L77))
has the following `if` statement:
```
llvm::Expected<int> lldb_private::process_linux::GetPtraceScope() {
  ErrorOr<std::unique_ptr<MemoryBuffer>> ptrace_scope_file =
      getProcFile("sys/kernel/yama/ptrace_scope");
  if (!*ptrace_scope_file)
    return errorCodeToError(ptrace_scope_file.getError());
  ...
}
```

The intention of the `if` statement is to check whether the
`ptrace_scope_file` is an `Error` or not, and return the error if it is.
However, the `operator*` of `ErrorOr` returns the value that is stored
(which is a `std::unique_ptr<MemoryBuffer>`), so what the `if` condition
actually do is to check if the unique pointer is non-null.

Note that the method `ErrorOr::getStorage()` ([called
by](https://github.com/llvm/llvm-project/blob/328f40f408c218f25695ea42c844e43bef38660b/llvm/include/llvm/Support/ErrorOr.h#L162-L164)
`ErrorOr::operator *`) **does** assert on whether or not `HasError` has
been set (see
[ErrorOr.h](https://github.com/llvm/llvm-project/blob/328f40f408c218f25695ea42c844e43bef38660b/llvm/include/llvm/Support/ErrorOr.h#L235-L243)).
However, it seems this wasn't executed, probably because the LLDB was a
release build.

# Fix

The fix is simply remove the `*` in the said `if` statement.

Author: royitaqi

lldb/source/Plugins/Process/Linux/Procfs.cpp

@@ -74,7 +74,7 @@ lldb_private::process_linux::GetAvailableLogicalCoreIDs() {
 llvm::Expected<int> lldb_private::process_linux::GetPtraceScope() {
   ErrorOr<std::unique_ptr<MemoryBuffer>> ptrace_scope_file =
       getProcFile("sys/kernel/yama/ptrace_scope");
-  if (!*ptrace_scope_file)
+  if (!ptrace_scope_file)
     return errorCodeToError(ptrace_scope_file.getError());
   // The contents should be something like "1\n". Trim it so we get "1".
   StringRef buffer = (*ptrace_scope_file)->getBuffer().trim();

lldb/unittests/Process/Linux/ProcfsTests.cpp

@@ -104,7 +104,7 @@ TEST(Perf, RealLogicalCoreIDs) {
   ASSERT_GT((int)cpu_ids->size(), 0) << "We must see at least one core";
 }
 
-TEST(Perf, RealPtraceScope) {
+TEST(Perf, RealPtraceScopeWhenExist) {
   // We first check we can read /proc/sys/kernel/yama/ptrace_scope
   auto buffer_or_error =
       errorOrToExpected(getProcFile("sys/kernel/yama/ptrace_scope"));
@@ -120,6 +120,21 @@ TEST(Perf, RealPtraceScope) {
       << "Sensible values of ptrace_scope are between 0 and 3";
 }
 
+TEST(Perf, RealPtraceScopeWhenNotExist) {
+  // We first check we can NOT read /proc/sys/kernel/yama/ptrace_scope
+  auto buffer_or_error =
+      errorOrToExpected(getProcFile("sys/kernel/yama/ptrace_scope"));
+  if (buffer_or_error)
+    GTEST_SKIP() << "In order for this test to run, "
+                    "/proc/sys/kernel/yama/ptrace_scope should not exist";
+  consumeError(buffer_or_error.takeError());
+
+  // At this point we should fail parsing the ptrace_scope value.
+  Expected<int> ptrace_scope = GetPtraceScope();
+  ASSERT_FALSE((bool)ptrace_scope);
+  consumeError(ptrace_scope.takeError());
+}
+
 #ifdef LLVM_ENABLE_THREADING
 TEST(Support, getProcFile_Tid) {
   auto BufferOrError = getProcFile(getpid(), llvm::get_threadid(), "comm");