[Lldb-commits] [lldb] [lldb] Fix SIGSEGV in `GetPtraceScope()` in `Procfs.cpp` (PR #142224)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-lldb

Author: None (royitaqi)

<details>
<summary>Changes</summary>

# 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).

A similar error (an unchecked `Error`) can be reproduced by running the newly added unit test without the fix on a Linux machine which doesn't have `/proc/sys/kernel/yama/ptrace_scope`. 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.

# Test

Adding a new unit test: `RealPtraceScopeWhenNotExist`.

On a Linux machine which doesn't have `/proc/sys/kernel/yama/ptrace_scope`, **run the unit test *without* the fix** reproduces this assertion error about an unchecked `Error`:
```
$ tools/lldb/unittests/Process/Linux/ProcessLinuxTests
[==========] Running 5 tests from 1 test suite.
...
[ RUN      ] Perf.RealPtraceScopeWhenNotExist
ProcessLinuxTests: /home/royshi/llvm-sand/external/llvm-project/llvm/include/llvm/Support/ErrorOr.h:236: storage_type *llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>>::getStorage() [T = std::unique_ptr<llvm::MemoryBuffer>]: Assertion `!HasError && "Cannot get value when an error exists!"' failed.
Stack dump without symbol names (ensure you have llvm-symbolizer in your PATH or set the environment var `LLVM_SYMBOLIZER_PATH` to point to it):
0  ProcessLinuxTests 0x0000560e7798d6e5
1  ProcessLinuxTests 0x0000560e7798ddf4
...
Aborted (core dumped)
```

**Run the unit test *with* the fix** show passed result:
```
[royshi@<!-- -->devvm24317.cln0 ~/llvm-sand/build/Debug/fbcode-x86_64/toolchain]$ tools/lldb/unittests/Process/Linux/ProcessLinuxTests
[==========] Running 5 tests from 1 test suite.
...
[ RUN      ] Perf.RealPtraceScopeWhenNotExist
[       OK ] Perf.RealPtraceScopeWhenNotExist (0 ms)
...
[  PASSED  ] 3 tests.
```


---
Full diff: https://github.com/llvm/llvm-project/pull/142224.diff


2 Files Affected:

- (modified) lldb/source/Plugins/Process/Linux/Procfs.cpp (+1-1) 
- (modified) lldb/unittests/Process/Linux/ProcfsTests.cpp (+15-1) 


``````````diff
diff --git a/lldb/source/Plugins/Process/Linux/Procfs.cpp b/lldb/source/Plugins/Process/Linux/Procfs.cpp
index d3bd396fbaeab..4349a2b1adb9d 100644
--- a/lldb/source/Plugins/Process/Linux/Procfs.cpp
+++ b/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();
diff --git a/lldb/unittests/Process/Linux/ProcfsTests.cpp b/lldb/unittests/Process/Linux/ProcfsTests.cpp
index e7af1f469c2bf..a795fa4e019c5 100644
--- a/lldb/unittests/Process/Linux/ProcfsTests.cpp
+++ b/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 +120,20 @@ 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");

``````````

</details>


https://github.com/llvm/llvm-project/pull/142224