[Lldb-commits] [lldb] [lldb][Linux] Fix checking of error values when attach fails (PR #161673)

[David Spickett via lldb-commits]

https://github.com/DavidSpickett created https://github.com/llvm/llvm-project/pull/161673

Relates to #161510

(it's not fixing it, it's just making the error not be an unhandled error)

When we fail to attach to a process we see if we can add more information about why it happened:
```
  if (status.GetError() == EPERM) {
    // Depending on the value of ptrace_scope, we can return a different
    // error that suggests how to fix it.
    return AddPtraceScopeNote(status.ToError());
  }
```
ToError creates a new error value and leaves the one in `status` unchecked. `status`'s error is ok because it will be checked by Status' destructor.

The problem happens in `AddPtraceScopeNote`. If we take certain return paths, this new error, or the one we get when trying to find the ptrace scope, may be unchecked on destruction when the function returns.

To fix this, in AddPtraceScopeNote, consume any errors that we are not going to return. Anything returned will be checked by some caller.

Reproducing this failure mode is difficult but it can be faked by calling AddPtraceScopeNote earlier. Which is what I did to prove the concept of the problem.

>From 2220f53721fc6ae3f6208799173935960182c271 Mon Sep 17 00:00:00 2001
From: David Spickett <david.spickett at linaro.org>
Date: Thu, 2 Oct 2025 14:05:22 +0000
Subject: [PATCH] [lldb][Linux] Fix checking of error values when attach fails

Relates to #161510

(it's not fixing it, it's just making the error not be an
unhandled error)

When we fail to attach to a process we see if we can add more
information about why it happpened:
```
  if (status.GetError() == EPERM) {
    // Depending on the value of ptrace_scope, we can return a different
    // error that suggests how to fix it.
    return AddPtraceScopeNote(status.ToError());
  }
```
ToError creates a new error value and leaves the one in `status`
unchecked. `status`'s error is ok because it will be checked
by Status' destructor.

The problem happens in `AddPtraceScopeNote`. If we take certain
return paths, this new error, or the one we get when trying to
find the ptrace scope, may be unchecked on destruction when the
function returns.

To fix this, in AddPtraceScopeNote, consume any errors that
we are not going to return. Anything returned will be checked
by some caller.

Reproducing this failure mode is difficult but it can be faked
by calling AddPtraceScopeNote earlier. Which is what I did to
prove the concept of the problem.
---
 lldb/source/Plugins/Process/Linux/NativeProcessLinux.cpp | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/lldb/source/Plugins/Process/Linux/NativeProcessLinux.cpp b/lldb/source/Plugins/Process/Linux/NativeProcessLinux.cpp
index 9c798cb1cc8f2..7d0bca738e2e5 100644
--- a/lldb/source/Plugins/Process/Linux/NativeProcessLinux.cpp
+++ b/lldb/source/Plugins/Process/Linux/NativeProcessLinux.cpp
@@ -219,7 +219,8 @@ static llvm::Error AddPtraceScopeNote(llvm::Error original_error) {
   Expected<int> ptrace_scope = GetPtraceScope();
   if (auto E = ptrace_scope.takeError()) {
     Log *log = GetLog(POSIXLog::Process);
-    LLDB_LOG(log, "error reading value of ptrace_scope: {0}", E);
+    LLDB_LOG(log, "error reading value of ptrace_scope: {0}",
+             llvm::toString(std::move(E)));
 
     // The original error is probably more interesting than not being able to
     // read or interpret ptrace_scope.
@@ -230,6 +231,7 @@ static llvm::Error AddPtraceScopeNote(llvm::Error original_error) {
   switch (*ptrace_scope) {
   case 1:
   case 2:
+    llvm::consumeError(std::move(original_error));
     return llvm::createStringError(
         std::error_code(errno, std::generic_category()),
         "The current value of ptrace_scope is %d, which can cause ptrace to "
@@ -239,6 +241,7 @@ static llvm::Error AddPtraceScopeNote(llvm::Error original_error) {
         "https://www.kernel.org/doc/Documentation/security/Yama.txt.",
         *ptrace_scope);
   case 3:
+    llvm::consumeError(std::move(original_error));
     return llvm::createStringError(
         std::error_code(errno, std::generic_category()),
         "The current value of ptrace_scope is 3, which will cause ptrace to "