[Linux] Add kernel.yama.ptrace_scope note when ptrace fails to attach.

A common reason for LLDB failing to attach to an already-running process on Linux is the Yama security module: https://www.kernel.org/doc/Documentation/security/Yama.txt. This patch adds an explaination and suggested fix when it detects that case happening.

This was previously proposed in D106226, but hasn't been updated in a while. The last request was to put the check in a target-specific location, which is the approach this patch takes. I believe Yama only exists on Linux, so it's put in that package.

This has no end-to-end test because I'm not sure how to set `ptrace_scope` in a test environment -- if there are suggestions on how to do that, I'd be happy to add it. (Also, setting it to `3` is comically irreversible). I tested this locally.

Reviewed By: DavidSpickett, labath

Differential Revision: https://reviews.llvm.org/D144904

1 files changed, 16 insertions, 0 deletions

diff --git a/lldb/unittests/Process/Linux/ProcfsTests.cpp b/lldb/unittests/Process/Linux/ProcfsTests.cpp
index c069091b7b06..d95de649ed57 100644
--- a/lldb/unittests/Process/Linux/ProcfsTests.cpp
+++ b/lldb/unittests/Process/Linux/ProcfsTests.cpp
@@ -102,3 +102,19 @@ TEST(Perf, RealLogicalCoreIDs) {
   ASSERT_TRUE((bool)cpu_ids);
   ASSERT_GT((int)cpu_ids->size(), 0) << "We must see at least one core";
 }
+
+TEST(Perf, RealPtraceScope) {
+  // We first check we can read /proc/sys/kernel/yama/ptrace_scope
+  auto buffer_or_error =
+      errorOrToExpected(getProcFile("sys/kernel/yama/ptrace_scope"));
+  if (!buffer_or_error)
+    GTEST_SKIP() << toString(buffer_or_error.takeError());
+
+  // At this point we shouldn't fail parsing the ptrace_scope value.
+  Expected<int> ptrace_scope = GetPtraceScope();
+  ASSERT_TRUE((bool)ptrace_scope) << ptrace_scope.takeError();
+  ASSERT_GE(*ptrace_scope, 0)
+      << "Sensible values of ptrace_scope are between 0 and 3";
+  ASSERT_LE(*ptrace_scope, 3)
+      << "Sensible values of ptrace_scope are between 0 and 3";
+}