From 3aed26ba1f598ce05863557342480b1e12a1b687 Mon Sep 17 00:00:00 2001 From: Jason Molenda Date: Fri, 12 Jul 2024 10:44:18 -0700 Subject: [PATCH] [lldb] Allow fetching of RA register when above fault handler (#98566) In RegisterContextUnwind::SavedLocationForRegister we have special logic for retrieving the Return Address register when it has the caller's return address in it. An example would be the lr register on AArch64. This register is never retrieved from a newer stack frame because it is necessarly overwritten by a normal ABI function call. We allow frame 0 to provide its lr value to get the caller's return address, if it has not been overwritten/saved to stack yet. When a function is interrupted asynchronously by a POSIX signal (sigtramp), or a fault handler more generally, the sigtramp/fault handler has the entire register context available. In this situation, if the fault handler is frame 0, the function that was async interrupted is frame 1 and frame 2's return address may still be stored in lr. We need to get the lr value for frame 1 from the fault handler in frame 0, to get the return address for frame 2. Without this fix, a frameless function that faults in a firmware environment (that's where we've seen this issue most commonly) hasn't spilled lr to stack, so we need to retrieve it from the fault handler's full-register-context to find the caller of the frameless function that faulted. It's an unsurprising fix, all of the work was finding exactly where in RegisterContextUnwind we were only allowing RA register use for frame 0, when it should have been frame 0 or above a fault handler function. rdar://127518945 (cherry picked from commit fd424179dcb3417fc0675f77d2bf06c750dd1c33) --- lldb/source/Target/RegisterContextUnwind.cpp | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/lldb/source/Target/RegisterContextUnwind.cpp b/lldb/source/Target/RegisterContextUnwind.cpp index 242c7d3f6b945..44531bef7322d 100644 --- a/lldb/source/Target/RegisterContextUnwind.cpp +++ b/lldb/source/Target/RegisterContextUnwind.cpp @@ -1401,7 +1401,7 @@ RegisterContextUnwind::SavedLocationForRegister( // it's still live in the actual register. Handle this specially. if (!have_unwindplan_regloc && return_address_reg.IsValid() && - IsFrameZero()) { + BehavesLikeZerothFrame()) { if (return_address_reg.GetAsKind(eRegisterKindLLDB) != LLDB_INVALID_REGNUM) { lldb_private::UnwindLLDB::RegisterLocation new_regloc;