================ @@ -229,3 +229,64 @@ bytes. `zt0`'s value and whether it is active or not will be saved prior to expression evaluation and restored afterwards. + +## Guarded Control Stack Extension (GCS) + +GCS support includes the following new registers: + +* `gcs_features_enabled` +* `gcs_features_locked` +* `gcspr_el0` + +These map to the registers ptrace provides. The first two have had a `gcs_` +prefix added as their names are too generic without it. + +When the GCS is enabled the kernel allocates a memory region for it. This region +has a special attribute that LLDB will detect and presents like this: +``` + (lldb) memory region --all + <...> + [0x0000fffff7a00000-0x0000fffff7e00000) rw- + shadow stack: yes + [0x0000fffff7e00000-0x0000fffff7e10000) --- +``` + +`shadow stack` is a generic term used in the kernel for secure stack +extensions like GCS. + +### Expression Evaluation + +To execute an expression when GCS is enabled, LLDB must push the return +address of the expression wrapper (usually the entry point of the program) +to the Guarded Control Stack. It does this by decrementing `gcspr_el0` and +writing to the location now pointed to by `gcspr_el0` (instead of using the +GCS push instructions). + +After an expression finishes, LLDB will restore the contents of all 3 +GCS registers, apart from the enable bit of `gcs_features_enabled`. This is +because there are limits on how often and from where you can set this +bit. + +We cannot enable GCS from ptrace at all and it is expected that a process +that has enabled GCS then disabled it, will not enable it again. The simplest +choice was to not restore the enable bit at all. It is up to the user or +program to manage that bit. + +The return address that LLDB pushed onto the Guarded Control Stack will be left +in place. As will any values that were pushed to the stack by functions run +during the expresison. ---------------- omjavaid wrote:
typo s/expresison/expression https://github.com/llvm/llvm-project/pull/117860 _______________________________________________ lldb-commits mailing list lldb-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/lldb-commits
