llvm-project

7281e0cb - [lldb] [debugserver] Use "full" x86_64 GPR state when available. (#108663)

[lldb] [debugserver] Use "full" x86_64 GPR state when available. (#108663) macOS 10.15 added a "full" x86_64 GPR thread state flavor, equivalent to the normal one but with DS, ES, SS, and GSbase added. This flavor can only be used with processes that install a custom LDT (functionality that was also added in 10.15 and is used by apps like Wine to execute 32-bit code). Along with allowing DS, ES, SS, and GSbase to be viewed/modified, using the full flavor is necessary when debugging a thread executing 32-bit code. If thread_set_state() is used with the regular thread state flavor, the kernel resets CS to the 64-bit code segment (see [set_thread_state64()](https://github.com/apple-oss-distributions/xnu/blob/94d3b452840153a99b38a3a9659680b2a006908e/osfmk/i386/pcb.c#L723), which makes debugging impossible. There's no way to detect whether the full flavor is available, try to use it and fall back to the regular one if it's not available. A downside is that this patch exposes the DS, ES, SS, and GSbase registers for all x86_64 processes, even though they are not populated unless the full thread state is available. I'm not sure if there's a way to tell LLDB that a register is unavailable. The classic GDB `g` command [allows returning `x`](https://sourceware.org/gdb/current/onlinedocs/gdb.html/Packets.html#Packets) to denote unavailable registers, but it seems like the debug server uses newer commands like `jThreadsInfo` and I'm not sure if those have the same support. Fixes #57591 (also filed as Apple FB11464104) @jasonmolenda