[lldb] [debugserver] Use "full" x86_64 GPR state when available. (#108663) #9324
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…m#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 llvm#57591 (also filed as Apple FB11464104) @jasonmolenda (cherry picked from commit 7281e0c)
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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(), 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
gcommand allows returningxto denote unavailable registers, but it seems like the debug server uses newer commands likejThreadsInfoand I'm not sure if those have the same support.Fixes llvm#57591
(also filed as Apple FB11464104)
@jasonmolenda
(cherry picked from commit 7281e0c)