> On May 3, 2021, at 3:56 PM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote: > > On Mon, May 03 2021 at 15:08, Linus Torvalds wrote: >>> On Mon, May 3, 2021 at 2:49 PM Andy Lutomirski <luto@xxxxxxxxxx> wrote: >>> >>> To be clear, I'm suggesting that we -EINVAL the PTRACE_GETREGS calls >>> and such, not the ATTACH. I have no idea what gdb will do if this >>> happens, though. >> >> I feel like the likelihood that it will make gdb work any better is >> basically zero. >> >> I think we should just do Stefan's patch - I assume it generates >> something like four instructions (two loads, two stores) on x86-64, >> and it "just works". >> >> Yeah, yeah, it presumably generates 8 instructions on 32-bit x86, and >> we could fix that by just using the constant __USER_CS/DS instead (no >> loads necessary) since 32-bit doesn't have any compat issues. >> >> But is it worth complicating the patch for a couple of instructions in >> a non-critical path? >> >> And I don't see anybody stepping up to say "yes, I will do the patch >> for gdb", so I really think the least pain is to just take the very >> straightforward and tested kernel patch. >> >> Yes, yes, that also means admitting to ourselves that the gdb >> situation isn't likely going to improve, but hey, if nobody in this >> thread is willing to work on the gdb side to fix the known issues >> there, isn't that the honest thing to do anyway? > > GDB is one thing. But is this setup actually correct under all > circumstances? > > It's all fine that we have lots of blurb about GDB, but there is no > reasoning why this does not affect regular kernel threads which take the > same code path. > > Neither is there an answer what happens in case of a signal delivered to > this thread and what any other GDB/ptraced induced poking might cause. > > This is a half setup user space thread which is assumed to behave like a > regular kernel thread, but is this assumption actually true? > > I’m personally concerned about FPU state. No one ever imagined when writing and reviewing the FPU state code that we were going to let ptrace poke the state on a kernel thread. Now admittedly kernel_execve() magically turns kernel threads into user threads, but, again, I see no evidence that anyone has thought through all the implications of letting ptrace go to town before doing so. (Is the io_uring thread a kthread style kernel thread? kthread does horrible, horrible things with the thread stack.)