Re: [REGRESSION] x86/entry: TIF_SINGLESTEP handling is still broken

[Linus Torvalds]

On Sun, Jan 31, 2021 at 3:35 PM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> I wonder if the simple solution is to just
>
>  (a) always set one of the SYSCALL_WORK_EXIT bits on the child in
> ptrace (exactly to catch the child on system call exit)
>
>  (b) basically revert 299155244770 ("entry: Drop usage of TIF flags in
> the generic syscall code") and have the syscall exit code check the
> TIF_SINGLESTEP flag

Actually, (b) looks unnecessary - as long as we get to
syscall_exit_work(), the current code will work fine.

So maybe just add a dummy SYSCALL_WORK_SYSCALL_EXIT_TRAP, and set that
flag whenever a singestep is requested for a process that is currently
in a system call?

IOW, make it a very explicit "do TF for system calls", rather than the
old code that was doing so implicitly and not very obviously. Hmm?

                    Linus