Re: [RFC PATCH] x86 single-step (TF) vs system calls & traps

[Roland McGrath <roland@redhat.com>]

> Davide's patch (which has been in -mm for 6-7 weeks) doesn't add
> fastpath overhead.

I am also dubious about exactly what it does.  That patch seems a bizarre
obfuscation of the code to me.  TIF_SINGLESTEP is really there to handle
the lazy TF clearing for sysenter entry, and that's all.  I don't think
that patch handles user-mode setting TF properly, unusual though that case
is.  How does that patch interact with PT_TRACESYSGOOD?  It appears to me
that PTRACE_SINGLESTEP will now generate a syscall trap instead of a
single-step trap, which is an undesireable change in behavior I would say.

I don't really care about user-mode setting of TF before executing int
$0x80.  If poeple have programs that use TF in user mode, they have never
complained about the issue before.  For PTRACE_SINGLESTEP, Davide's
approach of setting the kernel-work flag directly when PTRACE_SINGLESTEP
sets TF in the user flags word is the obvious way to avoid the test in the
fast path.  I am inclined to combine that approeach with what my patch
does, i.e. just take out the system call fast-path test and set
TIF_SINGLESTEP_TRAP in PTRACE_SINGLESTEP.  I think the way Davide's patch
uses TIF_SINGLESTEP is pretty questionable.


Thanks,
Roland