Re: More trouble with i386 EFLAGS and ptrace

[Roland McGrath <roland@redhat.com>]

> Is this semantically different from the patch I posted, i.e. is there
> any case which one of them covers and not the other?

Yes, the second case that I described when I said there were two cases!
(Sheesh.)  To repeat, when the process was doing PTRACE_SINGLESTEP and then
stops on some other signal rather than because of the single-step trap
(e.g. single-stepping an instruction that faults), ptrace will show TF set
in its registers.  With my patch, it will show TF clear.

> That is an inability to set breakpoints in the vsyscall page.  Andrew
> told me (last May, wow) that he thought this worked in Fedora, but I
> haven't seen any signs of the code.  It would certainly be a Good Thing
> if it is possible!

Fedora kernels use a normal mapping (with randomized location) for the
page, rather than the fixed high address in the vanilla kernel.  The
FIXADDR_USER_START area is globally mapped in a special way not using
normal vma data structures, and is permanently read-only in all tasks.  
COW via ptrace works normally for Fedora's flavor, but no writing is ever
possible to the fixmap page.


Thanks,
Roland