Re: [PATCH 4/4] sigaltstack: allow disabling and re-enabling sas within sighandler

On 02/01, Stas Sergeev wrote:

quoted

So the sequence is

// running on alt stack

sigaltstack(SS_DISABLE);

temporary_run_on_another_stack();

sigaltstack(SS_ONSTACK);

and SS_DISABLE saves us from another SA_ONSTACK signal, right?

Yes.
Note: there is a test-case in that patch serie from which
you can see or copy/paste the sample code.

OK, I wasn't cc'ed

quoted

But afaics it can only help after we change the stack. Suppose that SA_ONSTACK signal
comess before temporary_run_on_another_stack(). get_sigframe() should be fine after
your changes (afaics), it won't pick the alt stack after SS_DISABLE.

However, unless I missed something save_altstack_ex() will record SS_ONSTACK in
uc_stack->ss_flags, and after return from signal handler restore_altstack() will
enable alt stack again?

I don't think so. Please see the following hunk:

Yes, see another email, I already noticed this change.

So I understand this is very confusing, but I think the patch
is correct.

Not sure, but I can hardly read this patch and I can't apply it.

Do you think adding the SS_FORCE flag would be a better solution?

Yes, certainly. I see no point to remember that a thread actually has the alt stack
but it was disabled.

Oleg.