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

[Andy Lutomirski]

On Sun, Jan 31, 2016 at 8:28 AM, Stas Sergeev <stsp@xxxxxxx> wrote:
>
> linux implements the sigaltstack() in a way that makes it impossible to
> use with swapcontext(). Per the man page, sigaltstack is allowed to return
> EPERM if the process is altering its sigaltstack while running on
> sigaltstack.
> This is likely needed to consistently return oss->ss_flags, that indicates
> whether the process is being on sigaltstack or not.
> Unfortunately, linux takes that permission to return EPERM too literally:
> it returns EPERM even if you don't want to change to another sigaltstack,
> but only want to temporarily disable sigaltstack with SS_DISABLE.
> You can't use swapcontext() without disabling sigaltstack first, or the
> stack will be re-used and overwritten by a subsequent signal.
>
> With this patch, disabling sigaltstack inside a signal handler became
> possible, and the swapcontext() can then be used safely. After switching
> back to the sighandler, the app can re-enable the sigatlstack.
> The oss->ss_flags will correctly indicate the current use of sigaltstack,
> even if it is temporarily disabled. Any attempt to modify the sigaltstack
> (rather than to disable or re-enable it) within the sighandler, will still
> be punished with EPERM as suggested by POSIX.

This seems considerably more complicated than my previous proposal to
add an SS_FORCE flag to say "I know what I'm doing.  Ignore POSIX and
let me change the sigaltstack configuration even if it's in use".
What's the advantage?

--Andy