Subject: Re: [PATCH v9 3/6] signal: clear non-uapi flag bits when passing/returning sa_flags

From: Dave Martin <[email-masked]>
Date: Tue, 25 Aug 2020 15:25:14 +0100

On Mon, Aug 24, 2020 at 05:51:34PM -0700, Peter Collingbourne wrote:
> On Mon, Aug 24, 2020 at 6:40 AM Dave Martin <[email-masked]> wrote:
> >
> > On Wed, Aug 19, 2020 at 04:39:53PM -0700, Peter Collingbourne wrote:
> > > On Wed, Aug 19, 2020 at 3:39 AM Dave Martin <[email-masked]> wrote:
> > > >
> > > > On Mon, Aug 17, 2020 at 08:33:48PM -0700, Peter Collingbourne wrote:

[...]

> > > > > diff --git a/kernel/signal.c b/kernel/signal.c
> > > > > index 42b67d2cea37..348b7981f1ff 100644
> > > > > --- a/kernel/signal.c
> > > > > +++ b/kernel/signal.c
> > > > > @@ -3984,6 +3984,16 @@ int do_sigaction(int sig, struct k_sigaction *act, struct k_sigaction *oact)
> > > > >       if (oact)
> > > > >               *oact = *k;
> > > > >
> > > > > +     /*
> > > > > +      * Clear unknown flag bits in order to allow userspace to detect missing
> > > > > +      * support for flag bits and to allow the kernel to use non-uapi bits
> > > > > +      * internally.
> > > > > +      */
> > > > > +     if (act)
> > > > > +             act->sa.sa_flags &= SA_UAPI_FLAGS;
> > > > > +     if (oact)
> > > > > +             oact->sa.sa_flags &= SA_UAPI_FLAGS;
> > > > > +
> > > >
> > > > Seems reasonable.
> > >
> > > Thanks. I also decided to check how other operating systems handle
> > > unknown flag bits in sigaction.sa_flags. It looks like OpenBSD and
> > > illumos also accept unknown bits but (implicitly, as a result of using
> > > a different internal representation) end up clearing them in oldact:
> > >
> > > https://github.com/openbsd/src/blob/f634a6a4b5bf832e9c1de77f7894ae2625e74484/sys/kern/kern_sig.c#L278
> > > https://github.com/illumos/illumos-gate/blob/76f19f5fdc974fe5be5c82a556e43a4df93f1de1/usr/src/uts/common/syscall/sigaction.c#L86
> 
> XNU does the same:
> 
> https://github.com/apple/darwin-xnu/blob/a449c6a3b8014d9406c2ddbdc81795da24aa7443/bsd/kern/kern_sig.c#L480
> 
> > >
> > > and FreeBSD and NetBSD fail the syscall if unknown bits are set:
> > >
> > > https://github.com/freebsd/freebsd/blob/eded70c37057857c6e23fae51f86b8f8f43cd2d0/sys/kern/kern_sig.c#L699
> > > https://github.com/NetBSD/src/blob/3365779becdcedfca206091a645a0e8e22b2946e/sys/kern/sys_sig.c#L473
> > >
> > > So there is some precedent for doing what we're planning to do here,
> > > which makes it yet more likely that we'll be okay doing this.
> >
> > Ack, it's good to have that extra evidence to support this approach.
> >
> > This also means that other OSes could adopt the new Linux flag(s) with
> > comatible semantics, if they wanted to.  Or have I misunderstood
> > something there?
> 
> The other OSs could adopt SA_XFLAGS, but they would probably have no
> need for SA_UNSUPPORTED because they already have a protocol for
> detecting missing flag support in the kernel (Linux is really the odd
> one out here in not supporting such a protocol from the start).
> Userspace programs running on OpenBSD, illumos and XNU could use the
> Linux protocol without the SA_UNSUPPORTED part, while programs running
> on FreeBSD and NetBSD could do something like:
> 
> static bool has_xflags = true;
> [...]
> struct sigaction act;
> act.sa_flags = SA_SIGINFO | SA_XFLAGS;
> if (sigaction(SIGSEGV, &act, 0) != 0) {
>   has_xflags = false;
>   act.sa_flags = SA_SIGINFO;
>   if (sigaction(SIGSEGV, &act, 0) != 0)
>     perror("sigaction");
> }
> 
> It would probably be possible to write a unified function that would
> support all three protocols.

Ack, I think that's about as well as we can reasonably do in the
circumstances.

Cheers
---Dave

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