From: Christian Brauner <christian.brauner@ubuntu.com>
To: Florian Weimer <fweimer@redhat.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
libc-alpha <libc-alpha@sourceware.org>,
Joseph Myers <joseph@codesourcery.com>
Subject: Re: [RFC PATCH glibc] Linux: Use fixed rseq_len value for rseq registration
Date: Tue, 14 Jul 2020 15:33:52 +0200 [thread overview]
Message-ID: <20200714133352.ykv6qg6brzqlrc26@wittgenstein> (raw)
In-Reply-To: <87zh826x9s.fsf@oldenburg2.str.redhat.com>
On Tue, Jul 14, 2020 at 03:28:47PM +0200, Florian Weimer via Libc-alpha wrote:
> * Mathieu Desnoyers:
>
> > ----- On Jul 14, 2020, at 4:51 AM, Florian Weimer fweimer@redhat.com wrote:
> >
> >> * Mathieu Desnoyers:
> >>
> >>> + /* The rseq_len parameter does not allow extending struct rseq. Fix its
> >>> + value to 32 as expected by the Linux kernel. */
> >>> + ret = INTERNAL_SYSCALL_CALL (rseq, &__rseq_abi, 32, 0, RSEQ_SIG);
> >>
> >> If the layout of struct rseq can change in the kernel headers, than far
> >> more significant changes are needed. glibc cannot change its ABI
> >> depending on the version of the kernel headers it is compiled against.
> >
> > What I have in mind for struct rseq is that the current structure layout
> > is guaranteed as a "minimum" requirement. It can then be extended by the
> > kernel UAPI in a backward compatible way: if user-space provides enough
> > room for extension, and the kernel supports the additional features, then
> > user-space can use those additional features.
> >
> > Adding extra room in the __rseq_abi can be done though:
> >
> > - Upgrading glibc with knowledge of the extra layout,
> > - Preloading an interposition library which defines __rseq_abi with the
> > extended layout,
> > - Defining __rseq_abi with extended layout in the application binary.
> >
> > What I suspect is your concern here is that two glibc-2.32 builds against
> > different versions of kernel headers could lead to different __rseq_abi
> > size, which is unexpected.
>
> Indeed.
>
> > Would using the internal struct rseq all the time in glibc (rather than
> > uapi linux/rseq.h) resolve your concern ? This way, glibc could integrate
> > extended layouts on version bump (e.g. 2.33).
>
> It works reliably as long as glibc only ever uses the minimum rseq size.
> And since glibc monopolizes the rseq registration, applications cannot
> register a larger area. So there is no way to make use of any future
> kernel extensions.
But when you bump ABI in glibc you can switch to a new rseq size, no?
Christian
next prev parent reply other threads:[~2020-07-14 13:33 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-13 19:34 Mathieu Desnoyers
2020-07-14 8:51 ` Florian Weimer
2020-07-14 12:31 ` Mathieu Desnoyers
2020-07-14 13:28 ` Florian Weimer
2020-07-14 13:33 ` Christian Brauner [this message]
2020-07-14 13:54 ` Florian Weimer
2020-07-14 14:04 ` Mathieu Desnoyers
2020-07-14 15:07 ` Florian Weimer
2020-07-14 15:30 ` Mathieu Desnoyers
2020-07-14 16:01 ` Szabolcs Nagy
2020-07-14 16:12 ` Mathieu Desnoyers
2020-07-14 17:03 ` Florian Weimer
2020-07-14 17:12 ` Mathieu Desnoyers
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