On 2019-09-06, Jeff Layton <jlayton@xxxxxxxxxx> wrote: > On Fri, 2019-09-06 at 18:06 +0200, Mickaël Salaün wrote: > > On 06/09/2019 17:56, Florian Weimer wrote: > > > Let's assume I want to add support for this to the glibc dynamic loader, > > > while still being able to run on older kernels. > > > > > > Is it safe to try the open call first, with O_MAYEXEC, and if that fails > > > with EINVAL, try again without O_MAYEXEC? > > > > The kernel ignore unknown open(2) flags, so yes, it is safe even for > > older kernel to use O_MAYEXEC. > > > > Well...maybe. What about existing programs that are sending down bogus > open flags? Once you turn this on, they may break...or provide a way to > circumvent the protections this gives. It should be noted that this has been a valid concern for every new O_* flag introduced (and yet we still introduced new flags, despite the concern) -- though to be fair, O_TMPFILE actually does have a work-around with the O_DIRECTORY mask setup. The openat2() set adds O_EMPTYPATH -- though in fairness it's also backwards compatible because empty path strings have always given ENOENT (or EINVAL?) while O_EMPTYPATH is a no-op non-empty strings. > Maybe this should be a new flag that is only usable in the new openat2() > syscall that's still under discussion? That syscall will enforce that > all flags are recognized. You presumably wouldn't need the sysctl if you > went that route too. I'm also interested in whether we could add an UPGRADE_NOEXEC flag to how->upgrade_mask for the openat2(2) patchset (I reserved a flag bit for it, since I'd heard about this work through the grape-vine). -- Aleksa Sarai Senior Software Engineer (Containers) SUSE Linux GmbH <https://www.cyphar.com/>
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