Re: [PATCH v2 1/5] fs: Add support for an O_MAYEXEC flag on sys_open()

Aleksa Sarai
2019-09-06T18:45:26Z

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On 2019-09-06, Mickaël Salaün <mickael.salaun@ssi.gouv.fr> 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.

Depends on your definition of "safe" -- a security feature that you will
silently not enable on older kernels doesn't sound super safe to me.
Unfortunately this is a limitation of open(2) that we cannot change --
which is why the openat2(2) proposal I've been posting gives -EINVAL for
unknown O_* flags.

There is a way to probe for support (though unpleasant), by creating a
test O_MAYEXEC fd and then checking if the flag is present in
/proc/self/fdinfo/$n.

--
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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