On 2019-09-06, Mickaël Salaün [off-list ref] wrote:

On 06/09/2019 17:56, Florian Weimer wrote:

quoted

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/>