linux-kernel - Re: [PATCH v2 3/5] memfd: improve userspace warnings for missing exec-related flags

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Message-ID: <20230906.065211-swollen.overhaul.chosen.plan-vGnGFFTg39x8@cyphar.com>
Date:   Wed, 6 Sep 2023 16:58:41 +1000
From:   Aleksa Sarai <cyphar@...har.com>
To:     Florian Weimer <fweimer@...hat.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Damian Tometzki <dtometzki@...oraproject.org>,
        Shuah Khan <shuah@...nel.org>, Jeff Xu <jeffxu@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Daniel Verkamp <dverkamp@...omium.org>,
        Christian Brauner <brauner@...nel.org>,
        Dominique Martinet <asmadeus@...ewreck.org>,
        stable@...r.kernel.org, linux-api@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v2 3/5] memfd: improve userspace warnings for missing
 exec-related flags

On 2023-09-05, Florian Weimer <fweimer@...hat.com> wrote:
> * Andrew Morton:
> 
> > OK, thanks, I'll revert this.  Spamming everyone even harder isn't a
> > good way to get developers to fix their stuff.
> 
> Is this really buggy userspace?  Are future kernels going to require
> some of these flags?
> 
> That's going to break lots of applications which use memfd_create to
> enable run-time code generation on locked-down systems because it looked
> like a stable interface (“don't break userspace” and all that).

There is no userspace breakage with the current behaviour and obviously
actually requiring these flags to be passed by default would be a pretty
clear userspace breakage and would never be merged.

The original intention (as far as I can tell -- the logging behaviour
came from the original patchset) was to try to incentivise userspace to
start passing the flags so that if distributions decide to set
vm.memfd_noexec=1 as a default setting you won't end up with programs
that _need_ executable memfds (such as container runtimes) crashing
unexpectedly. I also suspect there was an aspect of "well, userspace
*should* be passing these flags after we've introduced them".

I'm sending a patch to just remove this part of the logging because I
don't think it makes sense if you can't rate-limit it sanely, and
there's probably an argument to be made that it doesn't make sense at
all (at least for the default vm.memfd_noexec=0 setting).

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

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