Subject: Re: [PATCH RFC v3 1/2] mm: Add personality flag to limit address to 47 bits

From: Charlie Jenkins <[email-masked]>
Date: Thu, 12 Sep 2024 14:15:59 -0700

On Thu, Sep 12, 2024 at 11:53:49AM +0100, Catalin Marinas wrote:
> On Wed, Sep 11, 2024 at 11:18:12PM -0700, Charlie Jenkins wrote:
> > Opting-in to the higher address space is reasonable. However, it is not
> > my preference, because the purpose of this flag is to ensure that
> > allocations do not exceed 47-bits, so it is a clearer ABI to have the
> > applications that want this guarantee to be the ones setting the flag,
> > rather than the applications that want the higher bits setting the flag.
> 
> Yes, this would be ideal. Unfortunately those applications don't know
> they need to set a flag in order to work.

It's not a regression, the applications never worked (on platforms that
do not have this default). The 47-bit default would allow applications
that didn't work to start working at the cost of a non-ideal ABI. That
doesn't seem like a reasonable tradeoff to me.  If applications want to
run on new hardware that has different requirements, shouldn't they be
required to update rather than expect the kernel will solve their
problems for them?

> 
> A slightly better option is to leave the default 47-bit at the kernel
> ABI level and have the libc/dynamic loader issue the prctl(). You can
> control the default with environment variables if needed.

Having glibc set the 47-bit requirement could make it slightly easier
for applications since they would only have to set the environment
variable. After the kernel interface is approved I can look into
supporting that.

- Charlie

> 
> We do something similar in glibc for arm64 MTE. When MTE is enabled, the
> top byte of an allocated pointer contains the tag that must not be
> corrupted. We left the decision to the C library via the
> glibc.mem.tagging tunable (Android has something similar via the app
> manifest). An app can change the default if it wants but if you run with
> old glibc or no environment variable to say otherwise, the default would
> be safe. Distros can set the environment to be the maximum range by
> default if they know the apps included have been upgraded and tested.
> 
> -- 
> Catalin