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.

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.

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