Re: [PATCH v4 1/2] arm64: Define Documentation/arm64/tagged-address-abi.txt

[Dave Martin <Dave.Martin@arm.com>]

On Thu, Jun 13, 2019 at 11:15:34AM +0100, Vincenzo Frascino wrote:
> Hi Catalin,
> 
> On 12/06/2019 16:35, Catalin Marinas wrote:
> > Hi Vincenzo,
> > 
> > Some minor comments below but it looks fine to me overall. Cc'ing
> > Szabolcs as well since I'd like a view from the libc people.
> > 
> 
> Thanks for this, I saw Szabolcs comments.
> 
> > On Wed, Jun 12, 2019 at 03:21:10PM +0100, Vincenzo Frascino wrote:
> >> diff --git a/Documentation/arm64/tagged-address-abi.txt b/Documentation/arm64/tagged-address-abi.txt
> >> new file mode 100644
> >> index 000000000000..96e149e2c55c
> >> --- /dev/null
> >> +++ b/Documentation/arm64/tagged-address-abi.txt

[...]

> >> +Since it is not desirable to relax the ABI to allow tagged user addresses
> >> +into the kernel indiscriminately, arm64 provides a new sysctl interface
> >> +(/proc/sys/abi/tagged_addr) that is used to prevent the applications from
> >> +enabling the relaxed ABI and a new prctl() interface that can be used to
> >> +enable or disable the relaxed ABI.
> >> +
> >> +The sysctl is meant also for testing purposes in order to provide a simple
> >> +way for the userspace to verify the return error checking of the prctl()
> >> +command without having to reconfigure the kernel.
> >> +
> >> +The ABI properties are inherited by threads of the same application and
> >> +fork()'ed children but cleared when a new process is spawn (execve()).
> > 
> > "spawned".

I'd just say "cleared by execve()."

"Spawn" suggests (v)fork+exec to me (at least, what's what "spawn" means on
certain other OSes).

> > 
> > I guess you could drop these three paragraphs here and mention the
> > inheritance properties when introducing the prctl() below. You can also
> > mention the global sysctl switch after the prctl() was introduced.
> > 
> 
> I will move the last two (rewording them) to the _section_ 2, but I would still
> prefer the Introduction to give an overview of the solution as well.
> 
> >> +
> >> +2. ARM64 Tagged Address ABI
> >> +---------------------------
> >> +
> >> +From the kernel syscall interface prospective, we define, for the purposes
> >> +of this document, a "valid tagged pointer" as a pointer that either it has
> > 
> > "either has" (no 'it') sounds slightly better but I'm not a native
> > English speaker either.
> > 
> >> +a zero value set in the top byte or it has a non-zero value, it is in memory
> >> +ranges privately owned by a userspace process and it is obtained in one of
> >> +the following ways:
> >> +  - mmap() done by the process itself, where either:
> >> +    * flags = MAP_PRIVATE | MAP_ANONYMOUS
> >> +    * flags = MAP_PRIVATE and the file descriptor refers to a regular
> >> +      file or "/dev/zero"
> >> +  - a mapping below sbrk(0) done by the process itself
> >> +  - any memory mapped by the kernel in the process's address space during
> >> +    creation and following the restrictions presented above (i.e. data, bss,
> >> +    stack).
> >> +
> >> +The ARM64 Tagged Address ABI is an opt-in feature, and an application can
> >> +control it using the following prctl()s:
> >> +  - PR_SET_TAGGED_ADDR_CTRL: can be used to enable the Tagged Address ABI.
> > 
> > enable or disable (not sure we need the latter but it doesn't heart).
> > 
> > I'd add the arg2 description here as well.
> > 
> 
> Good point I missed this.
> 
> >> +  - PR_GET_TAGGED_ADDR_CTRL: can be used to check the status of the Tagged
> >> +                             Address ABI.

For both prctls, you should also document the zeroed arguments up to
arg5 (unless we get rid of the enforcement and just ignore them).


Is there a canonical way to detect whether this whole API/ABI is
available?  (i.e., try to call this prctl / check for an HWCAP bit,
etc.)

[...]

Cheers
---Dave