Re: [PATCH v4 2/2] arm64: Relax Documentation/arm64/tagged-pointers.txt

[Szabolcs Nagy]

On 12/06/2019 15:21, Vincenzo Frascino wrote:
> On arm64 the TCR_EL1.TBI0 bit has been always enabled hence
> the userspace (EL0) is allowed to set a non-zero value in the
> top byte but the resulting pointers are not allowed at the
> user-kernel syscall ABI boundary.
> 
> With the relaxed ABI proposed in this set, it is now possible to pass
> tagged pointers to the syscalls, when these pointers are in memory
> ranges obtained by an anonymous (MAP_ANONYMOUS) mmap().
> 
> Relax the requirements described in tagged-pointers.txt to be compliant
> with the behaviours guaranteed by the ARM64 Tagged Address ABI.
> 
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Will Deacon <will.deacon@xxxxxxx>
> CC: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
> Signed-off-by: Vincenzo Frascino <vincenzo.frascino@xxxxxxx>
> ---
>  Documentation/arm64/tagged-pointers.txt | 23 ++++++++++++++++-------
>  1 file changed, 16 insertions(+), 7 deletions(-)
> 
> diff --git a/Documentation/arm64/tagged-pointers.txt b/Documentation/arm64/tagged-pointers.txt
> index a25a99e82bb1..db58a7e95805 100644
> --- a/Documentation/arm64/tagged-pointers.txt
> +++ b/Documentation/arm64/tagged-pointers.txt
> @@ -18,7 +18,8 @@ Passing tagged addresses to the kernel
>  --------------------------------------
>  
>  All interpretation of userspace memory addresses by the kernel assumes
> -an address tag of 0x00.
> +an address tag of 0x00, unless the userspace opts-in the ARM64 Tagged
> +Address ABI via the PR_SET_TAGGED_ADDR_CTRL prctl().
>  
>  This includes, but is not limited to, addresses found in:
>  
> @@ -31,18 +32,23 @@ This includes, but is not limited to, addresses found in:
>   - the frame pointer (x29) and frame records, e.g. when interpreting
>     them to generate a backtrace or call graph.
>  
> -Using non-zero address tags in any of these locations may result in an
> -error code being returned, a (fatal) signal being raised, or other modes
> -of failure.
> +Using non-zero address tags in any of these locations when the
> +userspace application did not opt-in to the ARM64 Tagged Address ABI,
> +may result in an error code being returned, a (fatal) signal being raised,
> +or other modes of failure.
>  
> -For these reasons, passing non-zero address tags to the kernel via
> -system calls is forbidden, and using a non-zero address tag for sp is
> -strongly discouraged.
> +For these reasons, when the userspace application did not opt-in, passing
> +non-zero address tags to the kernel via system calls is forbidden, and using
> +a non-zero address tag for sp is strongly discouraged.
>  
>  Programs maintaining a frame pointer and frame records that use non-zero
>  address tags may suffer impaired or inaccurate debug and profiling
>  visibility.
>  
> +A definition of the meaning of ARM64 Tagged Address ABI and of the
> +guarantees that the ABI provides when the userspace opts-in via prctl()
> +can be found in: Documentation/arm64/tagged-address-abi.txt.
> +

OK.

>  
>  Preserving tags
>  ---------------
> @@ -57,6 +63,9 @@ be preserved.
>  The architecture prevents the use of a tagged PC, so the upper byte will
>  be set to a sign-extension of bit 55 on exception return.
>  
> +This behaviours are preserved even when the the userspace opts-in the ARM64

these behaviours.

> +Tagged Address ABI via the PR_SET_TAGGED_ADDR_CTRL prctl().
> +
>  
>  Other considerations
>  --------------------
>