[PATCH 4/4] arm64: documentation: document tagged pointer stack constraints

[Dave P Martin]

On Thu, Apr 20, 2017 at 07:17:14PM +0100, Kristina Martsenko wrote:
> Some kernel features don't currently work if a task puts a non-zero
> address tag in its stack pointer, frame pointer, or frame record entries
> (FP, LR).
>
> For example, with a tagged stack pointer, the kernel can't deliver
> signals to the process, and the task is killed instead. As another
> example, with a tagged frame pointer or frame records, perf fails to
> generate call graphs or resolve symbols.
>
> For now, just document these limitations, instead of finding and fixing
> everything that doesn't work, as it's not known if anyone needs to use
> tags in these places anyway.
>
> In addition, as requested by Dave Martin, generalize the limitations
> into a general kernel address tag policy, and refactor
> tagged-pointers.txt to include it.
>
> Cc: Dave Martin <dave.martin at arm.com>
> Signed-off-by: Kristina Martsenko <kristina.martsenko at arm.com>

Looks OK to me.  It's not feasible to describe all possible scenarios
here, but this makes the kernel's intentions clearer.

Reviewed-by: Dave Martin <Dave.Martin at arm.com>

> ---
>  Documentation/arm64/tagged-pointers.txt | 62 +++++++++++++++++++++++++--------
>  1 file changed, 47 insertions(+), 15 deletions(-)
>
> diff --git a/Documentation/arm64/tagged-pointers.txt b/Documentation/arm64/tagged-pointers.txt
> index d9995f1f51b3..a25a99e82bb1 100644
> --- a/Documentation/arm64/tagged-pointers.txt
> +++ b/Documentation/arm64/tagged-pointers.txt
> @@ -11,24 +11,56 @@ in AArch64 Linux.
>  The kernel configures the translation tables so that translations made
>  via TTBR0 (i.e. userspace mappings) have the top byte (bits 63:56) of
>  the virtual address ignored by the translation hardware. This frees up
> -this byte for application use, with the following caveats:
> +this byte for application use.
>
> -     (1) The kernel requires that all user addresses passed to EL1
> -         are tagged with tag 0x00. This means that any syscall
> -         parameters containing user virtual addresses *must* have
> -         their top byte cleared before trapping to the kernel.
>
> -     (2) Non-zero tags are not preserved when delivering signals.
> -         This means that signal handlers in applications making use
> -         of tags cannot rely on the tag information for user virtual
> -         addresses being maintained for fields inside siginfo_t.
> -         One exception to this rule is for signals raised in response
> -         to watchpoint debug exceptions, where the tag information
> -         will be preserved.
> +Passing tagged addresses to the kernel
> +--------------------------------------
>
> -     (3) Special care should be taken when using tagged pointers,
> -         since it is likely that C compilers will not hazard two
> -         virtual addresses differing only in the upper byte.
> +All interpretation of userspace memory addresses by the kernel assumes
> +an address tag of 0x00.
> +
> +This includes, but is not limited to, addresses found in:
> +
> + - pointer arguments to system calls, including pointers in structures
> +   passed to system calls,
> +
> + - the stack pointer (sp), e.g. when interpreting it to deliver a
> +   signal,
> +
> + - 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.
> +
> +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.
> +
> +Programs maintaining a frame pointer and frame records that use non-zero
> +address tags may suffer impaired or inaccurate debug and profiling
> +visibility.
> +
> +
> +Preserving tags
> +---------------
> +
> +Non-zero tags are not preserved when delivering signals. This means that
> +signal handlers in applications making use of tags cannot rely on the
> +tag information for user virtual addresses being maintained for fields
> +inside siginfo_t. One exception to this rule is for signals raised in
> +response to watchpoint debug exceptions, where the tag information will
> +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.
> +
> +
> +Other considerations
> +--------------------
> +
> +Special care should be taken when using tagged pointers, since it is
> +likely that C compilers will not hazard two virtual addresses differing
> +only in the upper byte.
> --
> 2.1.4
>
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