[PATCH v10 0/1] arm64 tagged address ABI

[PATCH v10 0/1] arm64 tagged address ABI

[Catalin Marinas]

Hi,

Minor update to the arm64 tagged address ABI documentation since v9,
posted here:

http://lkml.kernel.org/r/20190821164730.47450-1-catalin.marinas@arm.com

The mmap/mremap/... patch (1/3) has been queued in the -mm tree and
removed from this series. The tagged-address-abi.rst patch (2/3) has
been queued in the arm64 for-next/core tree. There is only one patch
left in this series (keeping the cover letter for consistency).

Changes in v10:

- Remove the tag preservation paragraph since the new ABI does not
  change the behaviour we already have. The only difference is that now
  the kernel can access tagged addresses (e.g. delivering a signal on a
  tagged alternate stack).

Vincenzo Frascino (1):
  arm64: Relax Documentation/arm64/tagged-pointers.rst

 Documentation/arm64/tagged-pointers.rst | 21 ++++++++++++++-------
 1 file changed, 14 insertions(+), 7 deletions(-)


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[PATCH v10 1/1] arm64: Relax Documentation/arm64/tagged-pointers.rst

[Catalin Marinas]

From: Vincenzo Frascino <vincenzo.frascino@arm.com>

On AArch64 the TCR_EL1.TBI0 bit is set by default, allowing userspace
(EL0) to perform memory accesses through 64-bit pointers with a non-zero
top byte. However, such pointers were not allowed at the user-kernel
syscall ABI boundary.

With the Tagged Address ABI patchset, it is now possible to pass tagged
pointers to the syscalls. Relax the requirements described in
tagged-pointers.rst to be compliant with the behaviours guaranteed by
the AArch64 Tagged Address ABI.

Cc: Will Deacon <will.deacon@arm.com>
Cc: Szabolcs Nagy <szabolcs.nagy@arm.com>
Cc: Kevin Brodsky <kevin.brodsky@arm.com>
Acked-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Co-developed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
---
 Documentation/arm64/tagged-pointers.rst | 21 ++++++++++++++-------
 1 file changed, 14 insertions(+), 7 deletions(-)

diff --git a/Documentation/arm64/tagged-pointers.rst b/Documentation/arm64/tagged-pointers.rst
index 2acdec3ebbeb..eab4323609b9 100644
--- a/Documentation/arm64/tagged-pointers.rst
+++ b/Documentation/arm64/tagged-pointers.rst
@@ -20,7 +20,9 @@ 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 application enables the AArch64
+Tagged Address ABI explicitly
+(Documentation/arm64/tagged-address-abi.rst).
 
 This includes, but is not limited to, addresses found in:
 
@@ -33,13 +35,15 @@ 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 enable the AArch64 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 AArch64 Tagged Address ABI is disabled,
+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
@@ -59,6 +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 behaviour is maintained when the AArch64 Tagged Address ABI is
+enabled.
+
 
 Other considerations
 --------------------

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