[PATCH v8 5/5] arm64: Relax Documentation/arm64/tagged-pointers.rst

From: Catalin Marinas <catalin.marinas@arm.com>
To: linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Vincenzo Frascino <vincenzo.frascino@arm.com>,
	Will Deacon <will.deacon@arm.com>,
	Andrey Konovalov <andreyknvl@google.com>,
	Szabolcs Nagy <szabolcs.nagy@arm.com>,
	Kevin Brodsky <kevin.brodsky@arm.com>,
	Dave P Martin <Dave.Martin@arm.com>,
	Dave Hansen <dave.hansen@intel.com>,
	linux-doc@vger.kernel.org, linux-arch@vger.kernel.org
Subject: [PATCH v8 5/5] arm64: Relax Documentation/arm64/tagged-pointers.rst
Date: Thu, 15 Aug 2019 16:44:03 +0100	[thread overview]
Message-ID: <20190815154403.16473-6-catalin.marinas@arm.com> (raw)
In-Reply-To: <20190815154403.16473-1-catalin.marinas@arm.com>

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: Andrey Konovalov <andreyknvl@google.com>
Cc: Szabolcs Nagy <szabolcs.nagy@arm.com>
Cc: Kevin Brodsky <kevin.brodsky@arm.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 | 23 ++++++++++++++++-------
 1 file changed, 16 insertions(+), 7 deletions(-)

diff --git a/Documentation/arm64/tagged-pointers.rst b/Documentation/arm64/tagged-pointers.rst
index 2acdec3ebbeb..fd5306019e91 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 +63,11 @@ 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. In addition, with the exceptions above, the kernel will
+preserve any non-zero tags passed by the user via syscalls and stored in
+kernel data structures (e.g. set_robust_list(), sigaltstack()).
+
 
 Other considerations
 --------------------
