[PATCH v11 13/14] arm64: update Documentation/arm64/tagged-pointers.txt

[Andrey Konovalov]

This patch is a part of a series that extends arm64 kernel ABI to allow to
pass tagged user pointers (with the top byte set to something else other
than 0x00) as syscall arguments.

Document the ABI changes in Documentation/arm64/tagged-pointers.txt.

Signed-off-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
---
 Documentation/arm64/tagged-pointers.txt | 18 ++++++++----------
 1 file changed, 8 insertions(+), 10 deletions(-)

diff --git a/Documentation/arm64/tagged-pointers.txt b/Documentation/arm64/tagged-pointers.txt
index a25a99e82bb1..07fdddeacad0 100644
--- a/Documentation/arm64/tagged-pointers.txt
+++ b/Documentation/arm64/tagged-pointers.txt
@@ -17,13 +17,15 @@ this byte for application use.
 Passing tagged addresses to the kernel
 --------------------------------------
 
-All interpretation of userspace memory addresses by the kernel assumes
-an address tag of 0x00.
+The kernel supports tags in pointer arguments (including pointers in
+structures) of syscalls, however such pointers must point to memory ranges
+obtained by anonymous mmap() or brk().
 
-This includes, but is not limited to, addresses found in:
+The kernel supports tags in user fault addresses. However the fault_address
+field in the sigcontext struct will contain an untagged address.
 
- - pointer arguments to system calls, including pointers in structures
-   passed to system calls,
+All other interpretations of userspace memory addresses by the kernel
+assume an address tag of 0x00, in particular:
 
  - the stack pointer (sp), e.g. when interpreting it to deliver a
    signal,
@@ -33,11 +35,7 @@ This includes, but is not limited to, addresses found in:
 
 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.
+of failure. 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
-- 
2.21.0.360.g471c308f928-goog