[PATCH v9 7/8] arm64: update Documentation/arm64/tagged-pointers.txt

From: Andrey Konovalov
Date: Mon Dec 10 2018 - 07:51:32 EST

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

Signed-off-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
Documentation/arm64/tagged-pointers.txt | 25 +++++++++++++++----------
1 file changed, 15 insertions(+), 10 deletions(-)

diff --git a/Documentation/arm64/tagged-pointers.txt b/Documentation/arm64/tagged-pointers.txt
index a25a99e82bb1..f4cf1f5cf362 100644
--- a/Documentation/arm64/tagged-pointers.txt
+++ b/Documentation/arm64/tagged-pointers.txt
@@ -17,13 +17,22 @@ 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) for a limited set of syscalls, the exceptions are:

-This includes, but is not limited to, addresses found in:
+ - memory syscalls: brk, madvise, mbind, mincore, mlock, mlock2, move_pages,
+ mprotect, mremap, msync, munlock, munmap, pkey_mprotect, process_vm_readv,
+ process_vm_writev, remap_file_pages;

- - pointer arguments to system calls, including pointers in structures
- passed to system calls,
+ - ioctls that accept user pointers that describe virtual memory ranges;
+ - TCP_ZEROCOPY_RECEIVE setsockopt.
+The kernel supports tags in user fault addresses. However the fault_address
+field in the sigcontext struct will contain an untagged address.
+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
@@ -33,11 +42,7 @@ 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