arm64: mte: Fix PTRACE_{PEEK,POKE}MTETAGS error documentation

PTRACE_{PEEK,POKE}MTETAGS return -EIO rather than -EOPNOTSUPP (as
documented) when no tags are copied from/to a mapping without PROT_MTE.
This has been the behaviour since the interface was introduced, though
the original intent was to distinguish between address not being
accessible and mapped as untagged.

Update the documentation to match the implementation (de-facto ABI).
Since -EOPNOTSUPP was never returned, change the error assignment to
-EIO as well to avoid confusion.

Fixes: df9d7a22dd ("arm64: mte: Add Memory Tagging Extension documentation")
Fixes: 18ddbaa02b ("arm64: mte: ptrace: Add PTRACE_{PEEK,POKE}MTETAGS support")
Reported-by: Yury Khrustalev <yury.khrustalev@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>

Documentation/arch/arm64/memory-tagging-extension.rst

@@ -208,11 +208,10 @@ will use the corresponding aligned address.
   tracer's space cannot be accessed or does not have valid tags.
 - ``-EPERM`` - the specified process cannot be traced.
 - ``-EIO`` - the tracee's address range cannot be accessed (e.g. invalid
-  address) and no tags copied. ``iov_len`` not updated.
+  address) or does not have valid tags (not mapped with the ``PROT_MTE``
+  flag) and no tags copied. ``iov_len`` not updated.
 - ``-EFAULT`` - fault on accessing the tracer's memory (``struct iovec``
   or ``iov_base`` buffer) and no tags copied. ``iov_len`` not updated.
-- ``-EOPNOTSUPP`` - the tracee's address does not have valid tags (never
-  mapped with the ``PROT_MTE`` flag). ``iov_len`` not updated.
 
 **Note**: There are no transient errors for the requests above, so user
 programs should not retry in case of a non-zero system call return.

arch/arm64/kernel/mte.c

@@ -476,7 +476,7 @@ static int __access_remote_tags(struct mm_struct *mm, unsigned long addr,
 		 * was never mapped with PROT_MTE.
 		 */
 		if (!(vma->vm_flags & VM_MTE)) {
-			err = -EOPNOTSUPP;
+			err = -EIO;
 			put_page(page);
 			break;
 		}