On 03/04/2019 17:50, Catalin Marinas wrote:
On Fri, Mar 22, 2019 at 03:52:49PM +0000, Kevin Brodsky wrote:
On 18/03/2019 16:35, Vincenzo Frascino wrote:
+2. Features exposed via AT_FLAGS
+--------------------------------
+
+bit[0]: ARM64_AT_FLAGS_SYSCALL_TBI
+
+ On arm64 the TCR_EL1.TBI0 bit has been always enabled on the arm64
+ kernel, hence the userspace (EL0) is allowed to set a non-zero value
+ in the top byte but the resulting pointers are not allowed at the
+ user-kernel syscall ABI boundary.
+ When bit[0] is set to 1 the kernel is advertising to the userspace
+ that a relaxed ABI is supported hence this type of pointers are now
+ allowed to be passed to the syscalls, when these pointers are in
+ memory ranges privately owned by a process and obtained by the
+ process in accordance with the definition of "valid tagged pointer"
+ in paragraph 3.
+ In these cases the tag is preserved as the pointer goes through the
+ kernel. Only when the kernel needs to check if a pointer is coming
+ from userspace an untag operation is required.
I would leave this last sentence out, because:
1. It is an implementation detail that doesn't impact this user ABI.
2. It is not entirely accurate: untagging the pointer may be needed for
various kinds of address lookup (like finding the corresponding VMA), at
which point the kernel usually already knows it is a userspace pointer.
I fully agree, the above paragraph should not be part of the user ABI
document.
+3. ARM64_AT_FLAGS_SYSCALL_TBI
+-----------------------------
+
+From the kernel syscall interface prospective, we define, for the purposes
+of this document, a "valid tagged pointer" as a pointer that either it has
+a zero value set in the top byte or it has a non-zero value, it is in memory
+ranges privately owned by a userspace process and it is obtained in one of
+the following ways:
+ - mmap() done by the process itself, where either:
+ * flags = MAP_PRIVATE | MAP_ANONYMOUS
+ * flags = MAP_PRIVATE and the file descriptor refers to a regular
+ file or "/dev/zero"
+ - a mapping below sbrk(0) done by the process itself
I don't think that's very clear, this doesn't say how the mapping is
obtained. Maybe "a mapping obtained by the process using brk() or sbrk()"?
I think what we mean here is anything in the "[heap]" section as per
/proc/*/maps (in the kernel this would be start_brk to brk).
+ - any memory mapped by the kernel in the process's address space during
+ creation and following the restrictions presented above (i.e. data, bss,
+ stack).
With the rules above, the code section is included as well. Replacing "i.e."
with "e.g." would avoid having to list every single section (which is
probably not a good idea anyway).
We could mention [stack] explicitly as that's documented in the
Documentation/filesystems/proc.txt and it's likely considered ABI
already.
The code section is MAP_PRIVATE, and can be done by the dynamic loader
(user process), so it falls under the mmap() rules listed above. I guess
we could simply drop "done by the process itself" here and allow
MAP_PRIVATE|MAP_ANONYMOUS or MAP_PRIVATE of regular file. This would
cover the [heap] and [stack] and we won't have to debate the brk() case
at all.
That's probably the best option. I initially used this wording because I was worried
that there could be cases where the kernel allocates "magic" memory for userspace
that is MAP_PRIVATE|MAP_ANONYMOUS, but in fact it's probably not the case (presumably
such mapping should always be done via install_special_mapping(), which is definitely
not MAP_PRIVATE).
We probably mention somewhere (or we should in the tagged pointers doc)
that we don't support tagged PC.
I think that Documentation/arm64/tagged-pointers.txt already makes it reasonably
clear (anyway, with the architecture not supporting it, you can't expect much from
the kernel).
Kevin
