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:
- It is an implementation detail that doesn't impact this user ABI.
- 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.
We probably mention somewhere (or we should in the tagged pointers doc) that we don't support tagged PC.