On 27/10/2023 16:06, Kevin Brodsky wrote:
@@ -1019,8 +1075,13 @@ SYSCALL_DEFINE5(__retptr__(mremap), user_uintptr_t, addr, unsigned long, old_len /* old_len exactly to the end of the area.. */ if (old_len == vma->vm_end - addr) {
/* can we just expand the current mapping? */if (vma_expandable(vma, new_len - old_len)) {
/** can we just expand the current mapping?* PCuABI specification does not allow increasing reservation* size so just skip this path.Reservations cannot be expanded, but mappings still can. Sure, in most cases the mapping ends where the reservation does, but we shouldn't assume it. We should rather check whether there is enough free space after the mapping, within the reservation - I suppose this means modifying vma_expandable().
I'm realising there is some ambiguity on this point in the spec, as the Reservations section claims that unmapped space within a reservation cannot be reused. That's true when it comes to creating new mappings (e.g. mmap), but expanding mappings is still allowed. I will amend the spec to clarify this.
The spec is now amended to prevent only the creation of new mappings in unmapped space, allowing mremap() to grow mappings in-place [1].
Kevin
[1] https://git.morello-project.org/morello/kernel/linux/-/wikis/Morello-pure-ca...