On 30.03.23 17:48, stsp wrote:
Hello. The attached test-case demonstrates a bug in mremap(). If MREMAP_FIXED is used over an existing mapping and mremap() fails, destination area gets unmapped. AFAIK the failed syscall should have no observable effects.
I remember that holds for various mapping-related syscalls: if something goes wrong, the end result is not guaranteed to be what we had before the syscall.
For example, if you use mmap(MAP_FIXED) to replace part of an exiting mapping, we first munmap what's there and then try to mmap the new mapping. If something goes wrong while doing that, we cannot simple undo what we already did.
Long story short: the semantics of these syscalls has never been to leave the system in the state as it was before in case anything goes wrong.
As another example, if you do an mprotect() that covers multiple VMAS, and there is an issue with the last VMA, all but the last VMA will have their permissions changed.
-- Thanks, David / dhildenb
