Re: [PATCH] mm/gup: disallow GUP writing to file-backed mappings by default

[Simon Horman]

On Sat, Apr 22, 2023 at 02:37:05PM +0100, Lorenzo Stoakes wrote:
> It isn't safe to write to file-backed mappings as GUP does not ensure that
> the semantics associated with such a write are performed correctly, for
> instance filesystems which rely upon write-notify will not be correctly
> notified.
> 
> There are exceptions to this - shmem and hugetlb mappings are (in effect)
> anonymous mappings by other names so we do permit this operation in these
> cases.
> 
> In addition, if no pinning takes place (neither FOLL_GET nor FOLL_PIN is
> specified and neither flags gets implicitly set) then no writing can occur
> so we do not perform the check in this instance.
> 
> This is an important exception, as populate_vma_page_range() invokes
> __get_user_pages() in this way (and thus so does __mm_populate(), used by
> MAP_POPULATE mmap() and mlock() invocations).
> 
> There are GUP users within the kernel that do nevertheless rely upon this
> behaviour, so we introduce the FOLL_ALLOW_BROKEN_FILE_MAPPING flag to
> explicitly permit this kind of GUP access.
> 
> This is required in order to not break userspace in instances where the
> uAPI might permit file-mapped addresses - a number of RDMA users require
> this for instance, as do the process_vm_[read/write]v() system calls,
> /proc/$pid/mem, ptrace and SDT uprobes. Each of these callers have been
> updated to use this flag.
> 
> Making this change is an important step towards a more reliable GUP, and
> explicitly indicates which callers might encouter issues moving forward.

nit: s/encouter/encounter/