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5302e93f34
Right now fs/exec.c invokes expand_downwards(), an otherwise internal implementation detail of the VMA logic in order to ensure that an arg page can be obtained by get_user_pages_remote(). In order to be able to move the stack expansion logic into mm/vma.c to make it available to userland testing we need to find an alternative approach here. We do so by providing the mmap_read_lock_maybe_expand() function which also helpfully documents what get_arg_page() is doing here and adds an additional check against VM_GROWSDOWN to make explicit that the stack expansion logic is only invoked when the VMA is indeed a downward-growing stack. This allows expand_downwards() to become a static function. Importantly, the VMA referenced by mmap_read_maybe_expand() must NOT be currently user-visible in any way, that is place within an rmap or VMA tree. It must be a newly allocated VMA. This is the case when exec invokes this function. Link: https://lkml.kernel.org/r/5295d1c70c58e6aa63d14be68d4e1de9fa1c8e6d.1733248985.git.lorenzo.stoakes@oracle.com Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com> Cc: Al Viro <viro@zeniv.linux.org.uk> Cc: Christian Brauner <brauner@kernel.org> Cc: Eric W. Biederman <ebiederm@xmission.com> Cc: Jan Kara <jack@suse.cz> Cc: Jann Horn <jannh@google.com> Cc: Kees Cook <kees@kernel.org> Cc: Liam R. Howlett <Liam.Howlett@Oracle.com> Cc: Vlastimil Babka <vbabka@suse.cz> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>