mm: abstract initial stack setup to mm subsystem

There are peculiarities within the kernel where what is very clearly mm
code is performed elsewhere arbitrarily.

This violates separation of concerns and makes it harder to refactor code
to make changes to how fundamental initialisation and operation of mm
logic is performed.

One such case is the creation of the VMA containing the initial stack upon
execve()'ing a new process.  This is currently performed in
__bprm_mm_init() in fs/exec.c.

Abstract this operation to create_init_stack_vma().  This allows us to
limit use of vma allocation and free code to fork and mm only.

We previously did the same for the step at which we relocate the initial
stack VMA downwards via relocate_vma_down(), now we move the initial VMA
establishment too.

Take the opportunity to also move insert_vm_struct() to mm/vma.c as it's
no longer needed anywhere outside of mm.

Link: https://lkml.kernel.org/r/118c950ef7a8dd19ab20a23a68c3603751acd30e.1745853549.git.lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Acked-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Reviewed-by: Pedro Falcato <pfalcato@suse.de>
Reviewed-by: Kees Cook <kees@kernel.org>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Jann Horn <jannh@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 4 insertions, 0 deletions

diff --git a/mm/vma.h b/mm/vma.h
index 4413445e074b..151e292263fd 100644
--- a/mm/vma.h
+++ b/mm/vma.h
@@ -548,8 +548,12 @@ int expand_downwards(struct vm_area_struct *vma, unsigned long address);
 
 int __vm_munmap(unsigned long start, size_t len, bool unlock);
 
+int insert_vm_struct(struct mm_struct *mm, struct vm_area_struct *vma);
+
 /* vma_exec.c */
 #ifdef CONFIG_MMU
+int create_init_stack_vma(struct mm_struct *mm, struct vm_area_struct **vmap,
+			  unsigned long *top_mem_p);
 int relocate_vma_down(struct vm_area_struct *vma, unsigned long shift);
 #endif