linux-kernel - Re: [PATCH 1/4] mm: abstract initial stack setup to mm subsystem

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Message-ID: <beee409d-67d7-4754-91af-99b4e193ece2@lucifer.local>
Date: Fri, 25 Apr 2025 11:11:18 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: David Hildenbrand <david@...hat.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
        Pedro Falcato <pfalcato@...e.de>, Kees Cook <kees@...nel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
        Suren Baghdasaryan <surenb@...gle.com>, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4] mm: abstract initial stack setup to mm subsystem

On Thu, Apr 24, 2025 at 11:30:35PM +0200, David Hildenbrand wrote:
> On 24.04.25 23:15, Lorenzo Stoakes wrote:
> > 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.
> >
> > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> > ---
> ...
>
> > +/*
> > + * Establish the stack VMA in an execve'd process, located temporarily at the
> > + * maximum stack address provided by the architecture.
> > + *
> > + * We later relocate this downwards in relocate_vma_down().
> > + *
> > + * This function is almost certainly NOT what you want for anything other than
> > + * early executable initialisation.
> > + *
> > + * On success, returns 0 and sets *vmap to the stack VMA and *top_mem_p to the
> > + * maximum addressable location in the stack (that is capable of storing a
> > + * system word of data).
> > + *
> > + * on failure, returns an error code.
> > + */
>
> I was about to say, if you already write that much documentation, why not
> turn it into kerneldoc? :) But this function is clearly not intended to have
> more than one caller, so ... :)

Haha yeah, I felt for this case it's probably not necessary, bit of a blurry
line on this but as a one-off thing probably ok :P

>
> Acked-by: David Hildenbrand <david@...hat.com>

Thanks! Sorry I forgot to say thanks also to Suren for his tag in other email,
so will say here - also thanks Suren :)

>
> --
> Cheers,
>
> David / dhildenb
>