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

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Message-ID: <06a60411-d8ef-407b-b8ab-0e28e0b88037@lucifer.local>
Date: Fri, 25 Apr 2025 11:10:27 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Suren Baghdasaryan <surenb@...gle.com>
Cc: David Hildenbrand <david@...hat.com>,
        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>,
        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 05:55:20PM -0700, Suren Baghdasaryan wrote:
> On Thu, Apr 24, 2025 at 2:30 PM David Hildenbrand <david@...hat.com> 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.
>
> nit: s/on/On
> You could also skip this sentence altogether since it's kinda obvious
> but up to you.

Ack, and yeah probably best to just drop tbh :)

>
> > > + */
> >
> > 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 ... :)
> >
> > Acked-by: David Hildenbrand <david@...hat.com>
>
> Reviewed-by: Suren Baghdasaryan <surenb@...gle.com>
>
> >
> > --
> > Cheers,
> >
> > David / dhildenb
> >