linux-kernel - Re: [PATCH v3 2/5] mm/mseal: update madvise() logic

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Message-ID: <202507241528.A73E1178@keescook>
Date: Thu, 24 Jul 2025 15:29:35 -0700
From: Kees Cook <kees@...nel.org>
To: David Hildenbrand <david@...hat.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.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>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Jeff Xu <jeffxu@...omium.org>
Subject: Re: [PATCH v3 2/5] mm/mseal: update madvise() logic

On Thu, Jul 24, 2025 at 11:41:04PM +0200, David Hildenbrand wrote:
> On 24.07.25 23:32, David Hildenbrand wrote:
> > > As an aside, why should discard work in this case even without step 4?
> > > Wouldn't setting "read-only" imply you don't want the memory to change
> > > out from under you? I guess I'm not clear on the semantics: how do memory
> > > protection bits map to madvise actions like this?
> > 
> > They generally don't affect MADV_DONTNEED behavior. The only documented
> > (man page) reason for EPERM in the man page is related to MADV_HWPOISON.
> > 
> 
> (Exception: MADV_POPULATE_READ/MADV_POPULATE_WRITE requires corresponding
> permissions)

Shouldn't an MADV action that changes memory contents require the W bit
though? I mean, I assume the ship may have sailed on this, but it feels
mismatched to me.

-- 
Kees Cook