linux-kernel - Re: [PATCH] mm/mremap: fix regression in vrm->new

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Message-ID: <aK_YO90AEgow_iQt@google.com>
Date: Thu, 28 Aug 2025 04:16:59 +0000
From: Carlos Llamas <cmllamas@...gle.com>
To: "Liam R. Howlett" <Liam.Howlett@...cle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
	Pedro Falcato <pfalcato@...e.de>, kernel-team@...roid.com,
	linux-kernel@...r.kernel.org,
	"open list:MEMORY MAPPING" <linux-mm@...ck.org>
Subject: Re: [PATCH] mm/mremap: fix regression in vrm->new_addr check

On Thu, Aug 28, 2025 at 04:06:58AM +0000, Carlos Llamas wrote:
> On Wed, Aug 27, 2025 at 11:43:39PM -0400, Liam R. Howlett wrote:
> > * Carlos Llamas <cmllamas@...gle.com> [250827 23:27]:
> > > Commit 3215eaceca87 ("mm/mremap: refactor initial parameter sanity
> > > checks") moved the sanity check for vrm->new_addr from mremap_to() to
> > > check_mremap_params().
> > > 
> > > However, this caused a regression as vrm->new_addr is now checked even
> > > when MREMAP_FIXED and MREMAP_DONTUNMAP flags are not specified. In this
> > > case, vrm->new_addr can be garbage and create unexpected failures.
> > > 
> > > Fix this by moving the new_addr check after the vrm_implies_new_addr()
> > > guard. This ensures that the new_addr is only checked when the user has
> > > specified one explicitly.
> > > 
> > > Fixes: 3215eaceca87 ("mm/mremap: refactor initial parameter sanity checks")
> > > Signed-off-by: Carlos Llamas <cmllamas@...gle.com>
> > 
> > I assume this showed up with clang?
> 
> Right.
> 
> The specific test that broke on our end was this:
> https://android.googlesource.com/platform/bionic/+/HEAD/tests/__cxa_atexit_test.cpp
> Although I'm not exactly sure how __cxa_atexit() implementation uses
> mremap() underneath.

Ok, here is the specific call to mremap() in the test:
https://android.googlesource.com/platform/bionic/+/HEAD/libc/bionic/atexit.cpp#197