linux-kernel - [PATCH 0/2] mremap: Fix newaddr hint with MREMAP

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Message-ID: <20241206152032.1222067-1-bgeffon@google.com>
Date: Fri,  6 Dec 2024 10:20:30 -0500
From: Brian Geffon <bgeffon@...gle.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	Brian Geffon <bgeffon@...gle.com>
Subject: [PATCH 0/2] mremap: Fix newaddr hint with MREMAP_DONTUNMAP

mmap(2) allows for a destination address to be specified without
MAP_FIXED and in this situation it's a hint to get_unmapped_area().
This address need not be page aligned because get_unmapped_area() will
align the hint.

In the case of mremap(2) with MREMAP_DONTUNMAP it shares a code path
with MREMAP_FIXED in mremap_to(), which means this function can be
called in 3 different scenarios: MREMAP_FIXED only, MREMAP_DONTUNMAP
only, or MREMAP_FIXED | MREMAP_DONTUNMAP. In the second case when only
MREMAP_DONTUNMAP is specified we don't need to do alignment or size
checks on newaddr because they will be passed to get_unmapped_area() and
dealt with appropriately.

This patch corrects that behavior to match what non-MREMAP_DONTUNMAP
mremap(2) and mmap(2) do. This odd behavioral difference was reported by
Marco Vanotti. Additionally, I've included a self test to validate this
behavior.

Brian Geffon (2):
  mremap: Fix new_addr being used as a hint with MREMAP_DONTUNMAP
  selftests: mm: Add a new MREMAP_DONTUNMAP self test

 mm/mremap.c                                   | 26 ++++++++----
 tools/testing/selftests/mm/mremap_dontunmap.c | 41 ++++++++++++++++++-
 2 files changed, 59 insertions(+), 8 deletions(-)

-- 
2.47.0.338.g60cca15819-goog