The patch titled Subject: mm: only enforce minimum stack gap size if it's sensible has been added to the -mm mm-unstable branch. Its filename is mm-only-enforce-minimum-stack-gap-size-if-its-sensible.patch

This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...

This patch will later appear in the mm-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------ From: David Gow davidgow@google.com Subject: mm: only enforce minimum stack gap size if it's sensible Date: Sat, 3 Aug 2024 15:46:41 +0800

The generic mmap_base code tries to leave a gap between the top of the stack and the mmap base address, but enforces a minimum gap size (MIN_GAP) of 128MB, which is too large on some setups. In particular, on arm tasks without ADDR_LIMIT_32BIT, the STACK_TOP value is less than 128MB, so it's impossible to fit such a gap in.

Only enforce this minimum if MIN_GAP < MAX_GAP, as we'd prefer to honour MAX_GAP, which is defined proportionally, so scales better and always leaves us with both _some_ stack space and some room for mmap.

This fixes the usercopy KUnit test suite on 32-bit arm, as it doesn't set any personality flags so gets the default (in this case 26-bit) task size. This test can be run with: ./tools/testing/kunit/kunit.py run --arch arm usercopy --make_options LLVM=1

Link: https://lkml.kernel.org/r/20240803074642.1849623-2-davidgow@google.com Fixes: dba79c3df4a2 ("arm: use generic mmap top-down layout and brk randomization") Signed-off-by: David Gow davidgow@google.com Cc: Alexandre Ghiti alex@ghiti.fr Cc: Kees Cook kees@kernel.org Cc: Linus Walleij linus.walleij@linaro.org Cc: Luis Chamberlain mcgrof@kernel.org Cc: Mark Rutland mark.rutland@arm.com Cc: Russell King linux@armlinux.org.uk Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/util.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/mm/util.c~mm-only-enforce-minimum-stack-gap-size-if-its-sensible +++ a/mm/util.c @@ -463,7 +463,7 @@ static unsigned long mmap_base(unsigned if (gap + pad > gap) gap += pad;

- if (gap < MIN_GAP) + if (gap < MIN_GAP && MIN_GAP < MAX_GAP) gap = MIN_GAP; else if (gap > MAX_GAP) gap = MAX_GAP; _

Patches currently in -mm which might be from davidgow@google.com are

mm-only-enforce-minimum-stack-gap-size-if-its-sensible.patch