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Message-ID: <20260512193511.GF1262@brightrain.aerifal.cx>
Date: Tue, 12 May 2026 15:35:11 -0400
From: Rich Felker <dalias@...c.org>
To: Kalven Schraut <kalvens@...ision.com>
Cc: musl@...ts.openwall.com
Subject: Re: [PATCH] posix_memalign with 4 GiB alignment returns
pointer that crashes when free is called
On Tue, May 12, 2026 at 07:28:15PM +0000, Kalven Schraut wrote:
> I wasn't sure if you wanted a patch, so just made one real quick
> based on your response. LMK if any issues since first time
> submitting something to a mailing list
>
> >From 2f64081a0cbfbb8d05f986b9edd9fa9114cb9a42 Mon Sep 17 00:00:00 2001
> From: Kalven Schraut <kalvens@...ision.com>
> Date: Tue, 12 May 2026 14:18:26 -0500
> Subject: [PATCH] fix mallocng free of allocations with large alignment
>
> Signed-off-by: Kalven Schraut <kalvens@...ision.com>
> ---
> src/malloc/mallocng/meta.h | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/src/malloc/mallocng/meta.h b/src/malloc/mallocng/meta.h
> index 61ec53f9..6137729e 100644
> --- a/src/malloc/mallocng/meta.h
> +++ b/src/malloc/mallocng/meta.h
> @@ -129,13 +129,14 @@ static inline int get_slot_index(const unsigned char *p)
> static inline struct meta *get_meta(const unsigned char *p)
> {
> assert(!((uintptr_t)p & 15));
> - int offset = *(const uint16_t *)(p - 2);
> + size_t offset = *(const uint16_t *)(p - 2);
> int index = get_slot_index(p);
> if (p[-4]) {
> assert(!offset);
> offset = *(uint32_t *)(p - 8);
> assert(offset > 0xffff);
> }
> + assert(offset <= PTRDIFF_MAX/UNIT)
> const struct group *base = (const void *)(p - UNIT*offset - UNIT);
> const struct meta *meta = base->meta;
> assert(meta->mem == base);
I've got it already, but thanks. BTW the assert goes inside that if,
not outside, in case the compiler isn't smart enough to move it
inside (a 16-bit offset can never overflow and 32-bit ones are the
rare case that can only occur for extreme overalignment).
Rich
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