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Re: [PATCH 09/11] kexec: avoid effectively open-coding xzalloc_flex_struct()
- To: Jan Beulich <jbeulich@xxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Fri, 9 Apr 2021 13:54:09 +0100
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- Cc: George Dunlap <george.dunlap@xxxxxxxxxx>
- Delivery-date: Fri, 09 Apr 2021 12:54:31 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 08/04/2021 13:21, Jan Beulich wrote:
> There is a difference in generated code: xzalloc_bytes() forces
> SMP_CACHE_BYTES alignment. I think we not only don't need this here, but
> actually don't want it.
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
This honestly looks like a backwards step. In particular, ...
>
> --- a/xen/common/kexec.c
> +++ b/xen/common/kexec.c
> @@ -463,7 +463,10 @@ static void * alloc_from_crash_heap(cons
> /* Allocate a crash note buffer for a newly onlined cpu. */
> static int kexec_init_cpu_notes(const unsigned long cpu)
> {
> - Elf_Note * note = NULL;
> + struct elf_notes {
> + Elf_Note first;
> + unsigned char more[];
> + } *notes = NULL;
> int ret = 0;
> int nr_bytes = 0;
>
> @@ -477,7 +480,8 @@ static int kexec_init_cpu_notes(const un
>
> /* If we dont care about the position of allocation, malloc. */
> if ( low_crashinfo_mode == LOW_CRASHINFO_NONE )
> - note = xzalloc_bytes(nr_bytes);
> + notes = xzalloc_flex_struct(struct elf_notes, more,
> + nr_bytes - sizeof(notes->first));
... this is far more error prone than the code you replaced, seeing as
there is now a chance that it can underflow.
~Andrew
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