Re: [PATCH 09/11] kexec: avoid effectively open-coding xzalloc_flex_struct()

[Andrew Cooper <andrew.cooper3@citrix.com>]

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@suse.com>

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