Re: [REGRESSION] bpf verifier slowdown due to vrealloc() change since 6.15-rc6

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Thu, May 15, 2025 at 8:53 AM Kees Cook <kees@kernel.org> wrote:
>
> On Thu, May 15, 2025 at 08:47:47AM -0700, Kees Cook wrote:
> > On Thu, May 15, 2025 at 09:12:25PM +0800, Shung-Hsi Yu wrote:
> > > Bisect was done by Pawan and got to commit a0309faf1cb0 "mm: vmalloc:
> > > support more granular vrealloc() sizing"[2]. To further zoom in the
> >
> > Can you try this patch? It's a clear bug fix, but if it doesn't improve
> > things, I have another idea to rearrange the memset.
>
> Here's the patch (on top of the prior one) that relocates the memset:
>
>
> From 0bc71b78603500705aca77f82de8ed1fc595c4c3 Mon Sep 17 00:00:00 2001
> From: Kees Cook <kees@kernel.org>
> Date: Thu, 15 May 2025 08:48:24 -0700
> Subject: [PATCH] mm: vmalloc: Only zero-init on vrealloc shrink
>
> The common case is to grow reallocations, and since init_on_alloc will
> have already zeroed the whole allocation, we only need to zero when
> shrinking the allocation.
>
> Fixes: a0309faf1cb0 ("mm: vmalloc: support more granular vrealloc() sizing")
> Signed-off-by: Kees Cook <kees@kernel.org>
> ---
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Uladzislau Rezki <urezki@gmail.com>
> Cc: <linux-mm@kvack.org>
> ---
>  mm/vmalloc.c | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
>
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 74bd00fd734d..83bedb1559ac 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -4093,8 +4093,8 @@ void *vrealloc_noprof(const void *p, size_t size, gfp_t flags)
>          * would be a good heuristic for when to shrink the vm_area?
>          */
>         if (size <= old_size) {
> -               /* Zero out "freed" memory. */
> -               if (want_init_on_free())
> +               /* Zero out "freed" memory, potentially for future realloc. */
> +               if (want_init_on_free() || want_init_on_alloc(flags))
>                         memset((void *)p + size, 0, old_size - size);
>                 vm->requested_size = size;
>                 kasan_poison_vmalloc(p + size, old_size - size);
> @@ -4107,9 +4107,11 @@ void *vrealloc_noprof(const void *p, size_t size, gfp_t flags)
>         if (size <= alloced_size) {
>                 kasan_unpoison_vmalloc(p + old_size, size - old_size,
>                                        KASAN_VMALLOC_PROT_NORMAL);
> -               /* Zero out "alloced" memory. */
> -               if (want_init_on_alloc(flags))
> -                       memset((void *)p + old_size, 0, size - old_size);
> +               /*
> +                * No need to zero memory here, as unused memory will have
> +                * already been zeroed at initial allocation time or during
> +                * realloc shrink time.
> +                */
>                 vm->requested_size = size;

This vm->requested_size change you are adding should also fix the
kasan issue reported by syzbot ([0]).

  [0] https://lore.kernel.org/bpf/68213ddf.050a0220.f2294.0045.GAE@google.com/

>                 return (void *)p;
>         }
> --
> 2.34.1
>
>
>
> --
> Kees Cook