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

[Kees Cook <kees@kernel.org>]

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;
 		return (void *)p;
 	}
-- 
2.34.1



-- 
Kees Cook