Re: [PATCH v3] mm, slub: change run-time assertion in kmalloc_index() to compile-time

[Hyeonggon Yoo]

On Wed, May 12, 2021 at 08:40:24PM -0700, Andrew Morton wrote:
> On Thu, 13 May 2021 12:12:20 +0900 Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx> wrote:
> 
> > On Wed, May 12, 2021 at 07:52:27PM -0700, Andrew Morton wrote:
> > > This explodes in mysterious ways.  The patch as I have it is appended,
> > > for reference.
> > > 
> > > gcc-10.3.0 allmodconfig.
> > > 
> > > This patch suppresses the error:
> 
> Ah, yes, of course, your patch changes kmalloc_index() to require that
> it always is called with a constant `size'.  kfence_test doesn't do
> that.
> 
> kfence is being a bit naughty here - the other kmalloc_index() callers
> only comple up the call after verifying that `size' is a compile-time
> constant.
> 
> Would something like this work?
>  include/linux/slab.h    |   12 ++++++++----
>  mm/kfence/kfence_test.c |    4 ++--
>  2 files changed, 10 insertions(+), 6 deletions(-)
> 
> --- a/include/linux/slab.h~b
> +++ a/include/linux/slab.h
> @@ -374,7 +374,8 @@ static __always_inline enum kmalloc_cach
>   * Note: there's no need to optimize kmalloc_index because it's evaluated
>   * in compile-time.
>   */
> -static __always_inline unsigned int kmalloc_index(size_t size)
> +static __always_inline unsigned int kmalloc_index(size_t size,
> +						  bool size_is_constant)
>  {
>  	if (!size)
>  		return 0;
> @@ -410,7 +411,10 @@ static __always_inline unsigned int kmal
>  	if (size <=  16 * 1024 * 1024) return 24;
>  	if (size <=  32 * 1024 * 1024) return 25;
>  
> -	BUILD_BUG_ON_MSG(1, "unexpected size in kmalloc_index()");
> +	if (size_is_constant)
> +		BUILD_BUG_ON_MSG(1, "unexpected size in kmalloc_index()");
> +	else
> +		BUG();


kfence is randomly generating size. because kfence is using non-constant
size, we should do run-time assertion or compile-time assertion depending
on situation.

I think we can use __builtin_constant_p here. we don't need to modify
kmalloc_index's prototype.

so what about this?
if you think it makes sense, I'll send patch v4.

I used KMALLOC_MAX_CACHE_SIZE to assure it's safe size.
it's safer than putting BUILD_BUG_ON_MSG(1, ...) to below if statements
because KMALLOC_MAX_CACHE_SIZE can be less than 32MB.

--- include/linux/slab.h.orig	2021-05-12 17:56:54.504738768 +0900
+++ include/linux/slab.h	2021-05-13 15:06:25.724565850 +0900
@@ -346,9 +346,18 @@ static __always_inline enum kmalloc_cach
  * 1 =  65 .. 96 bytes
  * 2 = 129 .. 192 bytes
  * n = 2^(n-1)+1 .. 2^n
+ *
+ * Note: there's no need to optimize kmalloc_index because it's evaluated
+ * in compile-time.
  */
 static __always_inline unsigned int kmalloc_index(size_t size)
 {
+	if (__builtin_constant_p(size)) {
+		BUILD_BUG_ON_MSG(size > KMALLOC_MAX_CACHE_SIZE , "unexpected size in kmalloc_index()");
+	} else if (size > KMALLOC_MAX_CACHE_SIZE) {
+		BUG();
+	}
+
 	if (!size)
 		return 0;

@@ -382,8 +391,6 @@ static __always_inline unsigned int kmal
 	if (size <=  8 * 1024 * 1024) return 23;
 	if (size <=  16 * 1024 * 1024) return 24;
 	if (size <=  32 * 1024 * 1024) return 25;
-	if (size <=  64 * 1024 * 1024) return 26;
-	BUG();

 	/* Will never be reached. Needed because the compiler may complain */
 	return -1;