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

Andrew Morton
2021-05-13T03:40:29Z

On Thu, 13 May 2021 12:12:20 +0900 Hyeonggon Yoo <42.hyeyoo@gmail.com> 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:
> >
> > --- a/mm/kfence/kfence_test.c~a
> > +++ a/mm/kfence/kfence_test.c
> > @@ -318,13 +318,13 @@ static void test_out_of_bounds_read(stru
> >
> > /* Test both sides. */
> >
> > - buf = test_alloc(test, size, GFP_KERNEL, ALLOCATE_LEFT);
> > + buf = test_alloc(test, 32, GFP_KERNEL, ALLOCATE_LEFT);
> > expect.addr = buf - 1;
> > READ_ONCE(*expect.addr);
> > KUNIT_EXPECT_TRUE(test, report_matches(&expect));
> > test_free(buf);
> >
> > - buf = test_alloc(test, size, GFP_KERNEL, ALLOCATE_RIGHT);
> > + buf = test_alloc(test, 32, GFP_KERNEL, ALLOCATE_RIGHT);
> > expect.addr = buf + size;
> > READ_ONCE(*expect.addr);
> > KUNIT_EXPECT_TRUE(test, report_matches(&expect));
> > @@ -519,11 +519,11 @@ static void test_free_bulk(struct kunit
> > const size_t size = setup_test_cache(test, 8 + prandom_u32_max(300), 0,
> > (iter & 1) ? ctor_set_x : NULL);
> > void *objects[] = {
> > - test_alloc(test, size, GFP_KERNEL, ALLOCATE_RIGHT),
> > - test_alloc(test, size, GFP_KERNEL, ALLOCATE_NONE),
> > - test_alloc(test, size, GFP_KERNEL, ALLOCATE_LEFT),
> > - test_alloc(test, size, GFP_KERNEL, ALLOCATE_NONE),
> > - test_alloc(test, size, GFP_KERNEL, ALLOCATE_NONE),
> > + test_alloc(test, 32, GFP_KERNEL, ALLOCATE_RIGHT),
> > + test_alloc(test, 32, GFP_KERNEL, ALLOCATE_NONE),
> > + test_alloc(test, 32, GFP_KERNEL, ALLOCATE_LEFT),
> > + test_alloc(test, 32, GFP_KERNEL, ALLOCATE_NONE),
> > + test_alloc(test, 32, GFP_KERNEL, ALLOCATE_NONE),
> > };
> >
> > kmem_cache_free_bulk(test_cache, ARRAY_SIZE(objects), objects);
> >
> >
> > Is gcc-10.3.0 simply confused? test_out_of_bounds_read() is clearly
> > calling kmalloc_index(32) which is OK.
> >
> > Anyway, I'll drop this patch for now so I can compile a kernel!
> >
>
> The error messages isn't so clear to me.
> but one problem I can see is in kfence_test.c, there are many places that
> are using size which is not constant.

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();

/* Will never be reached. Needed because the compiler may complain */
return -1;
@@ -575,7 +579,7 @@ static __always_inline void *kmalloc(siz
if (size > KMALLOC_MAX_CACHE_SIZE)
return kmalloc_large(size, flags);
#ifndef CONFIG_SLOB
- index = kmalloc_index(size);
+ index = kmalloc_index(size, true);

if (!index)
return ZERO_SIZE_PTR;
@@ -593,7 +597,7 @@ static __always_inline void *kmalloc_nod
#ifndef CONFIG_SLOB
if (__builtin_constant_p(size) &&
size <= KMALLOC_MAX_CACHE_SIZE) {
- unsigned int i = kmalloc_index(size);
+ unsigned int i = kmalloc_index(size, true);

if (!i)
return ZERO_SIZE_PTR;
--- a/mm/kfence/kfence_test.c~b
+++ a/mm/kfence/kfence_test.c
@@ -197,7 +197,7 @@ static void test_cache_destroy(void)

static inline size_t kmalloc_cache_alignment(size_t size)
{
- return kmalloc_caches[kmalloc_type(GFP_KERNEL)][kmalloc_index(size)]->align;
+ return kmalloc_caches[kmalloc_type(GFP_KERNEL)][kmalloc_index(size, false)]->align;
}

/* Must always inline to match stack trace against caller. */
@@ -267,7 +267,7 @@ static void *test_alloc(struct kunit *te

if (is_kfence_address(alloc)) {
struct page *page = virt_to_head_page(alloc);
- struct kmem_cache *s = test_cache ?: kmalloc_caches[kmalloc_type(GFP_KERNEL)][kmalloc_index(size)];
+ struct kmem_cache *s = test_cache ?: kmalloc_caches[kmalloc_type(GFP_KERNEL)][kmalloc_index(size, false)];

/*
* Verify that various helpers return the right values
_