[PATCH] kasan: test: Silence GCC 12 warnings

[Kees Cook]

GCC 12 continues to get smarter about array accesses. The KASAN tests
are expecting to explicitly test out-of-bounds conditions at run-time,
so hide the variable from GCC, to avoid warnings like:

../lib/test_kasan.c: In function 'ksize_uaf':
../lib/test_kasan.c:790:61: warning: array subscript 120 is outside array bounds of 'void[120]' [-Warray-bounds]
790 | KUNIT_EXPECT_KASAN_FAIL(test, ((volatile char *)ptr)[size]);
| ~~~~~~~~~~~~~~~~~~~~~~^~~~~~
../lib/test_kasan.c:97:9: note: in definition of macro 'KUNIT_EXPECT_KASAN_FAIL'
97 | expression; \
| ^~~~~~~~~~

Cc: Andrey Ryabinin <ryabin...@gmail.com>
Cc: Alexander Potapenko <gli...@google.com>
Cc: Andrey Konovalov <andre...@gmail.com>
Cc: Dmitry Vyukov <dvy...@google.com>
Cc: Vincenzo Frascino <vincenzo...@arm.com>
Cc: kasa...@googlegroups.com
Signed-off-by: Kees Cook <kees...@chromium.org>
---
lib/test_kasan.c | 10 ++++++++++
1 file changed, 10 insertions(+)

diff --git a/lib/test_kasan.c b/lib/test_kasan.c
index c233b1a4e984..58c1b01ccfe2 100644
--- a/lib/test_kasan.c
+++ b/lib/test_kasan.c
@@ -131,6 +131,7 @@ static void kmalloc_oob_right(struct kunit *test)
ptr = kmalloc(size, GFP_KERNEL);
KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);

+ OPTIMIZER_HIDE_VAR(ptr);
/*
* An unaligned access past the requested kmalloc size.
* Only generic KASAN can precisely detect these.
@@ -159,6 +160,7 @@ static void kmalloc_oob_left(struct kunit *test)
ptr = kmalloc(size, GFP_KERNEL);
KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);

+ OPTIMIZER_HIDE_VAR(ptr);
KUNIT_EXPECT_KASAN_FAIL(test, *ptr = *(ptr - 1));
kfree(ptr);
}
@@ -171,6 +173,7 @@ static void kmalloc_node_oob_right(struct kunit *test)
ptr = kmalloc_node(size, GFP_KERNEL, 0);
KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);

+ OPTIMIZER_HIDE_VAR(ptr);
KUNIT_EXPECT_KASAN_FAIL(test, ptr[0] = ptr[size]);
kfree(ptr);
}
@@ -191,6 +194,7 @@ static void kmalloc_pagealloc_oob_right(struct kunit *test)
ptr = kmalloc(size, GFP_KERNEL);
KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);

+ OPTIMIZER_HIDE_VAR(ptr);
KUNIT_EXPECT_KASAN_FAIL(test, ptr[size + OOB_TAG_OFF] = 0);

kfree(ptr);
@@ -271,6 +275,7 @@ static void kmalloc_large_oob_right(struct kunit *test)
ptr = kmalloc(size, GFP_KERNEL);
KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);

+ OPTIMIZER_HIDE_VAR(ptr);
KUNIT_EXPECT_KASAN_FAIL(test, ptr[size] = 0);
kfree(ptr);
}
@@ -410,6 +415,8 @@ static void kmalloc_oob_16(struct kunit *test)
ptr2 = kmalloc(sizeof(*ptr2), GFP_KERNEL);
KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr2);

+ OPTIMIZER_HIDE_VAR(ptr1);
+ OPTIMIZER_HIDE_VAR(ptr2);
KUNIT_EXPECT_KASAN_FAIL(test, *ptr1 = *ptr2);
kfree(ptr1);
kfree(ptr2);
@@ -756,6 +763,8 @@ static void ksize_unpoisons_memory(struct kunit *test)
KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
real_size = ksize(ptr);

+ OPTIMIZER_HIDE_VAR(ptr);
+
/* This access shouldn't trigger a KASAN report. */
ptr[size] = 'x';

@@ -778,6 +787,7 @@ static void ksize_uaf(struct kunit *test)
KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
kfree(ptr);

+ OPTIMIZER_HIDE_VAR(ptr);
KUNIT_EXPECT_KASAN_FAIL(test, ksize(ptr));
KUNIT_EXPECT_KASAN_FAIL(test, ((volatile char *)ptr)[0]);
KUNIT_EXPECT_KASAN_FAIL(test, ((volatile char *)ptr)[size]);
--
2.32.0

[Marco Elver]

On Wed, 8 Jun 2022 at 23:40, Kees Cook <kees...@chromium.org> wrote:
>
> GCC 12 continues to get smarter about array accesses. The KASAN tests
> are expecting to explicitly test out-of-bounds conditions at run-time,
> so hide the variable from GCC, to avoid warnings like:
>
> ../lib/test_kasan.c: In function 'ksize_uaf':
> ../lib/test_kasan.c:790:61: warning: array subscript 120 is outside array bounds of 'void[120]' [-Warray-bounds]

Since this keeps happening, I wonder if we could just pass
'-Wno-array-bounds' ? We already have 'CFLAGS_test_kasan.o += $(call
cc-disable-warning, vla)'.

Although eventually I'd assume all the OPTIMIZE_HIDE_VAR() should be
in place, and hopefully it'll have been the last one. I leave it to
you.

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