[PATCH] kasan: Zero initialize tag in __kasan_kmalloc

[Nathan Chancellor]

When building with -Wuninitialized and CONFIG_KASAN_SW_TAGS unset, Clang
warns:

mm/kasan/common.c:484:40: warning: variable 'tag' is uninitialized when
used here [-Wuninitialized]
kasan_unpoison_shadow(set_tag(object, tag), size);
^~~

set_tag ignores tag in this configuration but clang doesn't realize it
at this point in its pipeline, as it points to arch_kasan_set_tag as
being the point where it is used, which will later be expanded to
(void *)(object) without a use of tag. Just zero initialize tag, as it
removes this warning and doesn't change the meaning of the code.

Link: https://github.com/ClangBuiltLinux/linux/issues/465
Signed-off-by: Nathan Chancellor <natecha...@gmail.com>
---
mm/kasan/common.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/kasan/common.c b/mm/kasan/common.c
index 36afcf64e016..4c5af68f2a8b 100644
--- a/mm/kasan/common.c
+++ b/mm/kasan/common.c
@@ -464,7 +464,7 @@ static void *__kasan_kmalloc(struct kmem_cache *cache, const void *object,
{
unsigned long redzone_start;
unsigned long redzone_end;
- u8 tag;
+ u8 tag = 0;

if (gfpflags_allow_blocking(flags))
quarantine_reduce();
--
2.21.0

[Andrey Konovalov]

Hi Nathan,

Could you change this value to 0xff? This doesn't make any difference,
since set_tag() ignores the tag anyway, but is less confusing, as all
the non-tagged kernel pointers have 0xff in the top byte.

Thanks!

>
> if (gfpflags_allow_blocking(flags))
> quarantine_reduce();
> --
> 2.21.0
>

> --
> You received this message because you are subscribed to the Google Groups "kasan-dev" group.
> To unsubscribe from this group and stop receiving emails from it, send an email to kasan-dev+...@googlegroups.com.
> To post to this group, send email to kasa...@googlegroups.com.
> To view this discussion on the web visit https://groups.google.com/d/msgid/kasan-dev/20190502153538.2326-1-natechancellor%40gmail.com.
> For more options, visit https://groups.google.com/d/optout.

[Nathan Chancellor]

When building with -Wuninitialized and CONFIG_KASAN_SW_TAGS unset, Clang
warns:

mm/kasan/common.c:484:40: warning: variable 'tag' is uninitialized when
used here [-Wuninitialized]
kasan_unpoison_shadow(set_tag(object, tag), size);
^~~

set_tag ignores tag in this configuration but clang doesn't realize it
at this point in its pipeline, as it points to arch_kasan_set_tag as
being the point where it is used, which will later be expanded to

(void *)(object) without a use of tag. Initialize tag to 0xff, as it

removes this warning and doesn't change the meaning of the code.

Link: https://github.com/ClangBuiltLinux/linux/issues/465
Signed-off-by: Nathan Chancellor <natecha...@gmail.com>
---

v1 -> v2:

* Initialize tag to 0xff at Andrey's request

mm/kasan/common.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/kasan/common.c b/mm/kasan/common.c

index 36afcf64e016..242fdc01aaa9 100644

--- a/mm/kasan/common.c
+++ b/mm/kasan/common.c
@@ -464,7 +464,7 @@ static void *__kasan_kmalloc(struct kmem_cache *cache, const void *object,
{
unsigned long redzone_start;
unsigned long redzone_end;
- u8 tag;

+ u8 tag = 0xff;

[Andrey Konovalov]

On Thu, May 2, 2019 at 6:31 PM Nathan Chancellor
<natecha...@gmail.com> wrote:
>
> When building with -Wuninitialized and CONFIG_KASAN_SW_TAGS unset, Clang
> warns:
>
> mm/kasan/common.c:484:40: warning: variable 'tag' is uninitialized when
> used here [-Wuninitialized]
> kasan_unpoison_shadow(set_tag(object, tag), size);
> ^~~
>
> set_tag ignores tag in this configuration but clang doesn't realize it
> at this point in its pipeline, as it points to arch_kasan_set_tag as
> being the point where it is used, which will later be expanded to
> (void *)(object) without a use of tag. Initialize tag to 0xff, as it
> removes this warning and doesn't change the meaning of the code.
>
> Link: https://github.com/ClangBuiltLinux/linux/issues/465
> Signed-off-by: Nathan Chancellor <natecha...@gmail.com>

Reviewed-by: Andrey Konovalov <andre...@google.com>

Thanks!

> ---
>
> v1 -> v2:
>
> * Initialize tag to 0xff at Andrey's request
>
> mm/kasan/common.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/kasan/common.c b/mm/kasan/common.c
> index 36afcf64e016..242fdc01aaa9 100644
> --- a/mm/kasan/common.c
> +++ b/mm/kasan/common.c
> @@ -464,7 +464,7 @@ static void *__kasan_kmalloc(struct kmem_cache *cache, const void *object,
> {
> unsigned long redzone_start;
> unsigned long redzone_end;
> - u8 tag;
> + u8 tag = 0xff;
>
> if (gfpflags_allow_blocking(flags))
> quarantine_reduce();
> --
> 2.21.0
>

> --
> You received this message because you are subscribed to the Google Groups "kasan-dev" group.
> To unsubscribe from this group and stop receiving emails from it, send an email to kasan-dev+...@googlegroups.com.
> To post to this group, send email to kasa...@googlegroups.com.

> To view this discussion on the web visit https://groups.google.com/d/msgid/kasan-dev/20190502163057.6603-1-natechancellor%40gmail.com.

[Nathan Chancellor]

On Thu, May 02, 2019 at 06:40:52PM +0200, Andrey Konovalov wrote:
> On Thu, May 2, 2019 at 6:31 PM Nathan Chancellor
> <natecha...@gmail.com> wrote:
> >
> > When building with -Wuninitialized and CONFIG_KASAN_SW_TAGS unset, Clang
> > warns:
> >
> > mm/kasan/common.c:484:40: warning: variable 'tag' is uninitialized when
> > used here [-Wuninitialized]
> > kasan_unpoison_shadow(set_tag(object, tag), size);
> > ^~~
> >
> > set_tag ignores tag in this configuration but clang doesn't realize it
> > at this point in its pipeline, as it points to arch_kasan_set_tag as
> > being the point where it is used, which will later be expanded to
> > (void *)(object) without a use of tag. Initialize tag to 0xff, as it
> > removes this warning and doesn't change the meaning of the code.
> >
> > Link: https://github.com/ClangBuiltLinux/linux/issues/465
> > Signed-off-by: Nathan Chancellor <natecha...@gmail.com>
>
> Reviewed-by: Andrey Konovalov <andre...@google.com>
>
> Thanks!
>

Thanks Andrey! Did anyone else have any other comments or can this be
picked up?

Cheers,
Nathan

[Andrey Ryabinin]

On 5/2/19 7:30 PM, Nathan Chancellor wrote:
> When building with -Wuninitialized and CONFIG_KASAN_SW_TAGS unset, Clang
> warns:
>
> mm/kasan/common.c:484:40: warning: variable 'tag' is uninitialized when
> used here [-Wuninitialized]
> kasan_unpoison_shadow(set_tag(object, tag), size);
> ^~~
>
> set_tag ignores tag in this configuration but clang doesn't realize it
> at this point in its pipeline, as it points to arch_kasan_set_tag as
> being the point where it is used, which will later be expanded to
> (void *)(object) without a use of tag. Initialize tag to 0xff, as it
> removes this warning and doesn't change the meaning of the code.
>
> Link: https://github.com/ClangBuiltLinux/linux/issues/465
> Signed-off-by: Nathan Chancellor <natecha...@gmail.com>

Fixes: 7f94ffbc4c6a ("kasan: add hooks implementation for tag-based mode")
Cc: <sta...@vger.kernel.org>
Reviewed-by: Andrey Ryabinin <arya...@virtuozzo.com>