[PATCH] kasan: avoid -Wmaybe-uninitialized warning

[PATCH] kasan: avoid -Wmaybe-uninitialized warning

[Arnd Bergmann]

gcc-7 produces this warning:

mm/kasan/report.c: In function 'kasan_report':
mm/kasan/report.c:351:3: error: 'info.first_bad_addr' may be used uninitialized in this function [-Werror=maybe-uninitialized]
   print_shadow_for_address(info->first_bad_addr);
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
mm/kasan/report.c:360:27: note: 'info.first_bad_addr' was declared here

The code seems fine as we only print info.first_bad_addr when there is a shadow,
and we always initialize it in that case, but this is relatively hard
for gcc to figure out after the latest rework. Adding an intialization
in the other code path gets rid of the warning.

Fixes: b235b9808664 ("kasan: unify report headers")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 mm/kasan/report.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/kasan/report.c b/mm/kasan/report.c
index 718a10a48a19..63de3069dceb 100644
--- a/mm/kasan/report.c
+++ b/mm/kasan/report.c
@@ -109,6 +109,8 @@ const char *get_wild_bug_type(struct kasan_access_info *info)
 {
 	const char *bug_type = "unknown-crash";
 
+	info->first_bad_addr = (void *)(-1ul);
+
 	if ((unsigned long)info->access_addr < PAGE_SIZE)
 		bug_type = "null-ptr-deref";
 	else if ((unsigned long)info->access_addr < TASK_SIZE)
-- 
2.9.0

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Re: [PATCH] kasan: avoid -Wmaybe-uninitialized warning

[Dmitry Vyukov]

On Thu, Mar 23, 2017 at 4:04 PM, Arnd Bergmann <arnd@arndb.de> wrote:
> gcc-7 produces this warning:
>
> mm/kasan/report.c: In function 'kasan_report':
> mm/kasan/report.c:351:3: error: 'info.first_bad_addr' may be used uninitialized in this function [-Werror=maybe-uninitialized]
>    print_shadow_for_address(info->first_bad_addr);
>    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> mm/kasan/report.c:360:27: note: 'info.first_bad_addr' was declared here
>
> The code seems fine as we only print info.first_bad_addr when there is a shadow,
> and we always initialize it in that case, but this is relatively hard
> for gcc to figure out after the latest rework. Adding an intialization
> in the other code path gets rid of the warning.
>
> Fixes: b235b9808664 ("kasan: unify report headers")
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
>  mm/kasan/report.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/mm/kasan/report.c b/mm/kasan/report.c
> index 718a10a48a19..63de3069dceb 100644
> --- a/mm/kasan/report.c
> +++ b/mm/kasan/report.c
> @@ -109,6 +109,8 @@ const char *get_wild_bug_type(struct kasan_access_info *info)
>  {
>         const char *bug_type = "unknown-crash";
>
> +       info->first_bad_addr = (void *)(-1ul);
> +
>         if ((unsigned long)info->access_addr < PAGE_SIZE)
>                 bug_type = "null-ptr-deref";
>         else if ((unsigned long)info->access_addr < TASK_SIZE)
> --
> 2.9.0
>


Acked-by: Dmitry Vyukov <dvyukov@google.com>

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Re: [PATCH] kasan: avoid -Wmaybe-uninitialized warning

[Andrew Morton]

On Thu, 23 Mar 2017 16:04:09 +0100 Arnd Bergmann <arnd@arndb.de> wrote:

> gcc-7 produces this warning:
> 
> mm/kasan/report.c: In function 'kasan_report':
> mm/kasan/report.c:351:3: error: 'info.first_bad_addr' may be used uninitialized in this function [-Werror=maybe-uninitialized]
>    print_shadow_for_address(info->first_bad_addr);
>    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> mm/kasan/report.c:360:27: note: 'info.first_bad_addr' was declared here
> 
> The code seems fine as we only print info.first_bad_addr when there is a shadow,
> and we always initialize it in that case, but this is relatively hard
> for gcc to figure out after the latest rework. Adding an intialization
> in the other code path gets rid of the warning.
> 
> ...
>
> --- a/mm/kasan/report.c
> +++ b/mm/kasan/report.c
> @@ -109,6 +109,8 @@ const char *get_wild_bug_type(struct kasan_access_info *info)
>  {
>  	const char *bug_type = "unknown-crash";
>  
> +	info->first_bad_addr = (void *)(-1ul);
> +
>  	if ((unsigned long)info->access_addr < PAGE_SIZE)
>  		bug_type = "null-ptr-deref";
>  	else if ((unsigned long)info->access_addr < TASK_SIZE)

A weird, ugly and seemingly-unneeded statement should have a comment
explaining its existence, no?

Fortunately it is no longer needed.  We now have:

static void print_error_description(struct kasan_access_info *info)
{
	const char *bug_type = "unknown-crash";
	u8 *shadow_addr;

	info->first_bad_addr = find_first_bad_addr(info->access_addr,
						info->access_size);

	shadow_addr = (u8 *)kasan_mem_to_shadow(info->first_bad_addr);

	...

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