[PATCH] kprobes: Replace zero-length array with flexible-array

[PATCH] kprobes: Replace zero-length array with flexible-array

[Gustavo A. R. Silva]

The current codebase makes use of the zero-length array language
extension to the C90 standard, but the preferred mechanism to declare
variable-length types such as these ones is a flexible array member[1][2],
introduced in C99:

struct foo {
        int stuff;
        struct boo array[];
};

By making use of the mechanism above, we will get a compiler warning
in case the flexible array does not occur last in the structure, which
will help us prevent some kind of undefined behavior bugs from being
inadvertently introduced[3] to the codebase from now on.

Also, notice that, dynamic memory allocations won't be affected by
this change:

"Flexible array members have incomplete type, and so the sizeof operator
may not be applied. As a quirk of the original implementation of
zero-length arrays, sizeof evaluates to zero."[1]

sizeof(flexible-array-member) triggers a warning because flexible array
members have incomplete type[1]. There are some instances of code in
which the sizeof operator is being incorrectly/erroneously applied to
zero-length arrays and the result is zero. Such instances may be hiding
some bugs. So, this work (flexible-array member conversions) will also
help to get completely rid of those sorts of issues.

This issue was found with the help of Coccinelle.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] https://github.com/KSPP/linux/issues/21
[3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
---
 include/linux/kprobes.h |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/kprobes.h b/include/linux/kprobes.h
index 04bdaf01112c..4477205e3e92 100644
--- a/include/linux/kprobes.h
+++ b/include/linux/kprobes.h
@@ -161,7 +161,7 @@ struct kretprobe_instance {
 	kprobe_opcode_t *ret_addr;
 	struct task_struct *task;
 	void *fp;
-	char data[0];
+	char data[];
 };
 
 struct kretprobe_blackpoint {

Re: [PATCH] kprobes: Replace zero-length array with flexible-array

[Masami Hiramatsu]

On Thu, 7 May 2020 13:57:33 -0500
"Gustavo A. R. Silva" <gustavoars@kernel.org> wrote:

> The current codebase makes use of the zero-length array language
> extension to the C90 standard, but the preferred mechanism to declare
> variable-length types such as these ones is a flexible array member[1][2],
> introduced in C99:
> 
> struct foo {
>         int stuff;
>         struct boo array[];
> };
> 
> By making use of the mechanism above, we will get a compiler warning
> in case the flexible array does not occur last in the structure, which
> will help us prevent some kind of undefined behavior bugs from being
> inadvertently introduced[3] to the codebase from now on.
> 
> Also, notice that, dynamic memory allocations won't be affected by
> this change:
> 
> "Flexible array members have incomplete type, and so the sizeof operator
> may not be applied. As a quirk of the original implementation of
> zero-length arrays, sizeof evaluates to zero."[1]
> 
> sizeof(flexible-array-member) triggers a warning because flexible array
> members have incomplete type[1]. There are some instances of code in
> which the sizeof operator is being incorrectly/erroneously applied to
> zero-length arrays and the result is zero. Such instances may be hiding
> some bugs. So, this work (flexible-array member conversions) will also
> help to get completely rid of those sorts of issues.
> 
> This issue was found with the help of Coccinelle.
> 
> [1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
> [2] https://github.com/KSPP/linux/issues/21
> [3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")
> 
> Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>

Thanks for the complete explanation :-D
This looks good to me.

Acked-by: Masami Hiramatsu <mhiramat@kernel.org>

> ---
>  include/linux/kprobes.h |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/kprobes.h b/include/linux/kprobes.h
> index 04bdaf01112c..4477205e3e92 100644
> --- a/include/linux/kprobes.h
> +++ b/include/linux/kprobes.h
> @@ -161,7 +161,7 @@ struct kretprobe_instance {
>  	kprobe_opcode_t *ret_addr;
>  	struct task_struct *task;
>  	void *fp;
> -	char data[0];
> +	char data[];
>  };
>  
>  struct kretprobe_blackpoint {
> 


-- 
Masami Hiramatsu <mhiramat@kernel.org>