Re: [PATCH v2 7/7] tools/testing/selftests/bpf: replace open-coded 16 with TASK_COMM_LEN

Hi,

David Hildenbrand [off-list ref] writes:

On 29.11.21 15:21, Sven Schnelle wrote:

quoted

Yafang Shao [off-list ref] writes:

quoted

Thanks for the report and debugging!
Seems we should explicitly define it as signed ?
Could you pls. help verify it?

diff --git a/include/linux/sched.h b/include/linux/sched.h
index cecd4806edc6..44d36c6af3e1 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h

@@ -278,7 +278,7 @@ struct task_group;
  * Define the task command name length as enum, then it can be visible to
  * BPF programs.
  */
-enum {
+enum SignedEnum {
        TASK_COMM_LEN = 16,
 };

Umm no. What you're doing here is to define the name of the enum as
'SignedEnum'. This doesn't change the type. I think before C++0x you
couldn't force an enum type.

I think there are only some "hacks" to modify the type with GCC. For
example, with "__attribute__((packed))" we can instruct GCC to use the
smallest type possible for the defined enum values.

Yes, i meant no way that the standard defines. You could force it to
signed by having a negative member.

I think with some fake entries one can eventually instruct GCC to use an
unsigned type in some cases:

https://stackoverflow.com/questions/14635833/is-there-a-way-to-make-an-enum-unsigned-in-the-c90-standard-misra-c-2004-compl

enum {
	TASK_COMM_LEN = 16,
	TASK_FORCE_UNSIGNED = 0x80000000,
};

Haven't tested it, though, and I'm not sure if we should really do that
... :)

TBH, i would vote for reverting the change. defining an array size as
enum feels really odd.

Regards
Sven