Re: [PATCH v3] proc,fcntl: introduce F_SET_DESCRIPTION

Thanks for reviewing, I added some questions inline below

On Mon, Jul 27, 2020 at 7:21 AM Alexey Dobriyan [off-list ref] wrote:

quoted

+     d = strndup_user(desc, MAX_FILE_DESC_SIZE);

This should be kmem accounted because allocation is persistent.
To make things more entertaining, strndup_user() doesn't have gfp_t argument.

I had to look it up so I might be very far from it, but is that
__GFP_ACCOUNT and would it be correct to assume memdup_user() should
use it unconditionally?

Otherwise my simple option would be to implement the logic in the
set_description, but the benefit would be very local.

Please let me know what you think is best, happy to read more doc if
there's a more productive way to address that

quoted

+     if (IS_ERR(d))
+             return PTR_ERR(d);
+
+     spin_lock(&file->f_lock);
+     kfree(file->f_description);
+     file->f_description = d;
+     spin_unlock(&file->f_lock);

Generally kfree under spinlock is not good idea.
You can replace the pointer and free without spinlock.

Sorry I also need some pointers here - do you suggest I move the kfree
out of the locked section or that there is a safe way other than
spinlock?

struct file is nicely aligned to 256 bytes on distro configs.
Will this break everything?

        $ cat /sys/kernel/slab/filp/object_size

Indeed on the config I'm using here this jumped to 264 bytes

Would it be better to move this to the inode struct? I don't know the
implications of this - any other option?

Thanks!