linux-kernel - Re: [RFC][PATCH 03/12] digest

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Date:   Sun, 27 Jun 2021 17:35:55 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Matthew Wilcox <willy@...radead.org>, Arnd Bergmann <arnd@...db.de>
Cc:     Roberto Sassu <roberto.sassu@...wei.com>, zohar@...ux.ibm.com,
        linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH 03/12] digest_lists: Basic definitions

On Sun, Jun 27, 2021 at 04:23:26PM +0100, Matthew Wilcox wrote:
> On Sun, Jun 27, 2021 at 12:53:47PM +0200, Greg KH wrote:
> > > +enum ops { DIGEST_LIST_ADD, DIGEST_LIST_DEL, DIGEST_LIST_OP__LAST };
> > > +
> > 
> > For enums you export to userspace, you need to specify the values so
> > that all compilers get them right.
> 
> I've never heard that rule before.  Where does it come from?
> https://en.cppreference.com/w/c/language/enum
> says:
> 
>   If enumeration-constant is not followed by = constant-expression,
>   its value is the value one greater than the value of the previous
>   enumerator in the same enumeration. The value of the first enumerator
>   (if it does not use = constant-expression) is zero.
> 

I thought it was in the Documentation/driver-api/ioctl.rst file, but I
can't find it right now.  Maybe it was something that Arnd said?

Arnd, is this still an issue?  For some reason I thought it was always
good to have uapi .h enums be explicit as to the value in them,
otherwise some C compilers might produce other values if they were not
specified?

thanks,

greg k-h