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Date:   Sun, 27 Jun 2021 16:23:26 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Greg KH <gregkh@linuxfoundation.org>
Cc:     Roberto Sassu <roberto.sassu@huawei.com>, zohar@linux.ibm.com,
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        linux-security-module@vger.kernel.org, linux-doc@vger.kernel.org,
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Subject: Re: [RFC][PATCH 03/12] digest_lists: Basic definitions
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References: <20210625165614.2284243-1-roberto.sassu@huawei.com>
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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.