Re: 'struct dynamic': struct tag leak in UAPI headers

["Eric W. Biederman" <ebiederm@xmission.com>]

Alejandro Colomar <alx@kernel.org> writes:

> On 2023-08-29 16:17, Matthew Wilcox wrote:
>> Ooh, story time!  Long, long ago, we did not separate "Linux headers"
>> from "libc headers".  It used to be somebody's job to take the files
>> in /usr/src/linux/include/linux and copy them to /usr/include/linux.
>> Bold people used a symlink.  People who cared about things like "Well,
>> POSIX says that" would edit the files after copying them to remove things
>> that POSIX said shouldn't be there or put _GNU_SOURCE markers around them.
>> 
>> At some point, we decided to split the headers to create the uapi headers
>> to make this job easier.  Refinements are, of course, possible, and now
>> easier than ever, but I think it's fair to say that anybody who included
>> <linux/elf.h> at any time in history got given a struct dynamic.
>
> Thanks for the story!  :-)
>
>
>> Well, it's a compile-time failure either way.  Code that depended on it
>> is compiling happily today, and the code that would want to use it
>> doesn't exist yet, so I'd rather err on the side of keeping code written
>> in the last 30 years working.
>
> Fair enough.

The kernel rules do allow removing the structure tag if we no one
is using it.  If someone is using it then the bug is a regression.

If you have the energy you can do a good faith search to see if there
is any likely hood that anyone is using it.  At a quick look I don't
see anything including linux/elf.h.  Add in a debian code search
(is there a github or gitlab code search?) and you can make a
pretty firm dent how widespread that is.

After a good faith search you can merge a patch to remove it,
and then if anyone reports a problem because you somehow missed
them you can revert the change.

If anything like libc that wants to very carefully control it's exported
symbols wants to use linux/elf.h it is probably worth it.  Otherwise it
is probably more work that it is worth.

Eric