linux-kernel - Re: [PATCH] userns: Add forward declaration of struct seq_file in user

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Message-ID: <87ldqegrf1.fsf@email.froward.int.ebiederm.org>
Date: Fri, 30 May 2025 08:27:14 -0500
From: "Eric W. Biederman" <ebiederm@...ssion.com>
To: Herbert Xu <herbert@...dor.apana.org.au>
Cc: Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,  Andrew Morton
 <akpm@...ux-foundation.org>
Subject: Re: [PATCH] userns: Add forward declaration of struct seq_file in
 user_namespace.h

Herbert Xu <herbert@...dor.apana.org.au> writes:

> Add forward declaration of struct seq_file before using it in
> function prototype.

I don't see anything wrong with doing that.

However:
> Fixes: 9cc46516ddf4 ("userns: Add a knob to disable setgroups on a per
> user namespace basis")

That description seems to be lacking something.  The code as has been
fine since 2014 and there is no conditional compilation involved.

Did some other change cause a declaration of seq_file not to be present?
Did user_namespace.h get included somewhere new and that new location
doesn't wind up declaring seq_file?

Especially since fixes tags tend to get code back ported I would
appreciate a bit more context on why this is a problem in 2025 when
it hasn't been a problem for the last decade.

Thank you,
Eric Biederman

> Signed-off-by: Herbert Xu <herbert@...dor.apana.org.au>
>
> diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
> index a0bb6d012137..5c7aa940b1aa 100644
> --- a/include/linux/user_namespace.h
> +++ b/include/linux/user_namespace.h
> @@ -16,6 +16,8 @@
>  #define UID_GID_MAP_MAX_BASE_EXTENTS 5
>  #define UID_GID_MAP_MAX_EXTENTS 340
>  
> +struct seq_file;
> +
>  struct uid_gid_extent {
>  	u32 first;
>  	u32 lower_first;