linux-kernel - Re: [PATCH 1/2 v4] user namespace: use union in {g,u}idmap struct

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Date:   Mon, 23 Oct 2017 09:39:25 +0300
From:   Nikolay Borisov <nborisov@...e.com>
To:     Christian Brauner <christian.brauner@...ntu.com>,
        ebiederm@...ssion.com, linux-kernel@...r.kernel.org
Cc:     serge@...lyn.com, stgraber@...ntu.com, tycho@...ho.ws
Subject: Re: [PATCH 1/2 v4] user namespace: use union in {g,u}idmap struct



On 19.10.2017 22:11, Christian Brauner wrote:
> - Add a struct containing two pointer to extents and wrap both the static extent
>   array and the struct into a union. This is done in preparation for bumping the
>   {g,u}idmap limits for user namespaces.
> - Add brackets around anonymous union when using designated initializers to
>   initialize members in order to please gcc <= 4.4.
> 
> Signed-off-by: Christian Brauner <christian.brauner@...ntu.com>
> ---
> Changelog 2017-10-19:
> * kernel/user.c: Use brackets around anonymous union when using designated
>   initializers to initialize members. This is done to please gcc <= 4.4.
> ---
>  include/linux/user_namespace.h | 18 +++++++++++++-----
>  kernel/user.c                  | 30 ++++++++++++++++++------------
>  2 files changed, 31 insertions(+), 17 deletions(-)
> 
> diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
> index c18e01252346..7c83d7f6289b 100644
> --- a/include/linux/user_namespace.h
> +++ b/include/linux/user_namespace.h
> @@ -12,13 +12,21 @@
>  
>  #define UID_GID_MAP_MAX_EXTENTS 5
>  
> +struct uid_gid_extent {
> +	u32 first;
> +	u32 lower_first;
> +	u32 count;
> +};
> +
>  struct uid_gid_map {	/* 64 bytes -- 1 cache line */
>  	u32 nr_extents;
> -	struct uid_gid_extent {
> -		u32 first;
> -		u32 lower_first;
> -		u32 count;
> -	} extent[UID_GID_MAP_MAX_EXTENTS];
> +	union {
> +		struct uid_gid_extent extent[UID_GID_MAP_MAX_EXTENTS];
> +		struct {
> +			struct uid_gid_extent *forward;
> +			struct uid_gid_extent *reverse;
> +		};
> +	};
>  };
>  
>  #define USERNS_SETGROUPS_ALLOWED 1UL
> diff --git a/kernel/user.c b/kernel/user.c
> index 00281add65b2..9a20acce460d 100644
> --- a/kernel/user.c
> +++ b/kernel/user.c
> @@ -26,26 +26,32 @@
>  struct user_namespace init_user_ns = {
>  	.uid_map = {
>  		.nr_extents = 1,
> -		.extent[0] = {
> -			.first = 0,
> -			.lower_first = 0,
> -			.count = 4294967295U,
> +		{
> +			.extent[0] = {
> +				.first = 0,
> +				.lower_first = 0,
> +				.count = 4294967295U,

nit: ULONG_MAX ?

> +			},
>  		},
>  	},
>  	.gid_map = {
>  		.nr_extents = 1,
> -		.extent[0] = {
> -			.first = 0,
> -			.lower_first = 0,
> -			.count = 4294967295U,
> +		{
> +			.extent[0] = {
> +				.first = 0,
> +				.lower_first = 0,
> +				.count = 4294967295U,
> +			},
>  		},
>  	},
>  	.projid_map = {
>  		.nr_extents = 1,
> -		.extent[0] = {
> -			.first = 0,
> -			.lower_first = 0,
> -			.count = 4294967295U,
> +		{
> +			.extent[0] = {
> +				.first = 0,
> +				.lower_first = 0,
> +				.count = 4294967295U,
> +			},
>  		},
>  	},
>  	.count = ATOMIC_INIT(3),
>