Re: [PATCH v4] net: netfilter: LLVMLinux: vlais-netfilter

[Behan Webster <behanw@converseincode.com>]

On 03/22/14 23:55, behanw@converseincode.com wrote:
> From: Mark Charlebois <charlebm@gmail.com>
>
> Replaced non-standard C use of Variable Length Arrays In Structs (VLAIS) in
> xt_repldata.h with a C99 compliant flexible array member and then calculated
> offsets to the other struct members. These other members aren't referenced by
> name in this code, however this patch maintains the same memory layout and
> padding as was previously accomplished using VLAIS.
>
> Had the original structure been ordered differently, with the entries VLA at
> the end, then it could have been a flexible member, and this patch would have
> been a lot simpler. However since the data stored in this structure is
> ultimately exported to userspace, the order of this structure can't be changed.
>
> This patch makes no attempt to change the existing behavior, merely the way in
> which the current layout is accomplished using standard C99 constructs. As such
> the code can now be compiled with either gcc or clang.
>
> This version of the patch removes the trailing alignment that the VLAIS
> structure would allocate in order to simplify the patch.
>
> Author: Mark Charlebois <charlebm@gmail.com>
> Signed-off-by: Mark Charlebois <charlebm@gmail.com>
> Signed-off-by: Behan Webster <behanw@converseincode.com>
> Signed-off-by: Vinícius Tinti <viniciustinti@gmail.com>
> ---
>   net/netfilter/xt_repldata.h | 22 +++++++++++++++++-----
>   1 file changed, 17 insertions(+), 5 deletions(-)
>
> diff --git a/net/netfilter/xt_repldata.h b/net/netfilter/xt_repldata.h
> index 6efe4e5..8fd3241 100644
> --- a/net/netfilter/xt_repldata.h
> +++ b/net/netfilter/xt_repldata.h
> @@ -5,23 +5,35 @@
>    * they serve as the hanging-off data accessed through repl.data[].
>    */
>   
> +/* tbl has the following structure equivalent, but is C99 compliant:
> + * struct {
> + *	struct type##_replace repl;
> + *	struct type##_standard entries[nhooks];
> + *	struct type##_error term;
> + * } *tbl;
> + */
> +
>   #define xt_alloc_initial_table(type, typ2) ({ \
>   	unsigned int hook_mask = info->valid_hooks; \
>   	unsigned int nhooks = hweight32(hook_mask); \
>   	unsigned int bytes = 0, hooknum = 0, i = 0; \
>   	struct { \
>   		struct type##_replace repl; \
> -		struct type##_standard entries[nhooks]; \
> -		struct type##_error term; \
> -	} *tbl = kzalloc(sizeof(*tbl), GFP_KERNEL); \
> +		struct type##_standard entries[]; \
> +	} *tbl; \
> +	struct type##_error *term; \
> +	size_t term_offset = (offsetof(typeof(*tbl), entries[nhooks]) + \
> +		__alignof__(*term) - 1) & ~(__alignof__(*term) - 1); \
> +	tbl = kzalloc(term_offset + sizeof(*term), GFP_KERNEL); \
>   	if (tbl == NULL) \
>   		return NULL; \
> +	term = (struct type##_error *)&(((char *)tbl)[term_offset]); \
>   	strncpy(tbl->repl.name, info->name, sizeof(tbl->repl.name)); \
> -	tbl->term = (struct type##_error)typ2##_ERROR_INIT;  \
> +	*term = (struct type##_error)typ2##_ERROR_INIT;  \
>   	tbl->repl.valid_hooks = hook_mask; \
>   	tbl->repl.num_entries = nhooks + 1; \
>   	tbl->repl.size = nhooks * sizeof(struct type##_standard) + \
> -	                 sizeof(struct type##_error); \
> +			 sizeof(struct type##_error); \
>   	for (; hook_mask != 0; hook_mask >>= 1, ++hooknum) { \
>   		if (!(hook_mask & 1)) \
>   			continue; \

Any further feedback about this patch?

Behan

-- 
Behan Webster
behanw@converseincode.com