[PATCH v2] net: netfilter: LLVMLinux: vlais-netfilter

[PATCH v2] net: netfilter: LLVMLinux: vlais-netfilter

[behanw]

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.

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 | 33 +++++++++++++++++++++++++++------
 1 file changed, 27 insertions(+), 6 deletions(-)

diff --git a/net/netfilter/xt_repldata.h b/net/netfilter/xt_repldata.h
index 6efe4e5..f83a519 100644
--- a/net/netfilter/xt_repldata.h
+++ b/net/netfilter/xt_repldata.h
@@ -5,29 +5,50 @@
  * 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); \
+		char data[0]; \
+	} *tbl; \
+	struct type##_standard *entries; \
+	struct type##_error *term; \
+	size_t entries_offset = (-sizeof(tbl->repl)) & \
+		(__alignof__(*entries)-1); \
+	size_t entries_end = entries_offset + nhooks * sizeof(*entries); \
+	size_t term_offset = entries_end + \
+		(-sizeof(tbl->repl) - entries_end) & (__alignof__(*term)-1); \
+	size_t term_end = term_offset + sizeof(*term); \
+	size_t data_sz = term_end + \
+		(-sizeof(tbl->repl) - term_end) & (__alignof__(tbl->repl)-1); \
+	size_t tbl_sz = sizeof(tbl->repl) + data_sz; \
+	tbl = kzalloc(tbl_sz, GFP_KERNEL); \
 	if (tbl == NULL) \
 		return NULL; \
+	entries = (struct type##_standard *)&tbl->data[entries_offset]; \
+	term = (struct type##_error *)&tbl->data[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; \
 		tbl->repl.hook_entry[hooknum] = bytes; \
 		tbl->repl.underflow[hooknum]  = bytes; \
-		tbl->entries[i++] = (struct type##_standard) \
+		entries[i++] = (struct type##_standard) \
 			typ2##_STANDARD_INIT(NF_ACCEPT); \
 		bytes += sizeof(struct type##_standard); \
 	} \
-- 
1.8.3.2

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RE: [PATCH v2] net: netfilter: LLVMLinux: vlais-netfilter

[David Laight]

From:  behanw@converseincode.com
> 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.

Why not just remove the last element and allocate space for it after the
structure?
That would reduce the complexity of the patch and the unreadability
of the new code.
I realise that the alignment of type##_error is 'tricky' to determine.

	David

Re: [PATCH v2] net: netfilter: LLVMLinux: vlais-netfilter

[Behan Webster]

On 03/18/14 02:41, David Laight wrote:
> From:  behanw@converseincode.com
>> 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.
> Why not just remove the last element and allocate space for it after the
> structure?
Because that would still be employing VLAIS to solve the problem. The 
last element may be a zero-length array (a flexible member), not a VLA. 
Sadly both the last 2 elements in the struct need to be manually 
calculated, which is what we've done.

> That would reduce the complexity of the patch and the unreadability
> of the new code.
No one is claiming this patch is more readable, merely that it is C99 
compliant (though strictly speaking this patch is C89, C99 and C11 
compliant). We tried to use macros to make it more readable in previous 
patches. The consensus was that macros were bad.

> I realise that the alignment of type##_error is 'tricky' to determine.
That is what makes it "unreadable". :(

Behan

-- 
Behan Webster
behanw@converseincode.com

RE: [PATCH v2] net: netfilter: LLVMLinux: vlais-netfilter

[David Laight]

From: Behan Webster 
> On 03/18/14 02:41, David Laight wrote:
> > From:  behanw@converseincode.com
> >> 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.
> > Why not just remove the last element and allocate space for it after the
> > structure?
> Because that would still be employing VLAIS to solve the problem. The
> last element may be a zero-length array (a flexible member), not a VLA.
> Sadly both the last 2 elements in the struct need to be manually
> calculated, which is what we've done.

So make the last element a 'flexible member' and then work out where
the final field goes.
Something like:
	struct p {
		struct a a;
		struct b b[];
	} p = malloc(sizeof *p + n * sizeof (struct b) + alignof (struct c)
		+ sizeof (struct c);
	struct c *c = (void *)&p->b[n] + (-offsetof(struct p, b[n]) & (alignof(struct c) - 1);

	David

Re: [PATCH v2] net: netfilter: LLVMLinux: vlais-netfilter

[Behan Webster]

On 03/18/14 08:24, David Laight wrote:
> From: Behan Webster
>> On 03/18/14 02:41, David Laight wrote:
>>> From:  behanw@converseincode.com
>>>> 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.
>>> Why not just remove the last element and allocate space for it after the
>>> structure?
>> Because that would still be employing VLAIS to solve the problem. The
>> last element may be a zero-length array (a flexible member), not a VLA.
>> Sadly both the last 2 elements in the struct need to be manually
>> calculated, which is what we've done.
> So make the last element a 'flexible member' and then work out where
> the final field goes.
> Something like:
> 	struct p {
> 		struct a a;
> 		struct b b[];
> 	} p = malloc(sizeof *p + n * sizeof (struct b) + alignof (struct c)
> 		+ sizeof (struct c);
> 	struct c *c = (void *)&p->b[n] + (-offsetof(struct p, b[n]) & (alignof(struct c) - 1);
Oh, I see. Will fix.

Thanks!

Behan

-- 
Behan Webster
behanw@converseincode.com

[PATCH v3] net: netfilter: LLVMLinux: vlais-netfilter

[behanw]

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.

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 | 27 ++++++++++++++++++++++-----
 1 file changed, 22 insertions(+), 5 deletions(-)

diff --git a/net/netfilter/xt_repldata.h b/net/netfilter/xt_repldata.h
index 6efe4e5..343599e 100644
--- a/net/netfilter/xt_repldata.h
+++ b/net/netfilter/xt_repldata.h
@@ -5,23 +5,40 @@
  * 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 entries_end = offsetof(typeof(*tbl), \
+		entries[nhooks-1]) + sizeof(tbl->entries[0]); \
+	size_t term_offset = (entries_end + __alignof__(*term) - 1) \
+		& ~(__alignof__(*term) - 1); \
+	size_t term_end = term_offset + sizeof(*term); \
+	size_t tbl_sz = (term_end + __alignof__(tbl->repl) - 1) \
+		& ~(__alignof__(tbl->repl) - 1); \
+	tbl = kzalloc(tbl_sz, 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; \
-- 
1.8.3.2

RE: [PATCH v3] net: netfilter: LLVMLinux: vlais-netfilter

[David Laight]

From: behanw@converseincode.com 
> 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.
> 
> 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 | 27 ++++++++++++++++++++++-----
>  1 file changed, 22 insertions(+), 5 deletions(-)
> 
> diff --git a/net/netfilter/xt_repldata.h b/net/netfilter/xt_repldata.h
> index 6efe4e5..343599e 100644
> --- a/net/netfilter/xt_repldata.h
> +++ b/net/netfilter/xt_repldata.h
> @@ -5,23 +5,40 @@
>   * 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 entries_end = offsetof(typeof(*tbl), \
> +		entries[nhooks-1]) + sizeof(tbl->entries[0]); \

Is the compiler complaining about:
	offsetof(typeof(*tbl), entries[nhooks])
If it does it is a PITA.

> +	size_t term_offset = (entries_end + __alignof__(*term) - 1) \
> +		& ~(__alignof__(*term) - 1); \

You've not tested this - the () are in the wrong places.

> +	size_t term_end = term_offset + sizeof(*term); \
> +	size_t tbl_sz = (term_end + __alignof__(tbl->repl) - 1) \
> +		& ~(__alignof__(tbl->repl) - 1); \
> +	tbl = kzalloc(tbl_sz, GFP_KERNEL); \

The number of temporary variables make the above hard to read.
I'm not at all sure you need to worry about the trailing alignment.
It rather depends on how the final data is used.
If the combined buffer is copied to userspace you may not
be copying all of the required data.
It might be easier to call copytouser() twice.

>  	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;  \

	David

Re: [PATCH v3] net: netfilter: LLVMLinux: vlais-netfilter

[Mark Charlebois]

On Wed, Mar 19, 2014 at 09:52:40AM +0000, David Laight wrote:
> From: behanw@converseincode.com 
> > 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.
> > 
> > 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 | 27 ++++++++++++++++++++++-----
> >  1 file changed, 22 insertions(+), 5 deletions(-)
> > 
> > diff --git a/net/netfilter/xt_repldata.h b/net/netfilter/xt_repldata.h
> > index 6efe4e5..343599e 100644
> > --- a/net/netfilter/xt_repldata.h
> > +++ b/net/netfilter/xt_repldata.h
> > @@ -5,23 +5,40 @@
> >   * 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 entries_end = offsetof(typeof(*tbl), \
> > +		entries[nhooks-1]) + sizeof(tbl->entries[0]); \
> 
> Is the compiler complaining about:
> 	offsetof(typeof(*tbl), entries[nhooks])
> If it does it is a PITA.
> 
> > +	size_t term_offset = (entries_end + __alignof__(*term) - 1) \
> > +		& ~(__alignof__(*term) - 1); \
> 
> You've not tested this - the () are in the wrong places.

I have tested it with both clang and gcc. Unit test is at http://git.linuxfoundation.org/?p=llvmlinux.git;a=blob;f=test/unit/vlais/netfilter.c;h=7adc255e47e15a252d2bda7af7ae217ac683c25e;hb=HEAD

Basic calulation of a new offset is:

new offset = (offset + align - 1) & ~(align - 1)

The parenthesis seem correct to me.
> 
> > +	size_t term_end = term_offset + sizeof(*term); \
> > +	size_t tbl_sz = (term_end + __alignof__(tbl->repl) - 1) \
> > +		& ~(__alignof__(tbl->repl) - 1); \
> > +	tbl = kzalloc(tbl_sz, GFP_KERNEL); \
> 
> The number of temporary variables make the above hard to read.
> I'm not at all sure you need to worry about the trailing alignment.
> It rather depends on how the final data is used.
> If the combined buffer is copied to userspace you may not
> be copying all of the required data.
> It might be easier to call copytouser() twice.

I can try to remove more variables if that is prefered.
The existing memory layout was preserved exactly so that however the
data currently used will be unaffected. I can remove the trailing
alignment but was not 100% sure it wasn't needed.

> 
> >  	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;  \
> 
> 	David
> 

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

[behanw]

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; \
-- 
1.8.3.2

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

[Behan Webster]

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