[next] pcmcia: synclink_cs: replace 1-element array with flex-array member

  One-element arrays are deprecated, and we are replacing them with
flexible array members instead. So, replace one-element array with
flexible-array member in struct RXBUF and refactor the rest of the code
accordingly.

It's worth mentioning that doing a build before/after this patch
results in no binary output differences.

This helps with the ongoing efforts to tighten the FORTIFY_SOURCE
routines on memcpy() and help us make progress towards globally
enabling -fstrict-flex-arrays=3 [1].

Link: https://github.com/KSPP/linux/issues/79
Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101836 [1]

Signed-off-by: Paulo Miguel Almeida <paulo.miguel.almeida.rodenas@gmail.com>
---
 drivers/char/pcmcia/synclink_cs.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)
  

On Wed, Dec 14, 2022 at 09:42:00PM +1300, Paulo Miguel Almeida wrote:
> One-element arrays are deprecated, and we are replacing them with
> flexible array members instead. So, replace one-element array with
> flexible-array member in struct RXBUF and refactor the rest of the code
> accordingly.
> 
> It's worth mentioning that doing a build before/after this patch
> results in no binary output differences.
> 
> This helps with the ongoing efforts to tighten the FORTIFY_SOURCE
> routines on memcpy() and help us make progress towards globally
> enabling -fstrict-flex-arrays=3 [1].
> 
> Link: https://github.com/KSPP/linux/issues/79
> Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101836 [1]
> 
> Signed-off-by: Paulo Miguel Almeida <paulo.miguel.almeida.rodenas@gmail.com>
> ---
>  drivers/char/pcmcia/synclink_cs.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/char/pcmcia/synclink_cs.c b/drivers/char/pcmcia/synclink_cs.c
> index b2735be81ab2..1ab2d552f498 100644
> --- a/drivers/char/pcmcia/synclink_cs.c
> +++ b/drivers/char/pcmcia/synclink_cs.c
> @@ -105,7 +105,7 @@ static MGSL_PARAMS default_params = {
>  typedef struct {
>  	int count;
>  	unsigned char status;
> -	char data[1];
> +	char data[];
>  } RXBUF;
>  
>  /* The queue of BH actions to be performed */
> @@ -2611,7 +2611,8 @@ static int mgslpc_proc_show(struct seq_file *m, void *v)
>  static int rx_alloc_buffers(MGSLPC_INFO *info)
>  {
>  	/* each buffer has header and data */
> -	info->rx_buf_size = sizeof(RXBUF) + info->max_frame_size;
> +	info->rx_buf_size = max(offsetof(typeof(RXBUF), data) + 1, sizeof(RXBUF))
> +		+ info->max_frame_size;

It seems like there is an existing size bug here, and likely should be
fixed separately?

i.e. this was already allocating 1 byte "too much". I'd expect this
first:

-	info->rx_buf_size = sizeof(RXBUF) + info->max_frame_size;
+	info->rx_buf_size = sizeof(RXBUF) - 1 + info->max_frame_size;

and then the next patch:

-	char data[1];
+	char data[];
...
-	info->rx_buf_size = sizeof(RXBUF) - 1 + info->max_frame_size;
+	info->rx_buf_size = sizeof(RXBUF) + info->max_frame_size;

The above would induce a binary output change, and the second would not.

Though this results in what you had for the v2 patch (but I can't
believe it had no binary changes...)
  

On Wed, Dec 14, 2022 at 11:29:37AM -0800, Kees Cook wrote:
> On Wed, Dec 14, 2022 at 09:42:00PM +1300, Paulo Miguel Almeida wrote:
> > One-element arrays are deprecated, and we are replacing them with
> > flexible array members instead. So, replace one-element array with
> > flexible-array member in struct RXBUF and refactor the rest of the code
> > accordingly.
> > 
> > It's worth mentioning that doing a build before/after this patch
> > results in no binary output differences.
> > 
> > This helps with the ongoing efforts to tighten the FORTIFY_SOURCE
> > routines on memcpy() and help us make progress towards globally
> > enabling -fstrict-flex-arrays=3 [1].
> > 
> > Link: https://github.com/KSPP/linux/issues/79
> > Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101836 [1]
> > 
> > Signed-off-by: Paulo Miguel Almeida <paulo.miguel.almeida.rodenas@gmail.com>
> > ---
> >  drivers/char/pcmcia/synclink_cs.c | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/char/pcmcia/synclink_cs.c b/drivers/char/pcmcia/synclink_cs.c
> > index b2735be81ab2..1ab2d552f498 100644
> > --- a/drivers/char/pcmcia/synclink_cs.c
> > +++ b/drivers/char/pcmcia/synclink_cs.c
> > @@ -105,7 +105,7 @@ static MGSL_PARAMS default_params = {
> >  typedef struct {
> >  	int count;
> >  	unsigned char status;
> > -	char data[1];
> > +	char data[];
> >  } RXBUF;
> >  
> >  /* The queue of BH actions to be performed */
> > @@ -2611,7 +2611,8 @@ static int mgslpc_proc_show(struct seq_file *m, void *v)
> >  static int rx_alloc_buffers(MGSLPC_INFO *info)
> >  {
> >  	/* each buffer has header and data */
> > -	info->rx_buf_size = sizeof(RXBUF) + info->max_frame_size;
> > +	info->rx_buf_size = max(offsetof(typeof(RXBUF), data) + 1, sizeof(RXBUF))
> > +		+ info->max_frame_size;
> 
> It seems like there is an existing size bug here, and likely should be
> fixed separately?
> 
> i.e. this was already allocating 1 byte "too much". I'd expect this
> first:
> 
> -	info->rx_buf_size = sizeof(RXBUF) + info->max_frame_size;
> +	info->rx_buf_size = sizeof(RXBUF) - 1 + info->max_frame_size;
> 
> and then the next patch:
> 
> -	char data[1];
> +	char data[];
> ...
> -	info->rx_buf_size = sizeof(RXBUF) - 1 + info->max_frame_size;
> +	info->rx_buf_size = sizeof(RXBUF) + info->max_frame_size;
> 
> The above would induce a binary output change, and the second would not.
> 
> Though this results in what you had for the v2 patch (but I can't
> believe it had no binary changes...)
> 
> -- 
> Kees Cook

Hi Kees, Hi Andy, Thanks for taking the time to review this patch.

As both of you had similar points, I will reply them here.

The reasons why it had no binary changes was because of the combination
of this 2 things:

1) Existing padding - so sizeof(RXBUF) returned 8 bytes in both cases.

pahole -C RXBUF gcc/before/drivers/char/pcmcia/synclink_cs.ko
typedef struct {
	int                        count;                /*     0     4 */
	unsigned char              status;               /*     4     1 */
	char                       data[1];              /*     5     1 */

	/* size: 8, cachelines: 1, members: 3 */
	/* padding: 2 */
	/* last cacheline: 8 bytes */
} RXBUF;

pahole -C RXBUF gcc/after/drivers/char/pcmcia/synclink_cs.ko
typedef struct {
	int                        count;                /*     0     4 */
	unsigned char              status;               /*     4     1 */
	char                       data[];               /*     5     0 */

	/* size: 8, cachelines: 1, members: 3 */
	/* padding: 3 */
	/* last cacheline: 8 bytes */
} RXBUF;

2) RXBUF (as implemented now) is just  like a pair of lenses from which a
developer can have access to one of the circular buffers in MGSLPC_INFO
struct called 'rx_buf'. 

2611 static int rx_alloc_buffers(MGSLPC_INFO *info)
2612 {
2613         /* each buffer has header and data */
2614         info->rx_buf_size = sizeof(RXBUF) + info->max_frame_size;
2615
2616         /* calculate total allocation size for 8 buffers */
2617         info->rx_buf_total_size = info->rx_buf_size * 8;
2618
2619         /* limit total allocated memory */
2620         if (info->rx_buf_total_size > 0x10000)
2621                 info->rx_buf_total_size = 0x10000;
2622
2623         /* calculate number of buffers */
2624         info->rx_buf_count = info->rx_buf_total_size / info->rx_buf_size;
2625
2626         info->rx_buf = kmalloc(info->rx_buf_total_size, GFP_KERNEL);

To be honest, char data[_1_] in RXBUF was never required to be there.
The code base seems to make sure that it doesn't run past its limits by
keeping track of size buffer on MGSLPC_INFO->rx_buf_size (and sometimes
RXBUF->count)

(Addressing one point made by Andy about using of of the macros in
overflow.h)
	struct_size(buf, data, 1) would return 9 bytes which could
	potentially break the existing driver as it produces binary
	changes.

Let me know your thoughts

thanks!

- Paulo A.
  

On Wed, Dec 14, 2022 at 11:29:37AM -0800, Kees Cook wrote:
> On Wed, Dec 14, 2022 at 09:42:00PM +1300, Paulo Miguel Almeida wrote:
> > One-element arrays are deprecated, and we are replacing them with
> > flexible array members instead. So, replace one-element array with
> > flexible-array member in struct RXBUF and refactor the rest of the code
> > accordingly.
> > 
> > It's worth mentioning that doing a build before/after this patch
> > results in no binary output differences.
> > 
> > This helps with the ongoing efforts to tighten the FORTIFY_SOURCE
> > routines on memcpy() and help us make progress towards globally
> > enabling -fstrict-flex-arrays=3 [1].
> > 
> > Link: https://github.com/KSPP/linux/issues/79
> > Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101836 [1]
> > 
> > Signed-off-by: Paulo Miguel Almeida <paulo.miguel.almeida.rodenas@gmail.com>
> > ---
> >  drivers/char/pcmcia/synclink_cs.c | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/char/pcmcia/synclink_cs.c b/drivers/char/pcmcia/synclink_cs.c
> > index b2735be81ab2..1ab2d552f498 100644
> > --- a/drivers/char/pcmcia/synclink_cs.c
> > +++ b/drivers/char/pcmcia/synclink_cs.c
> > @@ -105,7 +105,7 @@ static MGSL_PARAMS default_params = {
> >  typedef struct {
> >  	int count;
> >  	unsigned char status;
> > -	char data[1];
> > +	char data[];
> >  } RXBUF;
> >  
> >  /* The queue of BH actions to be performed */
> > @@ -2611,7 +2611,8 @@ static int mgslpc_proc_show(struct seq_file *m, void *v)
> >  static int rx_alloc_buffers(MGSLPC_INFO *info)
> >  {
> >  	/* each buffer has header and data */
> > -	info->rx_buf_size = sizeof(RXBUF) + info->max_frame_size;
> > +	info->rx_buf_size = max(offsetof(typeof(RXBUF), data) + 1, sizeof(RXBUF))
> > +		+ info->max_frame_size;
> 
> It seems like there is an existing size bug here, and likely should be
> fixed separately?
> 
> i.e. this was already allocating 1 byte "too much". I'd expect this
> first:
> 
> -	info->rx_buf_size = sizeof(RXBUF) + info->max_frame_size;
> +	info->rx_buf_size = sizeof(RXBUF) - 1 + info->max_frame_size;
> 
> and then the next patch:
> 
> -	char data[1];
> +	char data[];
> ...
> -	info->rx_buf_size = sizeof(RXBUF) - 1 + info->max_frame_size;
> +	info->rx_buf_size = sizeof(RXBUF) + info->max_frame_size;
> 
> The above would induce a binary output change, and the second would not.
> 
> Though this results in what you had for the v2 patch (but I can't
> believe it had no binary changes...)
> 
> -- 
> Kees Cook

Just realised that you made a comment on PATCH v1 and Andy made a
comment on PATCH v2. Please conside my answer for PATCH v2 as I have
abandoned the v1. Apologies for the confusion.

thanks!

- Paulo A.
  

On Thu, Dec 15, 2022 at 09:09:46AM +1300, Paulo Miguel Almeida wrote:
> On Wed, Dec 14, 2022 at 11:29:37AM -0800, Kees Cook wrote:
> > On Wed, Dec 14, 2022 at 09:42:00PM +1300, Paulo Miguel Almeida wrote:
> > > One-element arrays are deprecated, and we are replacing them with
> > > flexible array members instead. So, replace one-element array with
> > > flexible-array member in struct RXBUF and refactor the rest of the code
> > > accordingly.
> > > 
> > > It's worth mentioning that doing a build before/after this patch
> > > results in no binary output differences.
> > > 
> > > This helps with the ongoing efforts to tighten the FORTIFY_SOURCE
> > > routines on memcpy() and help us make progress towards globally
> > > enabling -fstrict-flex-arrays=3 [1].
> > > 
> > > Link: https://github.com/KSPP/linux/issues/79
> > > Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101836 [1]
> > > 
> > > Signed-off-by: Paulo Miguel Almeida <paulo.miguel.almeida.rodenas@gmail.com>
> > > ---
> > >  drivers/char/pcmcia/synclink_cs.c | 5 +++--
> > >  1 file changed, 3 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/drivers/char/pcmcia/synclink_cs.c b/drivers/char/pcmcia/synclink_cs.c
> > > index b2735be81ab2..1ab2d552f498 100644
> > > --- a/drivers/char/pcmcia/synclink_cs.c
> > > +++ b/drivers/char/pcmcia/synclink_cs.c
> > > @@ -105,7 +105,7 @@ static MGSL_PARAMS default_params = {
> > >  typedef struct {
> > >  	int count;
> > >  	unsigned char status;
> > > -	char data[1];
> > > +	char data[];
> > >  } RXBUF;
> > >  
> > >  /* The queue of BH actions to be performed */
> > > @@ -2611,7 +2611,8 @@ static int mgslpc_proc_show(struct seq_file *m, void *v)
> > >  static int rx_alloc_buffers(MGSLPC_INFO *info)
> > >  {
> > >  	/* each buffer has header and data */
> > > -	info->rx_buf_size = sizeof(RXBUF) + info->max_frame_size;
> > > +	info->rx_buf_size = max(offsetof(typeof(RXBUF), data) + 1, sizeof(RXBUF))
> > > +		+ info->max_frame_size;
> > 
> > It seems like there is an existing size bug here, and likely should be
> > fixed separately?
> > 
> > i.e. this was already allocating 1 byte "too much". I'd expect this
> > first:
> > 
> > -	info->rx_buf_size = sizeof(RXBUF) + info->max_frame_size;
> > +	info->rx_buf_size = sizeof(RXBUF) - 1 + info->max_frame_size;
> > 
> > and then the next patch:
> > 
> > -	char data[1];
> > +	char data[];
> > ...
> > -	info->rx_buf_size = sizeof(RXBUF) - 1 + info->max_frame_size;
> > +	info->rx_buf_size = sizeof(RXBUF) + info->max_frame_size;
> > 
> > The above would induce a binary output change, and the second would not.
> > 
> > Though this results in what you had for the v2 patch (but I can't
> > believe it had no binary changes...)
> > 
> > -- 
> > Kees Cook
> 
> Hi Kees, Hi Andy, Thanks for taking the time to review this patch.
> 
> As both of you had similar points, I will reply them here.
> 
> The reasons why it had no binary changes was because of the combination
> of this 2 things:
> 
> 1) Existing padding - so sizeof(RXBUF) returned 8 bytes in both cases.
> 
> pahole -C RXBUF gcc/before/drivers/char/pcmcia/synclink_cs.ko
> typedef struct {
> 	int                        count;                /*     0     4 */
> 	unsigned char              status;               /*     4     1 */
> 	char                       data[1];              /*     5     1 */
> 
> 	/* size: 8, cachelines: 1, members: 3 */
> 	/* padding: 2 */
> 	/* last cacheline: 8 bytes */
> } RXBUF;
> 
> pahole -C RXBUF gcc/after/drivers/char/pcmcia/synclink_cs.ko
> typedef struct {
> 	int                        count;                /*     0     4 */
> 	unsigned char              status;               /*     4     1 */
> 	char                       data[];               /*     5     0 */
> 
> 	/* size: 8, cachelines: 1, members: 3 */
> 	/* padding: 3 */
> 	/* last cacheline: 8 bytes */
> } RXBUF;

Ah-ha, now I see.

> 
> 2) RXBUF (as implemented now) is just  like a pair of lenses from which a
> developer can have access to one of the circular buffers in MGSLPC_INFO
> struct called 'rx_buf'. 
> 
> 2611 static int rx_alloc_buffers(MGSLPC_INFO *info)
> 2612 {
> 2613         /* each buffer has header and data */
> 2614         info->rx_buf_size = sizeof(RXBUF) + info->max_frame_size;
> 2615
> 2616         /* calculate total allocation size for 8 buffers */
> 2617         info->rx_buf_total_size = info->rx_buf_size * 8;
> 2618
> 2619         /* limit total allocated memory */
> 2620         if (info->rx_buf_total_size > 0x10000)
> 2621                 info->rx_buf_total_size = 0x10000;
> 2622
> 2623         /* calculate number of buffers */
> 2624         info->rx_buf_count = info->rx_buf_total_size / info->rx_buf_size;
> 2625
> 2626         info->rx_buf = kmalloc(info->rx_buf_total_size, GFP_KERNEL);
> 
> To be honest, char data[_1_] in RXBUF was never required to be there.
> The code base seems to make sure that it doesn't run past its limits by
> keeping track of size buffer on MGSLPC_INFO->rx_buf_size (and sometimes
> RXBUF->count)
> 
> (Addressing one point made by Andy about using of of the macros in
> overflow.h)
> 	struct_size(buf, data, 1) would return 9 bytes which could
> 	potentially break the existing driver as it produces binary
> 	changes.

Yeah, I think your v2 is fine. Perhaps explicitly repeat the notes about
struct size padding in a v3 commit log?
  

On Wed, Dec 14, 2022 at 10:09 PM Paulo Miguel Almeida
<paulo.miguel.almeida.rodenas@gmail.com> wrote:
> On Wed, Dec 14, 2022 at 11:29:37AM -0800, Kees Cook wrote:
> > On Wed, Dec 14, 2022 at 09:42:00PM +1300, Paulo Miguel Almeida wrote:
> > > One-element arrays are deprecated, and we are replacing them with
> > > flexible array members instead. So, replace one-element array with
> > > flexible-array member in struct RXBUF and refactor the rest of the code
> > > accordingly.
> > >
> > > It's worth mentioning that doing a build before/after this patch
> > > results in no binary output differences.
> > >
> > > This helps with the ongoing efforts to tighten the FORTIFY_SOURCE
> > > routines on memcpy() and help us make progress towards globally
> > > enabling -fstrict-flex-arrays=3 [1].

...

> > >  typedef struct {
> > >     int count;
> > >     unsigned char status;
> > > -   char data[1];
> > > +   char data[];
> > >  } RXBUF;

...

> As both of you had similar points, I will reply them here.
>
> The reasons why it had no binary changes was because of the combination
> of this 2 things:
>
> 1) Existing padding - so sizeof(RXBUF) returned 8 bytes in both cases.
>
> pahole -C RXBUF gcc/before/drivers/char/pcmcia/synclink_cs.ko
> typedef struct {
>         int                        count;                /*     0     4 */
>         unsigned char              status;               /*     4     1 */
>         char                       data[1];              /*     5     1 */
>
>         /* size: 8, cachelines: 1, members: 3 */
>         /* padding: 2 */
>         /* last cacheline: 8 bytes */
> } RXBUF;
>
> pahole -C RXBUF gcc/after/drivers/char/pcmcia/synclink_cs.ko
> typedef struct {
>         int                        count;                /*     0     4 */
>         unsigned char              status;               /*     4     1 */
>         char                       data[];               /*     5     0 */
>
>         /* size: 8, cachelines: 1, members: 3 */
>         /* padding: 3 */
>         /* last cacheline: 8 bytes */
> } RXBUF;

Yes, and Try to make it work with __packed. As I said, the problem is
that the code is relying on something which is architecture dependent
strictly speaking. And hence I disagree with Kees that v2 is okay to
go.

> 2) RXBUF (as implemented now) is just  like a pair of lenses from which a
> developer can have access to one of the circular buffers in MGSLPC_INFO
> struct called 'rx_buf'.

> 2611 static int rx_alloc_buffers(MGSLPC_INFO *info)
> 2612 {
> 2613         /* each buffer has header and data */
> 2614         info->rx_buf_size = sizeof(RXBUF) + info->max_frame_size;
> 2615
> 2616         /* calculate total allocation size for 8 buffers */
> 2617         info->rx_buf_total_size = info->rx_buf_size * 8;
> 2618
> 2619         /* limit total allocated memory */
> 2620         if (info->rx_buf_total_size > 0x10000)
> 2621                 info->rx_buf_total_size = 0x10000;
> 2622
> 2623         /* calculate number of buffers */
> 2624         info->rx_buf_count = info->rx_buf_total_size / info->rx_buf_size;
> 2625
> 2626         info->rx_buf = kmalloc(info->rx_buf_total_size, GFP_KERNEL);
>
> To be honest, char data[_1_] in RXBUF was never required to be there.
> The code base seems to make sure that it doesn't run past its limits by
> keeping track of size buffer on MGSLPC_INFO->rx_buf_size (and sometimes
> RXBUF->count)
>
> (Addressing one point made by Andy about using of of the macros in
> overflow.h)
>         struct_size(buf, data, 1) would return 9 bytes which could
>         potentially break the existing driver as it produces binary
>         changes.

You got it incorrectly. I believe you should use something different than 1.
In previous lines in the function it multiplies sizeof + max_frame_size by 8.

The full change should be something like

check_add(sizeof(), max_frame_size)
kcalloc(8, size)

Think about it.

> Let me know your thoughts
  

On Wed, Dec 14, 2022 at 10:39:52PM +0200, Andy Shevchenko wrote:
> Yes, and Try to make it work with __packed. As I said, the problem is
> that the code is relying on something which is architecture dependent
> strictly speaking. And hence I disagree with Kees that v2 is okay to
> go.

I meant that v2 is functionally identical to the existing code.

> The full change should be something like
> 
> check_add(sizeof(), max_frame_size)
> kcalloc(8, size)

Right -- this would fix the existing mistakes in size calculation (and
is certainly better).
  

On Wed, Dec 14, 2022 at 11:49 PM Kees Cook <keescook@chromium.org> wrote:
> On Wed, Dec 14, 2022 at 10:39:52PM +0200, Andy Shevchenko wrote:
> > Yes, and Try to make it work with __packed. As I said, the problem is
> > that the code is relying on something which is architecture dependent
> > strictly speaking. And hence I disagree with Kees that v2 is okay to
> > go.
>
> I meant that v2 is functionally identical to the existing code.

Ah, sorry for misunderstanding.

> > The full change should be something like
> >
> > check_add(sizeof(), max_frame_size)
> > kcalloc(8, size)
>
> Right -- this would fix the existing mistakes in size calculation (and
> is certainly better).

Glad to hear that we are on the same page.
  

On Thu, Dec 15, 2022 at 12:06:46AM +0200, Andy Shevchenko wrote:
> On Wed, Dec 14, 2022 at 11:49 PM Kees Cook <keescook@chromium.org> wrote:
> > On Wed, Dec 14, 2022 at 10:39:52PM +0200, Andy Shevchenko wrote:
> > > Yes, and Try to make it work with __packed. As I said, the problem is
> > > that the code is relying on something which is architecture dependent
> > > strictly speaking. And hence I disagree with Kees that v2 is okay to
> > > go.
> >
> > I meant that v2 is functionally identical to the existing code.
> 
> Ah, sorry for misunderstanding.
> 

I agree with using __packed attribute to remove the extra padding (and
for the reasons you mentioned before). That would reduce the sizeof(RXBUF)
from 8 to 5 (which is good) but that is still 1 byte "too much".

Piggying back on a suggestion Kees gave before:

-       info->rx_buf_size = sizeof(RXBUF) + info->max_frame_size;
+       info->rx_buf_size = sizeof(RXBUF) - 1 + info->max_frame_size;

That way RXBUF->data will point to the first byte of the frame_size 
(MGSLPC_INFO->max_frame_size) which is what is actually needed.

> > > The full change should be something like
> > >
> > > check_add(sizeof(), max_frame_size)
> > > kcalloc(8, size)
> >
> > Right -- this would fix the existing mistakes in size calculation (and
> > is certainly better).
> 
> Glad to hear that we are on the same page.
> 

That makes sense to me.

thanks!

- Paulo A.
  

On Thu, Dec 15, 2022 at 05:29:15PM +1300, Paulo Miguel Almeida wrote:
> On Thu, Dec 15, 2022 at 12:06:46AM +0200, Andy Shevchenko wrote:
> > On Wed, Dec 14, 2022 at 11:49 PM Kees Cook <keescook@chromium.org> wrote:
> > > On Wed, Dec 14, 2022 at 10:39:52PM +0200, Andy Shevchenko wrote:
> > > > Yes, and Try to make it work with __packed. As I said, the problem is
> > > > that the code is relying on something which is architecture dependent
> > > > strictly speaking. And hence I disagree with Kees that v2 is okay to
> > > > go.
> > >
> > > I meant that v2 is functionally identical to the existing code.
> > 
> > Ah, sorry for misunderstanding.
> > 
> 
> I agree with using __packed attribute to remove the extra padding (and
> for the reasons you mentioned before). That would reduce the sizeof(RXBUF)
> from 8 to 5 (which is good) but that is still 1 byte "too much".
> 
> Piggying back on a suggestion Kees gave before:
> 
> -       info->rx_buf_size = sizeof(RXBUF) + info->max_frame_size;
> +       info->rx_buf_size = sizeof(RXBUF) - 1 + info->max_frame_size;
> 
> That way RXBUF->data will point to the first byte of the frame_size 
> (MGSLPC_INFO->max_frame_size) which is what is actually needed.
> 

I chose my words poorly here... sorry my brain is a bit fried today.

Let me rephrase that last sentence. After that change (or similar
change), RXBUF->data will point to the first byte of the buffer
allocated during the initialisation process. (which is
limited/controlled by the size of MGSLPC_INFO->max_frame_size)...
so no 'extra byte/padding' will be there.

- Paulo A.

> > > > The full change should be something like
> > > >
> > > > check_add(sizeof(), max_frame_size)
> > > > kcalloc(8, size)
> > >
> > > Right -- this would fix the existing mistakes in size calculation (and
> > > is certainly better).
> > 
> > Glad to hear that we are on the same page.
> > 
> 
> That makes sense to me.
> 
> thanks!
> 
> - Paulo A.
  

On Thu, Dec 15, 2022 at 6:29 AM Paulo Miguel Almeida
<paulo.miguel.almeida.rodenas@gmail.com> wrote:
> On Thu, Dec 15, 2022 at 12:06:46AM +0200, Andy Shevchenko wrote:
> > On Wed, Dec 14, 2022 at 11:49 PM Kees Cook <keescook@chromium.org> wrote:
> > > On Wed, Dec 14, 2022 at 10:39:52PM +0200, Andy Shevchenko wrote:

...

> > > > Yes, and Try to make it work with __packed. As I said, the problem is
> > > > that the code is relying on something which is architecture dependent
> > > > strictly speaking. And hence I disagree with Kees that v2 is okay to
> > > > go.
> > >
> > > I meant that v2 is functionally identical to the existing code.
> >
> > Ah, sorry for misunderstanding.
>
> I agree with using __packed attribute to remove the extra padding (and
> for the reasons you mentioned before). That would reduce the sizeof(RXBUF)
> from 8 to 5 (which is good) but that is still 1 byte "too much".

What I meant with the above is that the code has to work properly with
or without __packed. It's just to show you that this code has flaws if
it relies on the padding.
  

On Thu, Dec 15, 2022 at 10:57:57AM +0200, Andy Shevchenko wrote:
> On Thu, Dec 15, 2022 at 6:29 AM Paulo Miguel Almeida
> <paulo.miguel.almeida.rodenas@gmail.com> wrote:
> > On Thu, Dec 15, 2022 at 12:06:46AM +0200, Andy Shevchenko wrote:
> > > On Wed, Dec 14, 2022 at 11:49 PM Kees Cook <keescook@chromium.org> wrote:
> > > > On Wed, Dec 14, 2022 at 10:39:52PM +0200, Andy Shevchenko wrote:
> 
> ...
> 
> > > > > Yes, and Try to make it work with __packed. As I said, the problem is
> > > > > that the code is relying on something which is architecture dependent
> > > > > strictly speaking. And hence I disagree with Kees that v2 is okay to
> > > > > go.
> > > >
> > > > I meant that v2 is functionally identical to the existing code.
> > >
> > > Ah, sorry for misunderstanding.
> >
> > I agree with using __packed attribute to remove the extra padding (and
> > for the reasons you mentioned before). That would reduce the sizeof(RXBUF)
> > from 8 to 5 (which is good) but that is still 1 byte "too much".
> 
> What I meant with the above is that the code has to work properly with
> or without __packed. It's just to show you that this code has flaws if
> it relies on the padding.
> 

Right - that would work just as well. I will work on v3 with the
suggestions given by you (sizing calculation amendments using overflow.h
macros) and kees (adding the notes regarding the padding) then.

- Paulo A.