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* [PATCH RESEND] tpm: eventlog: Replace zero-length array with flexible-array member
@ 2020-05-07  4:09 Gustavo A. R. Silva
  2020-05-07 18:02 ` Kees Cook
  2020-05-13 21:39 ` Jarkko Sakkinen
  0 siblings, 2 replies; 6+ messages in thread
From: Gustavo A. R. Silva @ 2020-05-07  4:09 UTC (permalink / raw)
  To: Peter Huewe, Jarkko Sakkinen, Jason Gunthorpe, Arnd Bergmann,
	Greg Kroah-Hartman
  Cc: linux-integrity, linux-kernel, Gustavo A. R. Silva

The current codebase makes use of the zero-length array language
extension to the C90 standard, but the preferred mechanism to declare
variable-length types such as these ones is a flexible array member[1][2],
introduced in C99:

struct foo {
        int stuff;
        struct boo array[];
};

By making use of the mechanism above, we will get a compiler warning
in case the flexible array does not occur last in the structure, which
will help us prevent some kind of undefined behavior bugs from being
inadvertently introduced[3] to the codebase from now on.

Also, notice that, dynamic memory allocations won't be affected by
this change:

"Flexible array members have incomplete type, and so the sizeof operator
may not be applied. As a quirk of the original implementation of
zero-length arrays, sizeof evaluates to zero."[1]

sizeof(flexible-array-member) triggers a warning because flexible array
members have incomplete type[1]. There are some instances of code in
which the sizeof operator is being incorrectly/erroneously applied to
zero-length arrays and the result is zero. Such instances may be hiding
some bugs. So, this work (flexible-array member conversions) will also
help to get completely rid of those sorts of issues.

Also, the following issue shows up due to the flexible-array member
having incomplete type[4]:

drivers/char/tpm/eventlog/tpm2.c: In function ‘tpm2_bios_measurements_start’:
drivers/char/tpm/eventlog/tpm2.c:54:46: error: invalid application of ‘sizeof’ to incomplete type ‘u8[]’ {aka ‘unsigned char[]’}
   54 |  size = sizeof(struct tcg_pcr_event) - sizeof(event_header->event)
      |                                              ^
drivers/char/tpm/eventlog/tpm2.c: In function ‘tpm2_bios_measurements_next’:
drivers/char/tpm/eventlog/tpm2.c:102:10: error: invalid application of ‘sizeof’ to incomplete type ‘u8[]’ {aka ‘unsigned char[]’}
  102 |    sizeof(event_header->event) + event_header->event_size;
      |          ^
drivers/char/tpm/eventlog/tpm2.c: In function ‘tpm2_binary_bios_measurements_show’:
drivers/char/tpm/eventlog/tpm2.c:140:10: error: invalid application of ‘sizeof’ to incomplete type ‘u8[]’ {aka ‘unsigned char[]’}
  140 |    sizeof(event_header->event) + event_header->event_size;
      |          ^
scripts/Makefile.build:266: recipe for target 'drivers/char/tpm/eventlog/tpm2.o' failed
make[3]: *** [drivers/char/tpm/eventlog/tpm2.o] Error 1

As mentioned above: "Flexible array members have incomplete type, and
so the sizeof operator may not be applied. As a quirk of the original
implementation of zero-length arrays, sizeof evaluates to zero."[1] So,
the sizeof(flexible-array) can be safely removed to fix the error above.

Lastly, prefer sizeof(*ptr) over sizeof(struct foo).

This issue was found with the help of Coccinelle.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] https://github.com/KSPP/linux/issues/21
[3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")
[4] https://github.com/KSPP/linux/issues/43

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
---
Hi,

I'm resending this because LKML is eating some messages, recently.
Sorry for the noise in case you've already received this patch.

Thanks

 drivers/char/tpm/eventlog/tpm2.c | 10 +++-------
 include/linux/tpm_eventlog.h     |  2 +-
 2 files changed, 4 insertions(+), 8 deletions(-)

diff --git a/drivers/char/tpm/eventlog/tpm2.c b/drivers/char/tpm/eventlog/tpm2.c
index e741b1157525..351a2989b3c6 100644
--- a/drivers/char/tpm/eventlog/tpm2.c
+++ b/drivers/char/tpm/eventlog/tpm2.c
@@ -51,8 +51,7 @@ static void *tpm2_bios_measurements_start(struct seq_file *m, loff_t *pos)
 	int i;
 
 	event_header = addr;
-	size = sizeof(struct tcg_pcr_event) - sizeof(event_header->event)
-		+ event_header->event_size;
+	size = sizeof(*event_header) + event_header->event_size;
 
 	if (*pos == 0) {
 		if (addr + size < limit) {
@@ -98,8 +97,7 @@ static void *tpm2_bios_measurements_next(struct seq_file *m, void *v,
 	event_header = log->bios_event_log;
 
 	if (v == SEQ_START_TOKEN) {
-		event_size = sizeof(struct tcg_pcr_event) -
-			sizeof(event_header->event) + event_header->event_size;
+		event_size = sizeof(*event_header) + event_header->event_size;
 		marker = event_header;
 	} else {
 		event = v;
@@ -136,9 +134,7 @@ static int tpm2_binary_bios_measurements_show(struct seq_file *m, void *v)
 	size_t size;
 
 	if (v == SEQ_START_TOKEN) {
-		size = sizeof(struct tcg_pcr_event) -
-			sizeof(event_header->event) + event_header->event_size;
-
+		size = sizeof(*event_header) + event_header->event_size;
 		temp_ptr = event_header;
 
 		if (size > 0)
diff --git a/include/linux/tpm_eventlog.h b/include/linux/tpm_eventlog.h
index c253461b1c4e..4f8c90c93c29 100644
--- a/include/linux/tpm_eventlog.h
+++ b/include/linux/tpm_eventlog.h
@@ -97,7 +97,7 @@ struct tcg_pcr_event {
 	u32 event_type;
 	u8 digest[20];
 	u32 event_size;
-	u8 event[0];
+	u8 event[];
 } __packed;
 
 struct tcg_event_field {
-- 
2.26.2


^ permalink raw reply	[flat|nested] 6+ messages in thread

* Re: [PATCH RESEND] tpm: eventlog: Replace zero-length array with flexible-array member
  2020-05-07  4:09 [PATCH RESEND] tpm: eventlog: Replace zero-length array with flexible-array member Gustavo A. R. Silva
@ 2020-05-07 18:02 ` Kees Cook
  2020-05-08 16:06   ` Gustavo A. R. Silva
  2020-05-13 21:39 ` Jarkko Sakkinen
  1 sibling, 1 reply; 6+ messages in thread
From: Kees Cook @ 2020-05-07 18:02 UTC (permalink / raw)
  To: Gustavo A. R. Silva
  Cc: Peter Huewe, Jarkko Sakkinen, Jason Gunthorpe, Arnd Bergmann,
	Greg Kroah-Hartman, linux-integrity, linux-kernel,
	Gustavo A. R. Silva

On Wed, May 06, 2020 at 11:09:12PM -0500, Gustavo A. R. Silva wrote:
> As mentioned above: "Flexible array members have incomplete type, and
> so the sizeof operator may not be applied. As a quirk of the original
> implementation of zero-length arrays, sizeof evaluates to zero."[1] So,
> the sizeof(flexible-array) can be safely removed to fix the error above.

As in "sizeof(event_header->event) always evaluated to 0, so removing it
has no effect".

> [...]
> diff --git a/drivers/char/tpm/eventlog/tpm2.c b/drivers/char/tpm/eventlog/tpm2.c
> index e741b1157525..351a2989b3c6 100644
> --- a/drivers/char/tpm/eventlog/tpm2.c
> +++ b/drivers/char/tpm/eventlog/tpm2.c
> @@ -51,8 +51,7 @@ static void *tpm2_bios_measurements_start(struct seq_file *m, loff_t *pos)
>  	int i;
>  
>  	event_header = addr;
> -	size = sizeof(struct tcg_pcr_event) - sizeof(event_header->event)
> -		+ event_header->event_size;
> +	size = sizeof(*event_header) + event_header->event_size;

That said, I think it would be better to stick to the struct_size()
idiom for dealing with flexible arrays here:

	size = struct_size(event_header, event, event_size);

-- 
Kees Cook

^ permalink raw reply	[flat|nested] 6+ messages in thread

* Re: [PATCH RESEND] tpm: eventlog: Replace zero-length array with flexible-array member
  2020-05-07 18:02 ` Kees Cook
@ 2020-05-08 16:06   ` Gustavo A. R. Silva
  0 siblings, 0 replies; 6+ messages in thread
From: Gustavo A. R. Silva @ 2020-05-08 16:06 UTC (permalink / raw)
  To: Kees Cook
  Cc: Peter Huewe, Jarkko Sakkinen, Jason Gunthorpe, Arnd Bergmann,
	Greg Kroah-Hartman, linux-integrity, linux-kernel,
	Gustavo A. R. Silva

On Thu, May 07, 2020 at 11:02:18AM -0700, Kees Cook wrote:
> On Wed, May 06, 2020 at 11:09:12PM -0500, Gustavo A. R. Silva wrote:
> > As mentioned above: "Flexible array members have incomplete type, and
> > so the sizeof operator may not be applied. As a quirk of the original
> > implementation of zero-length arrays, sizeof evaluates to zero."[1] So,
> > the sizeof(flexible-array) can be safely removed to fix the error above.
> 
> As in "sizeof(event_header->event) always evaluated to 0, so removing it
> has no effect".
> 

Thanks for this.  I wanted to make a more general statement, but I'll
update the changelog text. :)

> > [...]
> > diff --git a/drivers/char/tpm/eventlog/tpm2.c b/drivers/char/tpm/eventlog/tpm2.c
> > index e741b1157525..351a2989b3c6 100644
> > --- a/drivers/char/tpm/eventlog/tpm2.c
> > +++ b/drivers/char/tpm/eventlog/tpm2.c
> > @@ -51,8 +51,7 @@ static void *tpm2_bios_measurements_start(struct seq_file *m, loff_t *pos)
> >  	int i;
> >  
> >  	event_header = addr;
> > -	size = sizeof(struct tcg_pcr_event) - sizeof(event_header->event)
> > -		+ event_header->event_size;
> > +	size = sizeof(*event_header) + event_header->event_size;
> 
> That said, I think it would be better to stick to the struct_size()
> idiom for dealing with flexible arrays here:
> 
> 	size = struct_size(event_header, event, event_size);
> 

Yep, I agree. I'll add this and send v2, shortly.

Thanks
--
Gustavo


^ permalink raw reply	[flat|nested] 6+ messages in thread

* Re: [PATCH RESEND] tpm: eventlog: Replace zero-length array with flexible-array member
  2020-05-07  4:09 [PATCH RESEND] tpm: eventlog: Replace zero-length array with flexible-array member Gustavo A. R. Silva
  2020-05-07 18:02 ` Kees Cook
@ 2020-05-13 21:39 ` Jarkko Sakkinen
  2020-05-13 23:14   ` Gustavo A. R. Silva
  1 sibling, 1 reply; 6+ messages in thread
From: Jarkko Sakkinen @ 2020-05-13 21:39 UTC (permalink / raw)
  To: Gustavo A. R. Silva
  Cc: Peter Huewe, Jason Gunthorpe, Arnd Bergmann, Greg Kroah-Hartman,
	linux-integrity, linux-kernel, Gustavo A. R. Silva

On Wed, May 06, 2020 at 11:09:12PM -0500, Gustavo A. R. Silva wrote:
> The current codebase makes use of the zero-length array language
> extension to the C90 standard, but the preferred mechanism to declare
> variable-length types such as these ones is a flexible array member[1][2],
> introduced in C99:
> 
> struct foo {
>         int stuff;
>         struct boo array[];
> };
> 
> By making use of the mechanism above, we will get a compiler warning
> in case the flexible array does not occur last in the structure, which
> will help us prevent some kind of undefined behavior bugs from being
> inadvertently introduced[3] to the codebase from now on.
> 
> Also, notice that, dynamic memory allocations won't be affected by
> this change:
> 
> "Flexible array members have incomplete type, and so the sizeof operator
> may not be applied. As a quirk of the original implementation of
> zero-length arrays, sizeof evaluates to zero."[1]
> 
> sizeof(flexible-array-member) triggers a warning because flexible array
> members have incomplete type[1]. There are some instances of code in
> which the sizeof operator is being incorrectly/erroneously applied to
> zero-length arrays and the result is zero. Such instances may be hiding
> some bugs. So, this work (flexible-array member conversions) will also
> help to get completely rid of those sorts of issues.
> 
> Also, the following issue shows up due to the flexible-array member
> having incomplete type[4]:
> 
> drivers/char/tpm/eventlog/tpm2.c: In function ‘tpm2_bios_measurements_start’:
> drivers/char/tpm/eventlog/tpm2.c:54:46: error: invalid application of ‘sizeof’ to incomplete type ‘u8[]’ {aka ‘unsigned char[]’}
>    54 |  size = sizeof(struct tcg_pcr_event) - sizeof(event_header->event)
>       |                                              ^
> drivers/char/tpm/eventlog/tpm2.c: In function ‘tpm2_bios_measurements_next’:
> drivers/char/tpm/eventlog/tpm2.c:102:10: error: invalid application of ‘sizeof’ to incomplete type ‘u8[]’ {aka ‘unsigned char[]’}
>   102 |    sizeof(event_header->event) + event_header->event_size;
>       |          ^
> drivers/char/tpm/eventlog/tpm2.c: In function ‘tpm2_binary_bios_measurements_show’:
> drivers/char/tpm/eventlog/tpm2.c:140:10: error: invalid application of ‘sizeof’ to incomplete type ‘u8[]’ {aka ‘unsigned char[]’}
>   140 |    sizeof(event_header->event) + event_header->event_size;
>       |          ^
> scripts/Makefile.build:266: recipe for target 'drivers/char/tpm/eventlog/tpm2.o' failed
> make[3]: *** [drivers/char/tpm/eventlog/tpm2.o] Error 1
> 
> As mentioned above: "Flexible array members have incomplete type, and
> so the sizeof operator may not be applied. As a quirk of the original
> implementation of zero-length arrays, sizeof evaluates to zero."[1] So,
> the sizeof(flexible-array) can be safely removed to fix the error above.
> 
> Lastly, prefer sizeof(*ptr) over sizeof(struct foo).
> 
> This issue was found with the help of Coccinelle.
> 
> [1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
> [2] https://github.com/KSPP/linux/issues/21
> [3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")
> [4] https://github.com/KSPP/linux/issues/43
> 
> Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>

Reviewed-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>

/Jarkko

^ permalink raw reply	[flat|nested] 6+ messages in thread

* Re: [PATCH RESEND] tpm: eventlog: Replace zero-length array with flexible-array member
  2020-05-13 21:39 ` Jarkko Sakkinen
@ 2020-05-13 23:14   ` Gustavo A. R. Silva
  2020-05-14 11:05     ` Jarkko Sakkinen
  0 siblings, 1 reply; 6+ messages in thread
From: Gustavo A. R. Silva @ 2020-05-13 23:14 UTC (permalink / raw)
  To: Jarkko Sakkinen
  Cc: Peter Huewe, Jason Gunthorpe, Arnd Bergmann, Greg Kroah-Hartman,
	linux-integrity, linux-kernel, Gustavo A. R. Silva

On Thu, May 14, 2020 at 12:39:05AM +0300, Jarkko Sakkinen wrote:
> > 
> > [1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
> > [2] https://github.com/KSPP/linux/issues/21
> > [3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")
> > [4] https://github.com/KSPP/linux/issues/43
> > 
> > Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
> 
> Reviewed-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
> 

Hi Jarkko,

Thanks for your RB.

There is a v2 of this patch:

https://lore.kernel.org/lkml/20200508163826.GA768@embeddedor/

Thanks
--
Gustavo

^ permalink raw reply	[flat|nested] 6+ messages in thread

* Re: [PATCH RESEND] tpm: eventlog: Replace zero-length array with flexible-array member
  2020-05-13 23:14   ` Gustavo A. R. Silva
@ 2020-05-14 11:05     ` Jarkko Sakkinen
  0 siblings, 0 replies; 6+ messages in thread
From: Jarkko Sakkinen @ 2020-05-14 11:05 UTC (permalink / raw)
  To: Gustavo A. R. Silva
  Cc: Peter Huewe, Jason Gunthorpe, Arnd Bergmann, Greg Kroah-Hartman,
	linux-integrity, linux-kernel, Gustavo A. R. Silva

On Wed, 2020-05-13 at 18:14 -0500, Gustavo A. R. Silva wrote:
> On Thu, May 14, 2020 at 12:39:05AM +0300, Jarkko Sakkinen wrote:
> > > [1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
> > > [2] https://github.com/KSPP/linux/issues/21
> > > [3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")
> > > [4] https://github.com/KSPP/linux/issues/43
> > > 
> > > Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
> > 
> > Reviewed-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
> > 
> 
> Hi Jarkko,
> 
> Thanks for your RB.
> 
> There is a v2 of this patch:
> 
> https://lore.kernel.org/lkml/20200508163826.GA768@embeddedor/
> 
> Thanks
> --
> Gustavo

Yup,

http://git.infradead.org/users/jjs/linux-tpmdd.git/commit/47c18d91f4adb2ca164c8dbee861543b4466167d

Does this look correct?


/Jarkko


^ permalink raw reply	[flat|nested] 6+ messages in thread

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-- links below jump to the message on this page --
2020-05-07  4:09 [PATCH RESEND] tpm: eventlog: Replace zero-length array with flexible-array member Gustavo A. R. Silva
2020-05-07 18:02 ` Kees Cook
2020-05-08 16:06   ` Gustavo A. R. Silva
2020-05-13 21:39 ` Jarkko Sakkinen
2020-05-13 23:14   ` Gustavo A. R. Silva
2020-05-14 11:05     ` Jarkko Sakkinen

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