Re: [PATCH v2] tpm: eventlog: Replace zero-length array with flexible-array member

[Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>]

On Fri, 2020-05-08 at 11:38 -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] As
> in "sizeof(event_header->event) always evaluated to 0, so removing it
> has no effect".
> 
> Lastly, make use of the struct_size() helper to deal with the
> flexible array member and its host structure.
> 
> 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

> ---
> Changes in v2:
>  - Update changelog text.
>  - Make use of the struct_size() helper.
> 
>  drivers/char/tpm/eventlog/tpm2.c | 12 +++++-------
>  include/linux/tpm_eventlog.h     |  2 +-
>  2 files changed, 6 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/char/tpm/eventlog/tpm2.c b/drivers/char/tpm/eventlog/tpm2.c
> index e741b1157525..37a05800980c 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 = struct_size(event_header, event, event_header->event_size);
>  
>  	if (*pos == 0) {
>  		if (addr + size < limit) {
> @@ -98,8 +97,8 @@ 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 = struct_size(event_header, event,
> +					 event_header->event_size);
>  		marker = event_header;
>  	} else {
>  		event = v;
> @@ -136,9 +135,8 @@ 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 = struct_size(event_header, event,
> +				   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 {