tpm: eventlog: Replace zero-length array with flexible-array member

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 76497732 ("cxgb3/l2t: Fix undefined behaviour")
[4] https://github.com/KSPP/linux/issues/43Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>

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)

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 {