[SeaBIOS] [PATCH v2] std/tcg: Replace zero-length array with flexible-array member

[Paul Menzel]

Date: Tue, 3 Mar 2020 16:24:46 +0100

GCC 10 gives the warnings below:

In file included from out/ccode32flat.o.tmp.c:54:
    ./src/tcgbios.c: In function 'tpm20_write_EfiSpecIdEventStruct':
    ./src/tcgbios.c:290:30: warning: array subscript '(<unknown>) + 4294967295' is outside the bounds of an interior zero-length array 'struct TCG_EfiSpecIdEventAlgorithmSize[0]' [-Wzero-length-bounds]
      290 |         event.hdr.digestSizes[count].algorithmId = be16_to_cpu(sel->hashAlg);
          |         ~~~~~~~~~~~~~~~~~~~~~^~~~~~~
    In file included from ./src/tcgbios.c:22,
                     from out/ccode32flat.o.tmp.c:54:
    ./src/std/tcg.h:527:7: note: while referencing 'digestSizes'
      527 |     } digestSizes[0];
          |       ^~~~~~~~~~~
    In file included from out/ccode32flat.o.tmp.c:54:
    ./src/tcgbios.c:291:30: warning: array subscript '(<unknown>) + 4294967295' is outside the bounds of an interior zero-length array 'struct TCG_EfiSpecIdEventAlgorithmSize[0]' [-Wzero-length-bounds]
      291 |         event.hdr.digestSizes[count].digestSize = hsize;
          |         ~~~~~~~~~~~~~~~~~~~~~^~~~~~~
    In file included from ./src/tcgbios.c:22,
                     from out/ccode32flat.o.tmp.c:54:
    ./src/std/tcg.h:527:7: note: while referencing 'digestSizes'
      527 |     } digestSizes[0];
          |       ^~~~~~~~~~~

[Description copied from Gustavo A. R. Silva 
from his Linux kernel commits.]

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]

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")
    https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?i...

Signed-off-by: Paul Menzel 
Reviewed-by: Philippe Mathieu-Daudé 
---
v2: Cite all references, add Reviewed-by line

src/std/tcg.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/std/tcg.h b/src/std/tcg.h
index 1cc1c92..1c9eeb4 100644
--- a/src/std/tcg.h
+++ b/src/std/tcg.h
@@ -524,7 +524,7 @@ struct TCG_EfiSpecIdEventStruct {
     struct TCG_EfiSpecIdEventAlgorithmSize {
         u16 algorithmId;
         u16 digestSize;
-    } digestSizes[0];
+    } digestSizes[];
     /*
     u8 vendorInfoSize;
     u8 vendorInfo[0];