tree 76aa3289ca342b5bae7e3b838afbed954eacd490
parent bfdb3f86e9116fc79ce63c231373b084aad11218
author Paul Menzel <pmenzel@molgen.mpg.de> 1583329887 +0100
committer Kevin O'Connor <kevin@koconnor.net> 1584747083 -0400

std/tcg: Replace zero-length array with flexible-array member

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 <gustavo@embeddedor.com>
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/?id=76497732932f15e7323dc805e8ea8dc11bb587cf

Signed-off-by: Paul Menzel <pmenzel@molgen.mpg.de>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>