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    SubjectRe: [PATCH v2] tpm: eventlog: Replace zero-length array with flexible-array member
    On Mon, May 11, 2020 at 10:24:10PM -0700, Kees Cook wrote:
    > On Fri, May 08, 2020 at 11:38:26AM -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: Kees Cook <keescook@chromium.org>
    >

    Thanks, Kees.

    --
    Gustavo

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