char: hpet: Use flexible-array member
authorGustavo A. R. Silva <gustavo@embeddedor.com>
Mon, 20 Jan 2020 23:53:26 +0000 (17:53 -0600)
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Thu, 23 Jan 2020 18:54:26 +0000 (19:54 +0100)
commit987f028b8637cfa7658aa456ae73f8f21a7a7f6f
tree5d54f7aca3a7dc44b3efe82280e60c12215b0161
parenteb143f8756e77c8fcfc4d574922ae9efd3a43ca9
char: hpet: Use flexible-array member

Old code in the kernel uses 1-byte and 0-byte arrays to indicate the
presence of a "variable length array":

struct something {
    int length;
    u8 data[1];
};

struct something *instance;

instance = kmalloc(sizeof(*instance) + size, GFP_KERNEL);
instance->length = size;
memcpy(instance->data, source, size);

There is also 0-byte arrays. Both cases pose confusion for things like
sizeof(), CONFIG_FORTIFY_SOURCE, etc.[1] Instead, the preferred mechanism
to declare variable-length types such as the one above is a flexible array
member[2] which need to be the last member of a structure and empty-sized:

struct something {
        int stuff;
        u8 data[];
};

Also, 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
unadvertenly introduced[3] to the codebase from now on.

[1] https://github.com/KSPP/linux/issues/21
[2] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Link: https://lore.kernel.org/r/20200120235326.GA29231@embeddedor.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
drivers/char/hpet.c