lib/stackdepot: Use flexible-array member and struct_size() helper

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
inadvertenly introduced[3] to the codebase from now on.

Lastly, make use of the struct_size() helper to safely calculate the
allocation size for instances of struct n_hdlc_buf and avoid any potential
type mistakes[4][5].

[3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")
[5] commit 553d66cb1e86 ("iommu/vt-d: Use struct_size() helper")

Signed-off-by: Gustavo A. R. Silva <>
1 file changed