linux-kernel - Re: [PATCH v2] ACPICA: Replace one-element array with flexible-array

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Message-ID: <CAJZ5v0hd87G055A+ezkyF+UJQAeBvozFCvoC2Fxa1o-5YXrEhg@mail.gmail.com>
Date:   Wed, 3 Jun 2020 15:04:22 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Erik Kaneda <erik.kaneda@...el.com>
Cc:     Robert Moore <robert.moore@...el.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Len Brown <lenb@...nel.org>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        "open list:ACPI COMPONENT ARCHITECTURE (ACPICA)" <devel@...ica.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: Re: [PATCH v2] ACPICA: Replace one-element array with flexible-array

On Tue, Jun 2, 2020 at 11:34 PM Gustavo A. R. Silva
<gustavoars@...nel.org> wrote:
>
> The current codebase makes use of one-element arrays in the following
> form:
>
> 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);
>
> 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.
>
> This issue was found with the help of Coccinelle and audited _manually_.
>
> [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")
>
> Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>

Erik, can you take this to the upstream, please?

> ---
> Changes in v2:
>  - Don't use struct_size() for now.
>  - Update subject line and changelog text.
>
>  drivers/acpi/acpica/utids.c | 2 +-
>  include/acpi/actypes.h      | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/acpi/acpica/utids.c b/drivers/acpi/acpica/utids.c
> index 3bb06935a2ad3..225f3c60203c7 100644
> --- a/drivers/acpi/acpica/utids.c
> +++ b/drivers/acpi/acpica/utids.c
> @@ -263,7 +263,7 @@ acpi_ut_execute_CID(struct acpi_namespace_node *device_node,
>          * 3) Size of the actual CID strings
>          */
>         cid_list_size = sizeof(struct acpi_pnp_device_id_list) +
> -           ((count - 1) * sizeof(struct acpi_pnp_device_id)) +
> +           count * sizeof(struct acpi_pnp_device_id) +
>             string_area_size;
>
>         cid_list = ACPI_ALLOCATE_ZEROED(cid_list_size);
> diff --git a/include/acpi/actypes.h b/include/acpi/actypes.h
> index 4defed58ea338..c7bcda0ad366a 100644
> --- a/include/acpi/actypes.h
> +++ b/include/acpi/actypes.h
> @@ -1145,7 +1145,7 @@ struct acpi_pnp_device_id {
>  struct acpi_pnp_device_id_list {
>         u32 count;              /* Number of IDs in Ids array */
>         u32 list_size;          /* Size of list, including ID strings */
> -       struct acpi_pnp_device_id ids[1];       /* ID array */
> +       struct acpi_pnp_device_id ids[];        /* ID array */
>  };
>
>  /*
> --
> 2.27.0
>