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SubjectRe: [PATCH AUTOSEL 5.5 451/542] tty: n_hdlc: Use flexible-array member and struct_size() helper
On Fri, Feb 14, 2020 at 10:47:23AM -0500, Sasha Levin wrote:
> From: "Gustavo A. R. Silva" <gustavo@embeddedor.com>
>
> [ Upstream commit 85f4c95172d606dd66f7ee1fa50c45a245535ffd ]
>
> 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].
>
> [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")
> [4] https://lore.kernel.org/lkml/60e14fb7-8596-e21c-f4be-546ce39e7bdb@embeddedor.com/
> [5] commit 553d66cb1e86 ("iommu/vt-d: Use struct_size() helper")
>
> Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
> Reviewed-by: Jiri Slaby <jslaby@suse.cz>
> Link: https://lore.kernel.org/r/20200121172138.GA3162@embeddedor
> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Signed-off-by: Sasha Levin <sashal@kernel.org>
> ---
> drivers/tty/n_hdlc.c | 11 +++++------
> 1 file changed, 5 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/tty/n_hdlc.c b/drivers/tty/n_hdlc.c
> index 98361acd3053f..27b506bf03ced 100644
> --- a/drivers/tty/n_hdlc.c
> +++ b/drivers/tty/n_hdlc.c
> @@ -115,11 +115,9 @@
> struct n_hdlc_buf {
> struct list_head list_item;
> int count;
> - char buf[1];
> + char buf[];
> };
>
> -#define N_HDLC_BUF_SIZE (sizeof(struct n_hdlc_buf) + maxframe)
> -
> struct n_hdlc_buf_list {
> struct list_head list;
> int count;
> @@ -524,7 +522,8 @@ static void n_hdlc_tty_receive(struct tty_struct *tty, const __u8 *data,
> /* no buffers in free list, attempt to allocate another rx buffer */
> /* unless the maximum count has been reached */
> if (n_hdlc->rx_buf_list.count < MAX_RX_BUF_COUNT)
> - buf = kmalloc(N_HDLC_BUF_SIZE, GFP_ATOMIC);
> + buf = kmalloc(struct_size(buf, buf, maxframe),
> + GFP_ATOMIC);
> }
>
> if (!buf) {
> @@ -853,7 +852,7 @@ static struct n_hdlc *n_hdlc_alloc(void)
>
> /* allocate free rx buffer list */
> for(i=0;i<DEFAULT_RX_BUF_COUNT;i++) {
> - buf = kmalloc(N_HDLC_BUF_SIZE, GFP_KERNEL);
> + buf = kmalloc(struct_size(buf, buf, maxframe), GFP_KERNEL);
> if (buf)
> n_hdlc_buf_put(&n_hdlc->rx_free_buf_list,buf);
> else if (debuglevel >= DEBUG_LEVEL_INFO)
> @@ -862,7 +861,7 @@ static struct n_hdlc *n_hdlc_alloc(void)
>
> /* allocate free tx buffer list */
> for(i=0;i<DEFAULT_TX_BUF_COUNT;i++) {
> - buf = kmalloc(N_HDLC_BUF_SIZE, GFP_KERNEL);
> + buf = kmalloc(struct_size(buf, buf, maxframe), GFP_KERNEL);
> if (buf)
> n_hdlc_buf_put(&n_hdlc->tx_free_buf_list,buf);
> else if (debuglevel >= DEBUG_LEVEL_INFO)
> --
> 2.20.1
>

There's no need for any of these variable length array patches to be
backported anywhere.

thanks,

greg k-h

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