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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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