Re: [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... [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