linux-kernel - Re: [PATCH][next] cifs: cifspdu.h: Replace zero-length array with flexible-array member

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Date:   Sat, 7 Mar 2020 14:04:17 -0600
From:   Steve French <smfrench@...il.com>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Cc:     CIFS <linux-cifs@...r.kernel.org>,
        samba-technical <samba-technical@...ts.samba.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH][next] cifs: cifspdu.h: Replace zero-length array with
 flexible-array member

merged into cifs-2.6.git for-next

On Fri, Mar 6, 2020 at 5:01 PM Gustavo A. R. Silva
<gustavo@...eddedor.com> wrote:
>
> The current codebase makes use of the zero-length array language
> extension to the C90 standard, 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.
>
> Also, notice that, dynamic memory allocations won't be affected by
> this change:
>
> "Flexible array members have incomplete type, and so the sizeof operator
> may not be applied. As a quirk of the original implementation of
> zero-length arrays, sizeof evaluates to zero."[1]
>
> This issue was found with the help of Coccinelle.
>
> [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 <gustavo@...eddedor.com>
> ---
>  fs/cifs/cifspdu.h | 18 +++++++++---------
>  1 file changed, 9 insertions(+), 9 deletions(-)
>
> diff --git a/fs/cifs/cifspdu.h b/fs/cifs/cifspdu.h
> index 8e15887d1f1f..593d826820c3 100644
> --- a/fs/cifs/cifspdu.h
> +++ b/fs/cifs/cifspdu.h
> @@ -1021,7 +1021,7 @@ typedef struct smb_com_writex_req {
>         __le16 ByteCount;
>         __u8 Pad;               /* BB check for whether padded to DWORD
>                                    boundary and optimum performance here */
> -       char Data[0];
> +       char Data[];
>  } __attribute__((packed)) WRITEX_REQ;
>
>  typedef struct smb_com_write_req {
> @@ -1041,7 +1041,7 @@ typedef struct smb_com_write_req {
>         __le16 ByteCount;
>         __u8 Pad;               /* BB check for whether padded to DWORD
>                                    boundary and optimum performance here */
> -       char Data[0];
> +       char Data[];
>  } __attribute__((packed)) WRITE_REQ;
>
>  typedef struct smb_com_write_rsp {
> @@ -1306,7 +1306,7 @@ typedef struct smb_com_ntransact_req {
>         /* SetupCount words follow then */
>         __le16 ByteCount;
>         __u8 Pad[3];
> -       __u8 Parms[0];
> +       __u8 Parms[];
>  } __attribute__((packed)) NTRANSACT_REQ;
>
>  typedef struct smb_com_ntransact_rsp {
> @@ -1523,7 +1523,7 @@ struct file_notify_information {
>         __le32 NextEntryOffset;
>         __le32 Action;
>         __le32 FileNameLength;
> -       __u8  FileName[0];
> +       __u8  FileName[];
>  } __attribute__((packed));
>
>  /* For IO_REPARSE_TAG_SYMLINK */
> @@ -1536,7 +1536,7 @@ struct reparse_symlink_data {
>         __le16  PrintNameOffset;
>         __le16  PrintNameLength;
>         __le32  Flags;
> -       char    PathBuffer[0];
> +       char    PathBuffer[];
>  } __attribute__((packed));
>
>  /* Flag above */
> @@ -1553,7 +1553,7 @@ struct reparse_posix_data {
>         __le16  ReparseDataLength;
>         __u16   Reserved;
>         __le64  InodeType; /* LNK, FIFO, CHR etc. */
> -       char    PathBuffer[0];
> +       char    PathBuffer[];
>  } __attribute__((packed));
>
>  struct cifs_quota_data {
> @@ -1762,7 +1762,7 @@ struct set_file_rename {
>         __le32 overwrite;   /* 1 = overwrite dest */
>         __u32 root_fid;   /* zero */
>         __le32 target_name_len;
> -       char  target_name[0];  /* Must be unicode */
> +       char  target_name[];  /* Must be unicode */
>  } __attribute__((packed));
>
>  struct smb_com_transaction2_sfi_req {
> @@ -2451,7 +2451,7 @@ struct cifs_posix_acl { /* access conrol list  (ACL) */
>         __le16  version;
>         __le16  access_entry_count;  /* access ACL - count of entries */
>         __le16  default_entry_count; /* default ACL - count of entries */
> -       struct cifs_posix_ace ace_array[0];
> +       struct cifs_posix_ace ace_array[];
>         /* followed by
>         struct cifs_posix_ace default_ace_arraay[] */
>  } __attribute__((packed));  /* level 0x204 */
> @@ -2757,7 +2757,7 @@ typedef struct file_xattr_info {
>         /* BB do we need another field for flags? BB */
>         __u32 xattr_name_len;
>         __u32 xattr_value_len;
> -       char  xattr_name[0];
> +       char  xattr_name[];
>         /* followed by xattr_value[xattr_value_len], no pad */
>  } __attribute__((packed)) FILE_XATTR_INFO; /* extended attribute info
>                                               level 0x205 */
> --
> 2.25.0
>


-- 
Thanks,

Steve