linux-kernel - Re: [PATCH] [v2] smb: avoid field overflow warning

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Date:   Mon, 19 Jun 2023 08:26:29 -0400
From:   Tom Talpey <tom@...pey.com>
To:     Arnd Bergmann <arnd@...nel.org>,
        Namjae Jeon <linkinjeon@...nel.org>,
        Steve French <sfrench@...ba.org>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>,
        Christian Brauner <brauner@...nel.org>,
        Ronnie Sahlberg <lsahlber@...hat.com>,
        Hyunchul Lee <hyc.lee@...il.com>, linux-cifs@...r.kernel.org,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH] [v2] smb: avoid field overflow warning

On 6/19/2023 4:19 AM, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@...db.de>
> 
> clang warns about a possible field overflow in a memcpy:
> 
> In file included from fs/smb/server/smb_common.c:7:
> include/linux/fortify-string.h:583:4: error: call to '__write_overflow_field' declared with 'warning' attribute: detected write beyond size of field (1st parameter); maybe use struct_group()? [-Werror,-Wattribute-warning]
>                          __write_overflow_field(p_size_field, size);
> 
> It appears to interpret the "&out[baselen + 4]" as referring to a single
> byte of the character array, while the equivalen "out + baselen + 4" is
> seen as an offset into the array.
> 
> I don't see that kind of warning elsewhere, so just go with the simple
> rework.
> 
> Fixes: e2f34481b24db ("cifsd: add server-side procedures for SMB3")
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ----
> v2: fix typo in array length, and make sure it still addresses the warning
> ---
>   fs/smb/server/smb_common.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/smb/server/smb_common.c b/fs/smb/server/smb_common.c
> index a7e81067bc991..39c6c8d7d0623 100644
> --- a/fs/smb/server/smb_common.c
> +++ b/fs/smb/server/smb_common.c
> @@ -536,7 +536,7 @@ int ksmbd_extract_shortname(struct ksmbd_conn *conn, const char *longname,
>   	out[baselen + 3] = PERIOD;
>   
>   	if (dot_present)
> -		memcpy(&out[baselen + 4], extension, 4);
> +		memcpy(out + baselen + 4, extension, 4);
>   	else
>   		out[baselen + 4] = '\0';
>   	smbConvertToUTF16((__le16 *)shortname, out, PATH_MAX,

It'd be really confusing to have the other two out[baselen + foo] = bar
in the lines above and below.

Can this be fixed more clearly with a new pointer, like
	char *q = &out[baselen + 4];
	memcpy(q, extension, 4);
??

Tom.