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

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Date:   Fri, 16 Jun 2023 23:49:26 +0900
From:   Namjae Jeon <linkinjeon@...nel.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Arnd Bergmann <arnd@...nel.org>, Steve French <sfrench@...ba.org>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Tom Talpey <tom@...pey.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>,
        Christian Brauner <brauner@...nel.org>,
        Dave Chinner <dchinner@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Mickaël Salaün <mic@...ikod.net>,
        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] smb: avoid field overflow warning

2023-06-16 23:42 GMT+09:00, Arnd Bergmann <arnd@...db.de>:
> On Fri, Jun 16, 2023, at 16:40, Namjae Jeon wrote:
>> 2023-06-16 18:07 GMT+09:00, Arnd Bergmann <arnd@...nel.org>:
>>> 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>
>>> ---
>>>  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..e3273fa640b07 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, 1);
>> Is there any reason to change copy bytes from 4 bytes to 1 byte?
>>
>
> No, that was an accident, this patch is wrong.
>
> I have to revisit this one and check if my change actually still works
> after I change it back to the correct length.
Okay:)
Thanks for your check!
>
>       Arnd
>