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

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Message-Id: <6742b2ce-02f1-43c1-811f-de7fd0a74f6a@app.fastmail.com>
Date:   Fri, 16 Jun 2023 16:42:09 +0200
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "Namjae Jeon" <linkinjeon@...nel.org>,
        "Arnd Bergmann" <arnd@...nel.org>
Cc:     "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

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.

      Arnd