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Date:   Fri, 16 Jun 2023 16:42:09 +0200
From:   "Arnd Bergmann" <arnd@arndb.de>
To:     "Namjae Jeon" <linkinjeon@kernel.org>,
        "Arnd Bergmann" <arnd@kernel.org>
Cc:     "Steve French" <sfrench@samba.org>,
        "Sergey Senozhatsky" <senozhatsky@chromium.org>,
        "Tom Talpey" <tom@talpey.com>,
        "Nathan Chancellor" <nathan@kernel.org>,
        "Nick Desaulniers" <ndesaulniers@google.com>,
        "Tom Rix" <trix@redhat.com>,
        "Christian Brauner" <brauner@kernel.org>,
        "Dave Chinner" <dchinner@redhat.com>,
        "Kees Cook" <keescook@chromium.org>,
        =?UTF-8?Q?Micka=C3=ABl_Sala=C3=BCn?= <mic@digikod.net>,
        "Ronnie Sahlberg" <lsahlber@redhat.com>,
        "Hyunchul Lee" <hyc.lee@gmail.com>, linux-cifs@vger.kernel.org,
        linux-kernel@vger.kernel.org, llvm@lists.linux.dev
Subject: Re: [PATCH] smb: avoid field overflow warning
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On Fri, Jun 16, 2023, at 16:40, Namjae Jeon wrote:
> 2023-06-16 18:07 GMT+09:00, Arnd Bergmann <arnd@kernel.org>:
>> From: Arnd Bergmann <arnd@arndb.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@arndb.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