Re: [RFC PATCH 6/6] utf8.c: avoid char overflow

From: Beat Bolli <dev+git@drbeat.li>
Date: Jul 9, 2018, 15:45 UTC
Message-ID: <e3df2644b59b170e26b2a7c0d3978331@drbeat.li>
In-Reply-To: <0ceeb342fec1d0868b81cd64941df53c@drbeat.li>

Am 09.07.2018 16:48, schrieb Beat Bolli:

Show 60 quoted lines

> Hi Dscho
> 
> Am 09.07.2018 15:14, schrieb Johannes Schindelin:
>> Hi Beat,
>> 
>> On Sun, 8 Jul 2018, Beat Bolli wrote:
>> 
>>> In ISO C, char constants must be in the range -128..127. Change the 
>>> BOM
>>> constants to unsigned char to avoid overflow.
>>> 
>>> Signed-off-by: Beat Bolli <dev+git@drbeat.li>
>>> ---
>>>  utf8.c | 10 +++++-----
>>>  1 file changed, 5 insertions(+), 5 deletions(-)
>>> 
>>> diff --git a/utf8.c b/utf8.c
>>> index d55e20c641..833ce00617 100644
>>> --- a/utf8.c
>>> +++ b/utf8.c
>>> @@ -561,15 +561,15 @@ char *reencode_string_len(const char *in, int 
>>> insz,
>>>  #endif
>>> 
>>>  static int has_bom_prefix(const char *data, size_t len,
>>> -			  const char *bom, size_t bom_len)
>>> +			  const unsigned char *bom, size_t bom_len)
>>>  {
>>>  	return data && bom && (len >= bom_len) && !memcmp(data, bom, 
>>> bom_len);
>>>  }
>>> 
>>> -static const char utf16_be_bom[] = {0xFE, 0xFF};
>>> -static const char utf16_le_bom[] = {0xFF, 0xFE};
>>> -static const char utf32_be_bom[] = {0x00, 0x00, 0xFE, 0xFF};
>>> -static const char utf32_le_bom[] = {0xFF, 0xFE, 0x00, 0x00};
>>> +static const unsigned char utf16_be_bom[] = {0xFE, 0xFF};
>>> +static const unsigned char utf16_le_bom[] = {0xFF, 0xFE};
>>> +static const unsigned char utf32_be_bom[] = {0x00, 0x00, 0xFE, 
>>> 0xFF};
>>> +static const unsigned char utf32_le_bom[] = {0xFF, 0xFE, 0x00, 
>>> 0x00};
>> 
>> An alternative approach that might be easier to read (and avoids the
>> confusion arising from our use of (signed) chars for strings pretty 
>> much
>> everywhere):
>> 
>> #define FE ((char)0xfe)
>> #define FF ((char)0xff)
>> 
>> ...
> 
> I have tried this first (without the macros, though), and thought it 
> looked
> really ugly. That's why I chose this solution. The usage is pretty 
> local and
> close to function has_bom_prefix().
> 
> Would an explaining comment help?

I have found an even simpler solution. Use proper char literals.

I will put this into v2.

Regards, Beat

diff --git a/utf8.c b/utf8.c
index d55e20c641..982217eec9 100644
--- a/utf8.c
+++ b/utf8.c
@@ -566,10 +566,10 @@ static int has_bom_prefix(const char *data, size_t 
len,
         return data && bom && (len >= bom_len) && !memcmp(data, bom, 
bom_len);
  }

-static const char utf16_be_bom[] = {0xFE, 0xFF};
-static const char utf16_le_bom[] = {0xFF, 0xFE};
-static const char utf32_be_bom[] = {0x00, 0x00, 0xFE, 0xFF};
-static const char utf32_le_bom[] = {0xFF, 0xFE, 0x00, 0x00};
+static const char utf16_be_bom[] = {'\xFE', '\xFF'};
+static const char utf16_le_bom[] = {'\xFF', '\xFE'};
+static const char utf32_be_bom[] = {'\0', '\0', '\xFE', '\xFF'};
+static const char utf32_le_bom[] = {'\xFF', '\xFE', '\0', '\0'};

  int has_prohibited_utf_bom(const char *enc, const char *data, size_t 
len)
  {