Re: Fail to compile GDB with recent GCC trunk (-Werror=stringop-overflow=, -Werror=stringop-truncation)

[Eric Gallager <egall@gwmail.gwu.edu>]

On 11/20/17, Eric Gallager <egall@gwmail.gwu.edu> wrote:
> On 11/20/17, Yao Qi <qiyaoltc@gmail.com> wrote:
>>
>> Hi,
>> I failed to compile GDB with GCC trunk (8.0.0 20171117) because of some
>> -Werror=stringop-overflow= and -Werror=stringop-truncation warnings.
>> Some of them are not necessary to me,
>>
>> 1. ../../binutils-gdb/gdb/python/py-gdb-readline.c:79:15: error: ‘char*
>> strncpy(char*, const char*, size_t)’ output truncated before terminating
>> nul
>> copying as many bytes from a string as its length
>> [-Werror=stringop-truncation]
>>        strncpy (q, p, n);
>>        ~~~~~~~~^~~~~~~~~
>> ../../binutils-gdb/gdb/python/py-gdb-readline.c:73:14: note: length
>> computed
>> here
>>    n = strlen (p);
>>        ~~~~~~~^~~
>>
>> the code is simple,
>>
>>   n = strlen (p);
>>
>>   /* Copy the line to Python and return.  */
>>   q = (char *) PyMem_RawMalloc (n + 2);
>>   if (q != NULL)
>>     {
>>       strncpy (q, p, n);
>>       q[n] = '\n';
>>       q[n + 1] = '\0';
>>     }
>>
>> I don't see the point of warning here.
>>
>> 2. ../../binutils-gdb/gdb/cp-namespace.c:1071:11: error: ‘char*
>> strncpy(char*, const char*, size_t)’ output truncated before terminating
>> nul
>> copying 2 bytes from a string of the same length
>> [-Werror=stringop-truncation]
>>    strncpy (full_name + scope_length, "::", 2);
>>    ~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>
>>   full_name = (char *) alloca (scope_length + 2 + strlen (name) + 1);
>>   strncpy (full_name, scope, scope_length);
>>   strncpy (full_name + scope_length, "::", 2);
>>   strcpy (full_name + scope_length + 2, name);
>>
>> the code looks right to me,
>>
>> Likewise,
>>
>> ../../../binutils-gdb/gdb/gdbserver/remote-utils.c:1204:14: error: ‘char*
>> strncpy(char*, const char*, size_t)’ output truncated before terminating
>> nul
>> copying 6 bytes from a string of the same length
>> [-Werror=stringop-truncation]
>>       strncpy (buf, "watch:", 6);
>>       ~~~~~~~~^~~~~~~~~~~~~~~~~~
>>
>>             strncpy (buf, "watch:", 6);
>>             buf += 6;
>> ....
>>         *buf = '\0';
>>
>> I can "fix" these warnings by changing GDB code, use strcpy in 1) and
>> use memcpy in 2).  Do we expect all the users of GCC 8 changing their
>> correct code because GCC is not happy on the code?  The warning is too
>> aggressive to me.
>>
>> --
>> Yao (齐尧)
>>
>
> I thought there was a gcc bug open about this but now I can't seem to
> find it; please let me know if you come across the one I was trying to
> remember...
>

Never mind, I found the bug I was looking for (80354), but it was
about -Wformat-truncation, not -Wstringop-truncation, and it's closed,
not open, and the point I made in it was about sprintf vs. snprintf,
not strcpy vs. strncpy (although it applies equally as well in both
cases): https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80354