Re: [-Wstringop-overflow=] strncat(3)

[Alejandro Colomar <alx.manpages@gmail.com>]

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Hi Andrew,

On 12/14/22 23:57, Andrew Pinski wrote:
> On Wed, Dec 14, 2022 at 2:46 PM Alejandro Colomar via Libc-alpha
> <libc-alpha@sourceware.org> wrote:
>>
>> Hi,
>>
>> I was rewriting the strncat(3) manual page, and when I tried to compile the
>> example program, I got a surprise from the compiler.
>>
>> Here goes the page:
>>
>>
>>     strncat(3)               Library Functions Manual              strncat(3)
>>
>>     NAME
>>            strncat  -  concatenate  a  null‐padded  character sequence into a
>>            string
>>
>>     LIBRARY
>>            Standard C library (libc, -lc)
>>
>>     SYNOPSIS
>>            #include <string.h>
>>
>>            char *strncat(char *restrict dst, const char src[restrict .sz],
>>                           size_t sz);
>>
>>     DESCRIPTION
>>            This function catenates the input character sequence contained  in
>>            a  null‐padded  fixed‐width  buffer,  into  a string at the buffer
>>            pointed to by dst.  The programmer is responsible for allocating a
>>            buffer large enough, that is, strlen(dst) + strnlen(src, sz) + 1.
>>
>>            An implementation of this function might be:
>>
>>                char *
>>                strncat(char *restrict dst, const char *restrict src, size_t sz)
>>                {
>>                    int   len;
>>                    char  *end;
>>
>>                    len = strnlen(src, sz);
>>                    end = dst + strlen(dst);
>>                    end = mempcpy(end, src, len);
>>                    *end = '\0';
>>
>>                    return dst;
>>                }
>>
>>     RETURN VALUE
>>            strncat() returns dest.
>>
>>     ATTRIBUTES
>>            [...]
>>
>>     STANDARDS
>>            POSIX.1‐2001, POSIX.1‐2008, C89, C99, SVr4, 4.3BSD.
>>
>>     CAVEATS
>>            The  name of this function is confusing.  This function has no re‐
>>            lation with strncpy(3).
>>
>>            If the destination buffer is not large enough, the behavior is un‐
>>            defined.  See _FORTIFY_SOURCE in feature_test_macros(7).
>>
>>     BUGS
>>            This function  can  be  very  inefficient.   Read  about  Shlemiel
>>            the       painter      ⟨https://www.joelonsoftware.com/2001/12/11/
>>            back-to-basics/⟩.
>>
>>     EXAMPLES
>>            #include <stdio.h>
>>            #include <stdlib.h>
>>            #include <string.h>
>>
>>            int
>>            main(void)
>>            {
>>                char    buf[BUFSIZ];
>>                size_t  len;
>>
>>                buf[0] = '\0';  // There’s no ’cpy’ function to this ’cat’.
>>                strncat(buf, "Hello ", 6);
>>                strncat(buf, "world", 42);  // Padding null bytes ignored.
>>                strncat(buf, "!", 1);
>>                len = strlen(buf);
>>                printf("[len = %zu]: <%s>\n", len, buf);
>>
>>                exit(EXIT_SUCCESS);
>>            }
>>
>>     SEE ALSO
>>            string(3), string_copy(3)
>>
>>     Linux man‐pages (unreleased)      (date)                       strncat(3)
>>
>>
>> And when you compile that, you get:
>>
>> $ cc -Wall -Wextra ./strncat.c
>> ./strncat.c: In function ‘main’:
>> ./strncat.c:12:12: warning: ‘strncat’ specified bound 6 equals source length
>> [-Wstringop-overflow=]
>>      12 |            strncat(buf, "Hello ", 6);
>>         |            ^~~~~~~~~~~~~~~~~~~~~~~~~
>> ./strncat.c:14:12: warning: ‘strncat’ specified bound 1 equals source length
>> [-Wstringop-overflow=]
>>      14 |            strncat(buf, "!", 1);
>>         |            ^~~~~~~~~~~~~~~~~~~~
>>
>>
>> So, what?  Where's the problem?  This function does exactly that: "take an
>> unterminated character sequence and catenate it to an existing string".  Clang
>> seems to be fine with the code.
> 
> See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83404 and the
> background of why the warning was added here:
> 
> https://www.us-cert.gov/bsi/articles/knowledge/coding-practices/strncpy-and-strncat.

This document is bogus, since it's puting strncpy(3) and strncat(3) in the same 
sack, when they're in reality two completely different beasts.  I'll quote below 
some paragraphs of some new page I'm writing, which will show why.

The rationale behind GCC's warning is also fundamentally wrong.  Martin was 
wrong when he claimed that the right call for strncat(3) is the remaining space 
in the destination.

I admit that I didn't know what strncat(3) was useful for, and believed that it 
was simply a broken-by-design function until very recently (this week, more or 
less).  And to be honest, I still believe it's broken by design; it's just that 
it can be repurposed for a reasonable new purpose (which I found while digging 
in groff's source code; that's why the CC).


First I'll show an example program that I added to the strncat(3) manual page 
last week, which is based on the groff code that used it:


        #include <stdio.h>
        #include <stdlib.h>
        #include <string.h>

        #define nitems(arr)  (sizeof((arr)) / sizeof((arr)[0]))

        int
        main(void)
        {
            char pre[4] = "pre.";
            char *post = ".post";
            char *src = "some_long_body.post";
            char dest[100];

            dest[0] = '\0';
            strncat(dest, pre, nitems(pre));
            strncat(dest, src, strlen(src) - strlen(post));

            puts(dest);  // "pre.some_long_body"
            exit(EXIT_SUCCESS);
        }


And now I'll quote some text that I'm writing currently for the function:


    Null‐padded character sequences
        For historic reasons, some standard APIs, such as utmpx(5),  use  null‐
        padded  character  sequences in fixed‐width buffers.  To interface with
        them, specialized functions need to be used.

        To copy strings into them, use stpncpy(3).

        To copy from an unterminated string within a fixed‐width buffer into  a
        string,  ignoring  any  trailing  null  bytes in the source fixed‐width
        buffer, you should use strncat(3).

        [...]

        stpncpy(3)
               This function copies the input string  into  a  destination
               null‐padded character sequence in a fixed‐width buffer.  If
               the  destination  buffer,  limited by its size, isn’t large
               enough to hold the copy, the resulting  character  sequence
               is  truncated.   Since  it creates a character sequence, it
               doesn’t need to write a terminating null byte.  It’s impos‐
               sible to distinguish truncation  after  the  call,  from  a
               character  sequence  that just fits the destination buffer;
               truncation should be detected from the length of the origi‐
               nal string.

        strncpy(3)
               This function is identical to  stpncpy(3)  except  for  the
               useless return value.

               stpncpy(3) is a simpler alternative to this function.

        [...]

        strncat(3)
               Do not confuse this function with strncpy(3); they are  not
               related at all.

               This  function  catenates the input character sequence con‐
               tained in a null‐padded wixed‐width buffer, into a destina‐
               tion string.  The programmer is responsible for  allocating
               a buffer large enough.  The return value is useless.

               zustr2stp(3) is a faster alternative to this function.

               An implementation of this function might be:

                   char *
                   strncat(char *restrict dst, const char *restrict src,
                           size_t sz)
                   {
                       int   len;
                       char  *end;

                       len = strnlen(src, sz);
                       end = dst + strlen(dst);
                       end = mempcpy(end, src, len);
                       *end = '\0';

                       return dst;
                   }


Cheers,

Alex



> 
> Thanks,
> Andrew Pinski
> 
>>
>> Cheers,
>>
>> Alex
>>
>>
>> --
>> <http://www.alejandro-colomar.es/>

-- 
<http://www.alejandro-colomar.es/>

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