Re: Spurious warning for zero-sized array parameters to a function

[Richard Biener <richard.guenther@gmail.com>]

On Tue, Dec 6, 2022 at 5:22 PM Alejandro Colomar via Gcc
<gcc@gcc.gnu.org> wrote:
>
> Hi!
>
> In the following function, past_end is a pointer to one-past-the-end of the
> array.  Holding such a pointer is legal in C.  I use it as a sentinel value that
> helps (1) avoid overrunning the buffer, and (2) detect truncation.  I mark it as
> having a size of [0], which clearly states that it can't be dereferenced (and as
> you can see, I don't).
>
> /*
>   * This function copies an unterminated string into a string.
>   * -  It never overruns the dest buffer.
>   * -  It can be chained, to concatenate strings.
>   * -  It detects truncation.
>   * -  Truncation only needs to be tested once after all concatenations.
>   * -  The name is self-documenting, compared to its alternative: strncat(3).
>   */
> char *
> ustr2stpe(char *dst, const char *restrict src, size_t n, char past_end[0])
> {
>         bool       trunc;
>         char       *end;
>         ptrdiff_t  len;
>
>         if (dst == past_end)
>                 return past_end;
>
>         trunc = false;
>         len = strnlen(src, n);
>         if (len > past_end - dst - 1) {
>                 len = past_end - dst - 1;
>                 trunc = true;
>         }
>
>         end = mempcpy(dst, src, len);
>         *end = '\0';
>
>         return trunc ? past_end : end;
> }
>
>
> If I compile the code above, GCC considers the function definition to be fine.
> However, at call site, it always warns:
>
>
> #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];
>          char *p, *past_end;
>
>         past_end = dest + nitems(dest);
>         p = dest;
>         p = ustr2stpe(p, pre, nitems(pre), past_end);
>         p = ustr2stpe(p, src, strlen(src) - strlen(post), past_end);
>         p = ustr2stpe(p, "", 0, past_end);
>         if (p == past_end)
>                 fprintf(stderr, "truncation\n");
>
>         puts(dest);  // "pre.some-long-body"
> }
>
>
>
> $ cc -Wall -Wextra ustr2stpe.c
> ustr2stpe.c: In function ‘main’:
> ustr2stpe.c:43:13: warning: ‘ustr2stpe’ accessing 1 byte in a region of size 0
> [-Wstringop-overflow=]
>     43 |         p = ustr2stpe(p, pre, nitems(pre), past_end);
>        |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> ustr2stpe.c:43:13: note: referencing argument 4 of type ‘char[0]’
> ustr2stpe.c:10:1: note: in a call to function ‘ustr2stpe’
>     10 | ustr2stpe(char *dst, const char *restrict src, size_t n, char past_end[0])
>        | ^~~~~~~~~
> ustr2stpe.c:44:13: warning: ‘ustr2stpe’ accessing 1 byte in a region of size 0
> [-Wstringop-overflow=]
>     44 |         p = ustr2stpe(p, src, strlen(src) - strlen(post), past_end);
>        |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> ustr2stpe.c:44:13: note: referencing argument 4 of type ‘char[0]’
> ustr2stpe.c:10:1: note: in a call to function ‘ustr2stpe’
>     10 | ustr2stpe(char *dst, const char *restrict src, size_t n, char past_end[0])
>        | ^~~~~~~~~
> ustr2stpe.c:45:13: warning: ‘ustr2stpe’ accessing 1 byte in a region of size 0
> [-Wstringop-overflow=]
>     45 |         p = ustr2stpe(p, "", 0, past_end);
>        |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> ustr2stpe.c:45:13: note: referencing argument 4 of type ‘char[0]’
> ustr2stpe.c:10:1: note: in a call to function ‘ustr2stpe’
>     10 | ustr2stpe(char *dst, const char *restrict src, size_t n, char past_end[0])
>        | ^~~~~~~~~
> ustr2stpe.c:43:13: warning: ‘ustr2stpe’ accessing 1 byte in a region of size 0
> [-Wstringop-overflow=]
>     43 |         p = ustr2stpe(p, pre, nitems(pre), past_end);
>        |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> ustr2stpe.c:43:13: note: referencing argument 4 of type ‘char[0]’
> ustr2stpe.c:10:1: note: in a call to function ‘ustr2stpe’
>     10 | ustr2stpe(char *dst, const char *restrict src, size_t n, char past_end[0])
>        | ^~~~~~~~~
> ustr2stpe.c:44:13: warning: ‘ustr2stpe’ accessing 1 byte in a region of size 0
> [-Wstringop-overflow=]
>     44 |         p = ustr2stpe(p, src, strlen(src) - strlen(post), past_end);
>        |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> ustr2stpe.c:44:13: note: referencing argument 4 of type ‘char[0]’
> ustr2stpe.c:10:1: note: in a call to function ‘ustr2stpe’
>     10 | ustr2stpe(char *dst, const char *restrict src, size_t n, char past_end[0])
>        | ^~~~~~~~~
> ustr2stpe.c:45:13: warning: ‘ustr2stpe’ accessing 1 byte in a region of size 0
> [-Wstringop-overflow=]
>     45 |         p = ustr2stpe(p, "", 0, past_end);
>        |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> ustr2stpe.c:45:13: note: referencing argument 4 of type ‘char[0]’
> ustr2stpe.c:10:1: note: in a call to function ‘ustr2stpe’
>     10 | ustr2stpe(char *dst, const char *restrict src, size_t n, char past_end[0])
>        | ^~~~~~~~~
>
>
> The warnings are invalid.  While it's true that I'm referencing a pointer of
> size 0, it's false that I'm "accessing 1 byte" in that region.  I guess this is
> all about the bogus design of 'static' in ISO C, where you can have an array
> parameter of size 0, which is very useful in cases like this one.

It looks like we run into pass_waccess::maybe_check_access_sizes doing

      if (sizidx == -1)
        {
          /* If only the pointer attribute operand was specified and
             not size, set SIZE to the greater of MINSIZE or size of
             one element of the pointed to type to detect smaller
             objects (null pointers are diagnosed in this case only
             if the pointer is also declared with attribute nonnull.  */
          if (access.second.minsize
              && access.second.minsize != HOST_WIDE_INT_M1U)
            access_nelts = build_int_cstu (sizetype, access.second.minsize);
          else if (VOID_TYPE_P (argtype) && access.second.mode == access_none)
            /* Treat access mode none on a void* argument as expecting
               as little as zero bytes.  */
            access_nelts = size_zero_node;
          else
            access_nelts = size_one_node;

and use size_one_node as fallback - it either doesn't consider [0] "valid" or
for some reason chooses to interpret it as "unknown".  Can you file a bugreport
please?

Martin?

Richard.

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