[Bug c/97817] New: -Wformat-truncation=2 elicits invalid warning

[Bug c/97817] New: -Wformat-truncation=2 elicits invalid warning

[jim at meyering dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97817

            Bug ID: 97817
           Summary: -Wformat-truncation=2 elicits invalid warning
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jim at meyering dot net
  Target Milestone: ---

Here's the invalid warning.
The buffer's size is obviously not 6. It is AT LEAST 6.

$ gcc -Wformat-truncation=2 -O2 -c strerror_r.c
strerror_r.c: In function ‘strerror_r’:
strerror_r.c:12:35: warning: ‘Unknown error ’ directive output truncated
writing 14 bytes into a region of size 6 [-Wformat-truncation=]
   12 |     snprintf (buf, buflen, "Unknown error %d", errnum);
      |                             ~~~~~~^~~~~~~~
strerror_r.c:12:5: note: ‘snprintf’ output between 16 and 26 bytes into a
destination of size 6
   12 |     snprintf (buf, buflen, "Unknown error %d", errnum);
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Here's the reduced test case (from gnulib's strerror.c):

$ cat strerror_r.c
#define size_t unsigned long long
extern int snprintf (char *__restrict __s, size_t __maxlen,
                     const char *__restrict __format, ...)
  __attribute__ ((__format__ (__printf__, 3, 4)));
extern int __xpg_strerror_r (int errnum, char *buf, size_t buflen);
int strerror_r (int errnum, char *buf, size_t buflen)
{
  if (buflen <= 5)
    return 9;
  int ret = __xpg_strerror_r (errnum, buf, buflen);
  if (ret == 1 && !*buf)
    snprintf (buf, buflen, "Unknown error %d", errnum);
  return ret;
}

[Bug c/97817] -Wformat-truncation=2 elicits invalid warning

[jim at meyering dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97817

--- Comment #1 from jim at meyering dot net ---
I confirmed this happens both with the very latest built from git: gcc version
11.0.0 20201113 (experimental) (GCC), and Fedora 32's gcc version 10.2.1
20201016 (Red Hat 10.2.1-6) (GCC).

[Bug c/97817] -Wformat-truncation=2 elicits invalid warning

[schwab@linux-m68k.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97817

--- Comment #2 from Andreas Schwab <schwab@linux-m68k.org> ---
But when it's 6 it's truncated.

[Bug c/97817] -Wformat-truncation=2 elicits invalid warning

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97817

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |msebor at gcc dot gnu.org
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |INVALID

--- Comment #3 from Martin Sebor <msebor at gcc dot gnu.org> ---
Level 2 of the warning is documented to warn also about calls to bounded
functions whose return value is used and that might result in truncation given
an argument of sufficient length or magnitude.  The level is meant to help
write code with the least likelihood of truncation given unknown arguments.

In the test case, the output of the function will be truncated unless buflen is
at least 16.  It will also be truncated if buflen is 16 and errnum is either
negative or bigger than 9.  The note printed after the warning indicates the
minimum size of output (i.e., 16) and the maximum (26) beyond which truncation
is impossible.

[Bug c/97817] -Wformat-truncation=2 elicits invalid warning

[jim at meyering dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97817

--- Comment #4 from jim at meyering dot net ---
Thanks for explaining. It would be nice if the diagnostic were to say something
along the lines of "... writing into a region whose size may be as low as N".
Given the wording of the current diagnostic, I initially went looking for a
caller whose buffer really did have length 2 (in the original it was 2, not 6).

It's only when I finally noticed that initial "if" block in the implementation
that I understood where the "2" (6 in this example) was coming from.

[Bug c/97817] -Wformat-truncation=2 elicits invalid warning

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97817

--- Comment #5 from Martin Sebor <msebor at gcc dot gnu.org> ---
I agree that the text of the warning could be improved.  I'm hoping to make
changes along the lines you suggest for GCC 12 (it's too late for GCC 11),