[Bug libc/15410] New: Coverity complains about if (true || true) in bits/stdio2.h

["dank at kegel dot com" <sourceware-bugzilla at sourceware dot org>]

http://sourceware.org/bugzilla/show_bug.cgi?id=15410

             Bug #: 15410
           Summary: Coverity complains about if (true || true) in
                    bits/stdio2.h
           Product: glibc
           Version: 2.18
            Status: NEW
          Severity: normal
          Priority: P2
         Component: libc
        AssignedTo: unassigned@sourceware.org
        ReportedBy: dank@kegel.com
                CC: drepper.fsp@gmail.com
    Classification: Unclassified


This affects user programs, and searching for the warning with google
dredges up at least one instance,
http://lists.eecs.utk.edu/pipermail/perfapi-devel/2012-November/005854.html

To reproduce the problem (assuming you have Coverity), create try.c containing

#include <stdio.h>
int main(int argc, char **argv)
{
    char buf[128];
    fread(buf, 1, 1, stdin);
}

then 

PATH=/opt/coverity/cov-analysis-linux64-6.5.1/bin:$PATH
cov-configure --gcc
mkdir covlog
cov-build --dir covlog gcc -O2 try.c
cov-analyze --dir covlog

This outputs
...
Time taken by Coverity analysis : 00:00:01
Defect occurrences found        : 2 Total
                                  1 CHECKED_RETURN
                                  1 CONSTANT_EXPRESSION_RESULT
and leaves behind the error log

covlog/c/output/CONSTANT_EXPRESSION_RESULT.errors.xml

which says

<file>/usr/include/x86_64-linux-gnu/bits/stdio2.h</file>
<tag>pointless_expression</tag>
<description>{CovLStrv2{{t{The expression {0} does not accomplish anything
becau
se it evaluates to either of its identical operands, {1}.  Did you intend the
op
erands to be different?}{&quot;1 /* !0 */ || 1 /* !0 */&quot;}{&quot;1 /* !0
*/&
quot;}}}}</description>
<line>279</line>

That's

   279          if (!__builtin_constant_p (__size)
   280          || !__builtin_constant_p (__n)
   281          || (__size | __n) >= (((size_t) 1) << (8 * sizeof (size_t) /
2)))
   282        return __fread_chk (__ptr, __bos0 (__ptr), __size, __n,
__stream);

Those lines have remained unchanged in git in libio/bits/stdio2.h for the
last five years, so this applies to both git and deployed versions.

The warning can be suppressed by patching the installed header, (e.g.)
/usr/include/x86_64-linux-gnu/bits/stdio2.h
with

--- a/libio/bits/stdio2.h
+++ b/libio/bits/stdio2.h
@@ -284,6 +284,8 @@ fread (void *__restrict __ptr, size_t __size, size_t __n,
 {
   if (__bos0 (__ptr) != (size_t) -1)
     {
+      /* Suppress coverity warning about "if (true || true || foo)" */
+      /* coverity[pointless_expression] */
       if (!__builtin_constant_p (__size)
       || !__builtin_constant_p (__n)
       || (__size | __n) >= (((size_t) 1) << (8 * sizeof (size_t) / 2)))

It would be nice for Coverity users if glibc added this suppression comment.

Of course, then all the other static analysis tools would want special
treatment, too.

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