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From: "buczek at molgen dot mpg.de" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug analyzer/113333] New: analyzer: False positives with calloc()
Date: Thu, 11 Jan 2024 10:01:31 +0000
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X-Bugzilla-Component: analyzer
X-Bugzilla-Version: 14.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D113333

            Bug ID: 113333
           Summary: analyzer: False positives with calloc()
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: analyzer
          Assignee: dmalcolm at gcc dot gnu.org
          Reporter: buczek at molgen dot mpg.de
  Target Milestone: ---

Analyzer assumen that a pointer allocated by calloc() can be !=3D NULL.


** Code:

#include <stdlib.h>
char **f(void) {
    char **vec =3D calloc(1, sizeof(char *));
    if (vec)
        for (char **p=3Dvec ; *p ; p++);
    return vec;
}

** Result:

<source>: In function 'f':
<source>:5:29: warning: heap-based buffer over-read [CWE-126]
[-Wanalyzer-out-of-bounds]
    5 |         for (char **p=3Dvec ; *p ; p++);
      |                             ^~
  'f': events 1-6
    |
    |    3 |     char **vec =3D calloc(1, sizeof(char *));
    |      |                  ^~~~~~~~~~~~~~~~~~~~~~~~~
    |      |                  |
    |      |                  (1) capacity: 8 bytes
    |    4 |     if (vec)
    |      |        ~=20=20=20=20=20=20=20=20=20=20
    |      |        |
    |      |        (2) following 'true' branch (when 'vec' is non-NULL)...
    |    5 |         for (char **p=3Dvec ; *p ; p++);
    |      |                     ~       ~~   ~~~
    |      |                     |       |     |
    |      |                     |       |     (5) ...to here
    |      |                     |       (4) following 'true' branch...
    |      |                     |       (6) out-of-bounds read from byte 8
till byte 15 but region ends at byte 8
    |      |                     (3) ...to here
    |
<source>:5:29: note: read of 8 bytes from after the end of the region
    5 |         for (char **p=3Dvec ; *p ; p++);
      |                             ^~
<source>:5:29: warning: use of uninitialized value '*p' [CWE-457]
[-Wanalyzer-use-of-uninitialized-value]
  'f': events 1-6
    |
    |    3 |     char **vec =3D calloc(1, sizeof(char *));
    |      |                  ^~~~~~~~~~~~~~~~~~~~~~~~~
    |      |                  |
    |      |                  (1) region created on heap here
    |    4 |     if (vec)
    |      |        ~=20=20=20=20=20=20=20=20=20=20
    |      |        |
    |      |        (2) following 'true' branch (when 'vec' is non-NULL)...
    |    5 |         for (char **p=3Dvec ; *p ; p++);
    |      |                     ~       ~~   ~~~
    |      |                     |       |     |
    |      |                     |       |     (5) ...to here
    |      |                     |       (4) following 'true' branch...
    |      |                     |       (6) use of uninitialized value '*p'
here
    |      |                     (3) ...to here
    |
Compiler returned: 0

https://gcc.godbolt.org/z/h6bPeYc3T=