[Bug analyzer/113329] New: analyzer: False positive analyzer-fd-use-without-check with dup2()

[Bug analyzer/113329] New: analyzer: False positive analyzer-fd-use-without-check with dup2()

[buczek at molgen dot mpg.de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113329

            Bug ID: 113329
           Summary: analyzer: False positive analyzer-fd-use-without-check
                    with dup2()
           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: ---

Using a closed file descriptor as the second argument of dup2() is a valid
usage. It makes sense, because the implicit close of dup2() doesn't return
close errors to the caller, so an explicit close might be preferred.

** Code:

#include <unistd.h>
void f(int oldfd, int newfd) {
    close(newfd);
    dup2(oldfd, newfd);
}

** Result:

<source>: In function 'f':
<source>:4:5: warning: 'dup2' on possibly invalid file descriptor 'newfd'
[-Wanalyzer-fd-use-without-check]
    4 |     dup2(oldfd, newfd);
      |     ^~~~~~~~~~~~~~~~~~
  'f': events 1-2
    |
    |    3 |     close(newfd);
    |      |     ^~~~~~~~~~~~
    |      |     |
    |      |     (1) closed here
    |    4 |     dup2(oldfd, newfd);
    |      |     ~~~~~~~~~~~~~~~~~~
    |      |     |
    |      |     (2) 'newfd' could be invalid
    |
Compiler returned: 0

https://gcc.godbolt.org/z/8zac19e15

[Bug analyzer/113329] analyzer: False positive analyzer-fd-use-without-check with dup2()

[alan.coopersmith at oracle dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113329

Alan Coopersmith <alan.coopersmith at oracle dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |alan.coopersmith at oracle dot com

--- Comment #1 from Alan Coopersmith <alan.coopersmith at oracle dot com> ---
Created attachment 58057
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=58057&action=edit
dup2.c test case

[Bug analyzer/113329] analyzer: False positive analyzer-fd-use-without-check with dup2()

[alan.coopersmith at oracle dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113329

--- Comment #2 from Alan Coopersmith <alan.coopersmith at oracle dot com> ---
I've hit this as well with gcc 13.2 when building xfs with -fanalyzer.

I extracted this test case from
https://gitlab.freedesktop.org/xorg/app/xfs/-/blob/master/os/daemon.c
and when built with "gcc -fanalyzer -c dup2.c" it reports:

dup2.c: In function ‘DetachStdio’:
dup2.c:23:13: warning: ‘dup2’ on possibly invalid file descriptor ‘0’
[-Wanalyzer-fd-use-without-check]
   23 |         if (dup2(nullfd, 0) == -1) {
      |             ^~~~~~~~~~~~~~~
  ‘DetachStdio’: events 1-6
    |
    |   10 |     close (0);
    |      |     ^~~~~~~~~
    |      |     |
    |      |     (1) closed here
    |......
    |   18 |     if (nullfd == -1) {
    |      |        ~
    |      |        |
    |      |        (2) following ‘false’ branch (when ‘nullfd != -1’)...
    |......
    |   22 |     if (nullfd != 0) {
    |      |        ~
    |      |        |
    |      |        (3) ...to here
    |      |        (4) following ‘true’ branch (when ‘nullfd != 0’)...
    |   23 |         if (dup2(nullfd, 0) == -1) {
    |      |             ~~~~~~~~~~~~~~~
    |      |             |
    |      |             (5) ...to here
    |      |             (6) ‘0’ could be invalid
    |