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util: Don't use errno after a successful call in __daemon()

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I thought we could just set errno to 0, do a bunch of stuff, and check
that errno didn't change to infer we succeeded. But clang-tidy,
starting with LLVM 19, reports:

/home/sbrivio/passt/util.c:465:6: error: An undefined value may be read from 'errno' [clang-analyzer-unix.Errno,-warnings-as-errors]
  465 |         if (errno)
      |             ^
/usr/include/errno.h:38:16: note: expanded from macro 'errno'
   38 | # define errno (*__errno_location ())
      |                ^~~~~~~~~~~~~~~~~~~~~~
/home/sbrivio/passt/util.c:446:6: note: Assuming the condition is false
  446 |         if (pid == -1) {
      |             ^~~~~~~~~
/home/sbrivio/passt/util.c:446:2: note: Taking false branch
  446 |         if (pid == -1) {
      |         ^
/home/sbrivio/passt/util.c:451:6: note: Assuming 'pid' is 0
  451 |         if (pid) {
      |             ^~~
/home/sbrivio/passt/util.c:451:2: note: Taking false branch
  451 |         if (pid) {
      |         ^
/home/sbrivio/passt/util.c:463:2: note: Assuming that 'close' is successful; 'errno' becomes undefined after the call
  463 |         close(devnull_fd);
      |         ^~~~~~~~~~~~~~~~~
/home/sbrivio/passt/util.c:465:6: note: An undefined value may be read from 'errno'
  465 |         if (errno)
      |             ^
/usr/include/errno.h:38:16: note: expanded from macro 'errno'
   38 | # define errno (*__errno_location ())
      |                ^~~~~~~~~~~~~~~~~~~~~~

And the LLVM documentation for the unix.Errno checker, 1.1.8.3
unix.Errno (C), mentions, at:

  https://clang.llvm.org/docs/analyzer/checkers.html#unix-errno

that:

  The C and POSIX standards often do not define if a standard library
  function may change value of errno if the call does not fail.
  Therefore, errno should only be used if it is known from the return
  value of a function that the call has failed.

which is, somewhat surprisingly, the case for close().

Instead of using errno, check the actual return values of the calls
we issue here.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
This commit is contained in:
Stefano Brivio 2024-10-25 00:57:58 +02:00
parent b1a607fba1
commit ee7d0b62a7
1 changed files with 5 additions and 10 deletions
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15
util.c
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@ -444,16 +444,11 @@ int __daemon(int pidfile_fd, int devnull_fd)
exit(EXIT_SUCCESS); exit(EXIT_SUCCESS);
} }
errno = 0; if (setsid() < 0 ||
dup2(devnull_fd, STDIN_FILENO) < 0 ||
setsid(); dup2(devnull_fd, STDOUT_FILENO) < 0 ||
dup2(devnull_fd, STDERR_FILENO) < 0 ||
dup2(devnull_fd, STDIN_FILENO); close(devnull_fd))
dup2(devnull_fd, STDOUT_FILENO);
dup2(devnull_fd, STDERR_FILENO);
close(devnull_fd);
if (errno)
exit(EXIT_FAILURE); exit(EXIT_FAILURE);
return 0; return 0;