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From: Stefano Brivio <sbrivio@redhat.com>
To: passt-dev@passt.top
Cc: David Gibson <david@gibson.dropbear.id.au>
Subject: [PATCH v4 8/8] util: Don't use errno after a successful call in __daemon()
Date: Tue, 29 Oct 2024 12:28:23 +0100	[thread overview]
Message-ID: <20241029112823.1386613-9-sbrivio@redhat.com> (raw)
In-Reply-To: <20241029112823.1386613-1-sbrivio@redhat.com>

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>
---
 util.c | 15 +++++----------
 1 file changed, 5 insertions(+), 10 deletions(-)

diff --git a/util.c b/util.c
index 1ad3b5c..2ae5f1a 100644
--- a/util.c
+++ b/util.c
@@ -444,16 +444,11 @@ int __daemon(int pidfile_fd, int devnull_fd)
 		exit(EXIT_SUCCESS);
 	}
 
-	errno = 0;
-
-	setsid();
-
-	dup2(devnull_fd, STDIN_FILENO);
-	dup2(devnull_fd, STDOUT_FILENO);
-	dup2(devnull_fd, STDERR_FILENO);
-	close(devnull_fd);
-
-	if (errno)
+	if (setsid()				< 0 ||
+	    dup2(devnull_fd, STDIN_FILENO)	< 0 ||
+	    dup2(devnull_fd, STDOUT_FILENO)	< 0 ||
+	    dup2(devnull_fd, STDERR_FILENO)	< 0 ||
+	    close(devnull_fd))
 		exit(EXIT_FAILURE);
 
 	return 0;
-- 
@@ -444,16 +444,11 @@ int __daemon(int pidfile_fd, int devnull_fd)
 		exit(EXIT_SUCCESS);
 	}
 
-	errno = 0;
-
-	setsid();
-
-	dup2(devnull_fd, STDIN_FILENO);
-	dup2(devnull_fd, STDOUT_FILENO);
-	dup2(devnull_fd, STDERR_FILENO);
-	close(devnull_fd);
-
-	if (errno)
+	if (setsid()				< 0 ||
+	    dup2(devnull_fd, STDIN_FILENO)	< 0 ||
+	    dup2(devnull_fd, STDOUT_FILENO)	< 0 ||
+	    dup2(devnull_fd, STDERR_FILENO)	< 0 ||
+	    close(devnull_fd))
 		exit(EXIT_FAILURE);
 
 	return 0;
-- 
2.43.0


      parent reply	other threads:[~2024-10-29 11:28 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2024-10-29 11:28 [PATCH v4 0/8] Take care of clang-tidy warnings with LLVM >= 16 Stefano Brivio
2024-10-29 11:28 ` [PATCH v4 1/8] Makefile: Exclude qrap.c from clang-tidy checks Stefano Brivio
2024-10-29 11:28 ` [PATCH v4 2/8] treewide: Comply with CERT C rule ERR33-C for snprintf() Stefano Brivio
2024-10-29 11:28 ` [PATCH v4 3/8] treewide: Silence cert-err33-c clang-tidy warnings for fprintf() Stefano Brivio
2024-10-29 11:28 ` [PATCH v4 4/8] Makefile: Disable readability-math-missing-parentheses clang-tidy check Stefano Brivio
2024-10-29 11:28 ` [PATCH v4 5/8] treewide: Suppress clang-tidy warning if we already use O_CLOEXEC Stefano Brivio
2024-10-30  0:34   ` David Gibson
2024-10-29 11:28 ` [PATCH v4 6/8] treewide: Address cert-err33-c clang-tidy warnings for clock and timer functions Stefano Brivio
2024-10-29 11:28 ` [PATCH v4 7/8] udp: Take care of cert-int09-c clang-tidy warning for enum udp_iov_idx Stefano Brivio
2024-10-29 11:28 ` Stefano Brivio [this message]

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