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(a)redhat.com>
Reviewed-by: David Gibson <david(a)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;
--
2.43.0