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Message-ID: <ZsdCumxt_DuZl9Rp@voyager>
Date: Thu, 22 Aug 2024 15:52:58 +0200
From: Markus Wichmann <nullplan@....net>
To: musl@...ts.openwall.com
Subject: fcntl: Purpose of second DUPFD_CLOEXEC?
Hi all,
I just stumbled upon the code to emulate F_DUPFD_CLOEXEC on old kernels.
At the moment, it looks like this:
|int ret = __syscall(SYS_fcntl, fd, F_DUPFD_CLOEXEC, arg);
|if (ret != -EINVAL) {
| if (ret >= 0)
| __syscall(SYS_fcntl, ret, F_SETFD, FD_CLOEXEC);
| return __syscall_ret(ret);
|}
|ret = __syscall(SYS_fcntl, fd, F_DUPFD_CLOEXEC, 0);
|if (ret != -EINVAL) {
| if (ret >= 0) __syscall(SYS_close, ret);
| return __syscall_ret(-EINVAL);
|}
|ret = __syscall(SYS_fcntl, fd, F_DUPFD, arg);
|if (ret >= 0) __syscall(SYS_fcntl, ret, F_SETFD, FD_CLOEXEC);
|return __syscall_ret(ret);
And I'm wondering what the point of the second call to F_DUPFD_CLOEXEC
is. If it is just to test that the argument is valid, then why not just
get rid of the block? F_DUPFD has the same constraints on the argument.
Getting rid of that block also has the advantage of being able to factor
out the F_SETFD call. Then it would simply be
int ret = __syscall(SYS_fcntl, fd, F_DUPFD_CLOEXEC, arg);
if (ret == -EINVAL) ret = __syscall(SYS_fcntl, fd, F_DUPFD, arg);
if (ret >= 0) __syscall(SYS_fcntl, fd, F_SETFL, FD_CLOEXEC);
return __syscall_ret(ret);
Much nicer, isn't it? And it doesn't even allocate a file descriptor
uselessly.
Ciao,
Markus
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