Current codes using a brute-force traversal of all file descriptors
do not scale on a system where the maximum number of file descriptors
is set to a very large value (e.g.: in a Docker container of Manjaro
distribution it is set to 1073741816). QEMU just looks frozen during
start-up.
The close-on-exec flag (O_CLOEXEC) was introduced since Linux kernel
2.6.23, FreeBSD 8.3, OpenBSD 5.0, Solaris 11. While it's true QEMU
doesn't need to manually close the fds for child process as the proper
O_CLOEXEC flag should have been set properly on files with its own
codes, QEMU uses a huge number of 3rd party libraries and we don't
trust them to reliably be using O_CLOEXEC on everything they open.
Modern Linux and BSDs have the close_range() call we can use to do the
job, and on Linux we have one more way to walk through /proc/self/fd
to complete the task efficiently, which is what qemu_close_range()
does, a new API we add in util/osdep.c.
V1
link:https://lore.kernel.org/qemu-devel/20230406112041.798585-1-bmeng@tinylab.org/
Changes in v3:
- fix win32 build failure
- limit the last_fd of qemu_close_range() to sysconf(_SC_OPEN_MAX)