Thanks for the quick response!
On 02/23/2010 03:32 PM, Eric Blake wrote:
According to Laine Stump on 2/23/2010 12:54 PM:
Minor nits (I have not validated the overall patch in context; this is
just from a quick high-level review)
If qemudDomainRestore fails to open the domain save file, create a
pipe, then fork a process that does setuid(qemu_user) and opens the
file, then reads this file and stuffs it into the pipe. the parnet
s/. the parnet/. The parent/
libvirtd process will use the other end of the pipe as its fd, then
reap the child process after its done reading.
s/its/it's/
Ah, yes. Those terrible typos that never seem worth the effort to fix
after they're already checked in. Thanks for nipping it in the bud!
This makes domain restore work on a root-squash NFS share that is only
visible to the qemu user.
---
src/qemu/qemu_driver.c | 158 +++++++++++++++++++++++++++++++++++++++++++++++-
1 files changed, 155 insertions(+), 3 deletions(-)
diff --git a/src/qemu/qemu_driver.c b/src/qemu/qemu_driver.c
index ac6ef6a..f909a59 100644
--- a/src/qemu/qemu_driver.c
+++ b/src/qemu/qemu_driver.c
@@ -4659,6 +4659,135 @@ cleanup:
return ret;
}
+/* qemudOpenAsUID() - pipe/fork/setuid/open a file, and return the
+ pipe fd to caller, so that it can read from the file. Also return
+ the pid of the child process, so the caller can wait for it to exit
+ after it's finished reading (to avoid a zombie, if nothing
+ else). */
+
+static int qemudOpenAsUID(const char *path, uid_t uid, pid_t *childpid) {
+ int pipefd[2] = {-1, -1};
Why the explicit initialization...
Mostly because I was following the example of util.c:__virExec(), which
does the same thing. Following examples seems to get me into nearly as
much trouble as ignoring them ;-)
+ int fd = -1;
+
+ *childpid = -1;
+
+ if (pipe(pipefd)< 0) {
...since I see nothing in POSIX that guarantees that pipe() will preserve
your initialized values across failure. I think it's better to explicitly
set things to -1 on the failure path, rather than initializing that way.
Interesting point. If we can't rely on that, then __virExec() should be
changed too. Of course this makes maintenance a bit more error prone,
since we would have to make sure that *every* jump to the cleanup that
happened prior to creating the pipe would set the pipe fds to -1 (or
just leave the initialization in, and re-set them to -1 when pipe
failed; that's probably the safest thing to do).
+ virReportSystemError(errno,
+ _("failed to create pipe to read '%s'"),
+ path);
+ goto parentcleanup;
+ }
+
+ int forkRet = virFork(childpid);
Unrelated to your patch, but we should probably start using pipe2 and
atomically marking pipes as CLOEXEC, so that use of libvirt in a
multithreaded environment does not have a race window where unrelated
threads can leak libvirt's fds across an exec (well, for new enough
kernels anyway; at least gnulib has a pipe2 fallback for older kernels).
...
I think I just heard someone volunteering! ;-)
+ /* child */
+
+ /* setuid to the qemu user, then open the file, read it,
+ and stuff it into the pipe for the parent process to
+ read */
+ int exit_code;
+ char *buf = NULL;
+ int bufsize = 1024 * 1024;
+ int bytesread;
+
+ /* child doesn't need the read side of the pipe */
+ close(pipefd[0]);
+
+ if (forkRet< 0) {
+ exit_code = errno;
+ virReportSystemError(errno,
+ _("failed in child after forking to read '%s'"),
+ path);
+ goto childcleanup;
+ }
+
+ if (setuid(uid) != 0) {
+ exit_code = errno;
+ virReportSystemError(errno,
+ _("cannot setuid(%d) to read '%s'"),
+ getuid(), path);
Is that the right uid to be reporting in the error message?
What's the proper emoticon for blushing in embarrassment? Bonehead typo
;-) Thanks for pointing it out.
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