so I think to summarize the logic we require:
1) if we can determine that the attacher and attachee occupy the same
mnt ns (/proc/<attacher>/ns/mnt == /proc/<attachee>/ns/mnt), return "/tmp"
2) if they are not in the same mnt ns:
- test the /proc/<attachee>/root/tmp path for readability, if it
is, return that
- if it is not readable (its either secured with elevated privs and
it exists) return "/tmp" which may still fail because they are in fact
not in the same mnt ns
what about /proc/<attachee>/cwd?
- Larry
On 5/3/24 9:43 AM, Sebastian Lövdahl wrote:
Thanks for the patch @larry-cable
<https://urldefense.com/v3/__https://github.com/larry-cable__;!!ACWV5N9M2RV99hQ!J0KoDI4DkLNZ2JvWqbMeoXL5IFCiU97HRtB_b7i_B3cyEAzJPRDxISuGqYNLzapuMPcnSApQyL_CNtLiA5xDccS1BA$>,
much appreciated! I really like this idea.
I tried it out a bit locally. These cases seem to work:
* attaching to a process running on the same host (PID and mount
namespace the same)
* attaching as root from the host to a JVM inside a container
* attaching from a sidecar container to a JVM in another container
Unfortunately, attaching to a JVM process running as the same user in
the same PID and mount namespace but one that has elevated privileges
no longer works (the case that JDK-8226919 fixed). That case ends up
failing like this with the patch:
|slovdahl@ubuntu2204:~/reproducer$
/home/slovdahl/dev/external/jdk/build/linux-x86_64-server-release/images/jdk/bin/jcmd
1751545 VM.version 1751545: java.io.IOException: Unable to access root
directory /proc/1751545/root of target process 1751545 at
jdk.attach/sun.tools.attach.VirtualMachineImpl.findTargetProcessTmpDirectory(VirtualMachineImpl.java:284)
at
jdk.attach/sun.tools.attach.VirtualMachineImpl.findSocketFile(VirtualMachineImpl.java:229)
at
jdk.attach/sun.tools.attach.VirtualMachineImpl.<init>(VirtualMachineImpl.java:86)
at
jdk.attach/sun.tools.attach.AttachProviderImpl.attachVirtualMachine(AttachProviderImpl.java:58)
at
jdk.attach/com.sun.tools.attach.VirtualMachine.attach(VirtualMachine.java:207)
at jdk.jcmd/sun.tools.jcmd.JCmd.executeCommandForPid(JCmd.java:113) at
jdk.jcmd/sun.tools.jcmd.JCmd.main(JCmd.java:97) |
I /think/ it boils down to the same reason as why the fix for
JDK-8226919 was needed in the first place - a non-root user cannot
read the symlinks in |/proc/<pid>/ns| for a process running with more
privileges even though it's run by the same non-root user.
|slovdahl@ubuntu2204:/proc/1751545/ns$ ls -lh ls: cannot read symbolic
link 'net': Permission denied ls: cannot read symbolic link 'uts':
Permission denied ls: cannot read symbolic link 'ipc': Permission
denied ls: cannot read symbolic link 'pid': Permission denied ls:
cannot read symbolic link 'pid_for_children': Permission denied ls:
cannot read symbolic link 'user': Permission denied ls: cannot read
symbolic link 'mnt': Permission denied ls: cannot read symbolic link
'cgroup': Permission denied ls: cannot read symbolic link 'time':
Permission denied ls: cannot read symbolic link 'time_for_children':
Permission denied total 0 lrwxrwxrwx 1 slovdahl slovdahl 0 maj 3 19:28
cgroup lrwxrwxrwx 1 slovdahl slovdahl 0 maj 3 19:28 ipc lrwxrwxrwx 1
slovdahl slovdahl 0 maj 3 19:27 mnt lrwxrwxrwx 1 slovdahl slovdahl 0
maj 3 19:28 net lrwxrwxrwx 1 slovdahl slovdahl 0 maj 3 19:28 pid
lrwxrwxrwx 1 slovdahl slovdahl 0 maj 3 19:28 pid_for_children
lrwxrwxrwx 1 slovdahl slovdahl 0 maj 3 19:28 time lrwxrwxrwx 1
slovdahl slovdahl 0 maj 3 19:28 time_for_children lrwxrwxrwx 1
slovdahl slovdahl 0 maj 3 19:28 user lrwxrwxrwx 1 slovdahl slovdahl 0
maj 3 19:28 uts slovdahl@ubuntu2204:/proc/1751545/ns$ sudo ls -lh
total 0 lrwxrwxrwx 1 slovdahl slovdahl 0 maj 3 19:28 cgroup ->
'cgroup:[4026531835]' lrwxrwxrwx 1 slovdahl slovdahl 0 maj 3 19:28 ipc
-> 'ipc:[4026531839]' lrwxrwxrwx 1 slovdahl slovdahl 0 maj 3 19:27 mnt
-> 'mnt:[4026533862]' lrwxrwxrwx 1 slovdahl slovdahl 0 maj 3 19:28 net
-> 'net:[4026531840]' lrwxrwxrwx 1 slovdahl slovdahl 0 maj 3 19:28 pid
-> 'pid:[4026531836]' lrwxrwxrwx 1 slovdahl slovdahl 0 maj 3 19:28
pid_for_children -> 'pid:[4026531836]' lrwxrwxrwx 1 slovdahl slovdahl
0 maj 3 19:28 time -> 'time:[4026531834]' lrwxrwxrwx 1 slovdahl
slovdahl 0 maj 3 19:28 time_for_children -> 'time:[4026531834]'
lrwxrwxrwx 1 slovdahl slovdahl 0 maj 3 19:28 user ->
'user:[4026531837]' lrwxrwxrwx 1 slovdahl slovdahl 0 maj 3 19:28 uts
-> 'uts:[4026531838]' |
FWIW, my IntelliJ also highlighted the fact that the suggested patch
contains unreachable code. The |else throw new
IOException(String.format("target process:%d and this do not share
common mount namespace for: %s attach failed", pid, tmpdir));| path
can never be taken, because either the |if| statement evaluates to
|true|, or the |else if|.
I'm not sure if we can do any better than always falling back to
|/tmp|? But if anyone has suggestions, I'm certainly happy to try it out.
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