Re: [PATCHv3] gdb: linux-namespaces: enter user namespace when appropriate

[Andrew Burgess <aburgess@redhat.com>]

Tom de Vries <tdevries@suse.de> writes:

> On 6/23/25 15:56, Andrew Burgess wrote:
>> Andrew Burgess <aburgess@redhat.com> writes:
>> 
>>> From: Benjamin Berg <benjamin@sipsolutions.net>
>>>
>>> In v2:
>>>
>>>    - Update the test to ignore a warning seen when running the test on
>>>      a machine with libc debug information installed, but without the
>>>      libc source being available, e.g.:
>>>
>>>      warning: 46     ../sysdeps/unix/sysv/linux/arm/libc-do-syscall.S: No such file or directory
>>>
>>>      This was causing some CI failures to be reported from Linaro.
>>>
>>>    - Rebased to current upstream/master.
>>>
>>> In v3:
>>>
>>>    - Same as V2, but fix the test pattern correctly this time.
>>>
>>> --
>>>
>>> The use of user namespaces is required for normal users to use mount
>>> namespaces.  Consider trying this as an unprivileged user:
>>>
>>>    $ unshare --mount /bin/true
>>>    unshare: unshare failed: Operation not permitted
>>>
>>> The problem here is that an unprivileged user doesn't have the
>>> required permissions to create a new mount namespace.  If, instead, we
>>> do this:
>>>
>>>    $ unshare --mount --map-root-user /bin/true
>>>
>>> then this will succeed.  The new option causes unshare to create a
>>> user namespace in which the unprivileged user is mapped to UID/GID 0,
>>> and so gains all privileges (inside the namespace), the user is then
>>> able to create the mount namespace as required.
>>>
>>> So, how does this relate to GDB?
>>>
>>> When a user attaches to a process running in a separate mount
>>> namespace, GDB makes use of a separate helper process (see
>>> linux_mntns_get_helper in nat/linux-namespaces.c), which will then use
>>> the `setns` function to enter (or try to enter) the mount namespace of
>>> the process GDB is attaching too.  The helper process will then handle
>>> file I/O requests received from GDB, and return the results back to
>>> GDB, this allows GDB to access files within the mount namespace.
>>>
>>> The problem here is that, switching to a mount namespace requires that
>>> a process hold CAP_SYS_CHROOT and CAP_SYS_ADMIN capabilities within
>>> its user namespace (actually it's a little more complex, see 'man 2
>>> setns').  Assuming GDB is running as an unprivileged user, then GDB
>>> will not have the required permissions.
>>>
>>> However, if GDB enters the user namespace that the `unshare` process
>>> created, then the current user will be mapped to UID/GID 0, and will
>>> have the required permissions.
>>>
>>> And so, this patch extends linux_mntns_access_fs (in
>>> nat/linux-namespace.c) to first try and switch to the user namespace
>>> of the inferior before trying to switch to the mount namespace.  If
>>> the inferior does have a user namespace, and does have elevated
>>> privileges within that namespace, then this first switch by GDB will
>>> mean that the second step, into the mount namespace, will succeed.
>>>
>>> If there is no user namespace, or the inferior doesn't have elevated
>>> privileges within the user namespace, then the switch into the mount
>>> namespace will fail, just as it currently does, and the user will need
>>> to give elevated privileges to GDB via some other mechanism (e.g. run
>>> as root).
>>>
>>> This work was originally posted here:
>>>
>>>    https://inbox.sourceware.org/gdb-patches/20230321120126.1418012-1-benjamin@sipsolutions.net
>>>
>>> I (Andrew Burgess) have made some cleanups to the code to comply with
>>> GDB's coding standard, and the test is entirely mine.  This commit
>>> message is also entirely mine -- the original message was very terse
>>> and required the reader to understand how the various namespaces
>>> work and interact.  The above is my attempt to document what I now
>>> understand about the problem being fixed.
>>>
>>> I've left the original author in place as the core of the GDB change
>>> itself is largely as originally presented, but any inaccuracies in the
>>> commit message, or problems with the test, are all mine.
>>>
>>> Co-Authored-by: Andrew Burgess <aburgess@redhat.com>
>> 
>> I've pushed this patch.
>> 
>
> The new test-case fails on arm32 (Linaro CI reported this, and I was 
> able to reproduce) due to insufficient permissions:
> ...
> (gdb) attach 184732
> Attaching to process 184732
> warning: process 184732 is a zombie - the process has already terminated
> ptrace: Operation not permitted.
> (gdb) FAIL: gdb.base/user-namespace-attach.exp: flags=--mount 
> --map-root-user: attach to inferior
> ...
>
> In essence, the test-case assumes:
> ...
> $ unshare --mount --map-root-user /bin/true; echo $?
> 0
> ...
> but we get instead:
> ...
> $ unshare --mount --map-root-user /bin/true; echo $?
> unshare: unshare failed: Operation not permitted
> 1
> ...
>
> Filed here ( https://sourceware.org/bugzilla/show_bug.cgi?id=33108 ).

Hi!

Thanks for raising this issue.

What do you think to the patch below?

I've tested this by passing a bogus flag to `unshare`, e.g. "unshare
-blahblah", which has the same effect of causing the `unshare` process
to exit immediately with an exit code of 1.  I now see the test reported
as unsupported.

Thanks,
Andrew

---

commit 1c61ee90c22666a6f33a474c27a901d03bbc5da9
Author: Andrew Burgess <aburgess@redhat.com>
Date:   Wed Jun 25 11:24:30 2025 +0100

    gdb/testsuite: handle failure to start process to later attach to
    
    Commit:
    
      commit b23903836007d1acaf7f8c059ab000ee83fcebfa
      Date:   Tue Mar 21 13:01:26 2023 +0100
    
          gdb: linux-namespaces: enter user namespace when appropriate
    
    added a new test gdb.base/user-namespace-attach.exp.  It has been
    reported that this test will sometimes fail, like this:
    
      (gdb) attach 184732
      Attaching to process 184732
      warning: process 184732 is a zombie - the process has already terminated
      ptrace: Operation not permitted.
      (gdb) FAIL: gdb.base/user-namespace-attach.exp: flags=--mount --map-root-user: attach to inferior
    
    the test tries to run the 'unshare' application.  Sometimes though,
    the application is present, but the set of flags used is not
    supported (maybe due to restrictions on the local machine), so we see
    behaviour like this:
    
      $ unshare --mount --map-root-user /bin/true; echo $?
      unshare: unshare failed: Operation not permitted
      1
    
    Handle this case by checking for the warning:
    
      warning: process 184732 is a zombie - the process has already terminated
    
    when GDB tries to attach.  If we see this warning then we assume the
    'unshare' process failed to start correctly, in which case we report
    the test as unsupported and perform an early return.
    
    Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33108

diff --git a/gdb/testsuite/gdb.base/user-namespace-attach.exp b/gdb/testsuite/gdb.base/user-namespace-attach.exp
index 9936bb998eb..01f3dae1693 100644
--- a/gdb/testsuite/gdb.base/user-namespace-attach.exp
+++ b/gdb/testsuite/gdb.base/user-namespace-attach.exp
@@ -69,12 +69,18 @@ proc run_test { flags } {
 
     clean_restart
 
+    set saw_failure_to_start false
     set saw_bad_warning false
     gdb_test_multiple "attach $inferior_pid" "attach to inferior" {
 	-re "^attach $::decimal\r\n" {
 	    exp_continue
 	}
 
+	-re "^warning: process $::decimal is a zombie - the process has already terminated\r\n" {
+	    set saw_failure_to_start true
+	    exp_continue
+	}
+
 	-re "^warning: \[^\r\n\]+: could not open as an executable file: \[^\r\n\]+\r\n" {
 	    set saw_bad_warning true
 	    exp_continue
@@ -112,6 +118,11 @@ proc run_test { flags } {
 	}
 
 	-re "^$::gdb_prompt $" {
+	    if { $saw_failure_to_start } {
+		unsupported "unshare process failed to start"
+		return
+	    }
+
 	    gdb_assert { !$saw_bad_warning } $gdb_test_name
 	}