ERROR: invalid ancestor arch/arm64/kernel/vdso/vdso.so.dbg for arch/arm64/kernel/vdso/vgettimeofday.o.

[Conleyfree]

In kernel 5.10, I built a livepatch for vmlinux with kpatch, and give errors as following:
ERROR: invalid ancestor arch/arm64/kernel/vdso/vdso.so.dbg for arch/arm64/kernel/vdso/vgettimeofday.o.
Q1, What does this error mean?

I learn a way to resolve this probelm from this issue https://github.com/dynup/kpatch/issues/518
Then I ignored the "arch/arm64/kernel/vdso/*" in kpatch-gcc, and it works!
Q2, Can I really safly ignored thie path?

[joe-lawrence]

Hi @Conleyfree ,

Q1: Ancestor refers to the "parent" object that a given object file is build for. For example, kpatch-build detects a change in net/netlink/af_netlink.o and then needs to know for which kernel object it belongs to. In this case it's built into vmlinux, so it proceeds to create a livepatch for vmlinux. Other cases may result in changes to a module, in which case the kernel object to be patched will be foo.ko.

Q2: Well, according to the arm64 vdso Makefile, it builds its only C file without $(CC_FLAGS_FTRACE) so nothing in the vdso is livepatchable. Ignoring the path will result in kpatch-build ignoring the changes to that file... which raises a few follow up questions:

• What does the input patch look like? Maybe it can be better targeted so it doesn't affect the vdso (and perhaps other) files. AFAIK the vdso code runs entirely in userspace, so I'm not really sure how the livepatch consistency model and/or stack unwinding works in that context.
• Which kpatch-build are you using that supports the arm architecture?

[Conleyfree]

Thanks for your answer， @joe-lawrence

I used kpatch version 0.6.1 for creating livepatches. In my patch, I just add a "pr_info" statement in function "do_task_stat" at fs/proc/array.c. I'm not sure why it affects vdso files. And I tried to modify other files, and the same error was reported.
This is one of my toy patch:

diff --git a/fs/proc/array.c b/fs/proc/array.c
index 18a4588c35be..170458bbc806 100644
--- a/fs/proc/array.c
+++ b/fs/proc/array.c
@@ -450,6 +450,7 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
        unsigned long rsslim = 0;
        unsigned long flags;

+       pr_info("test klp ...\n");
        state = *get_task_state(task);
        vsize = eip = esp = 0;
        permitted = ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS | PTRACE_MODE_NOAUDIT);

According your answer, when we create a livepatch for vmlinux, it seems that arm64 vdso is not supported livepatchable.
Can I submit a patch to the community to ignore arm64 vsdo?

[joe-lawrence]

Thanks for your answer， @joe-lawrence

I used kpatch version 0.6.1 for creating livepatches. In my patch, I just add a "pr_info" statement in function "do_task_stat" at fs/proc/array.c. I'm not sure why it affects vdso files. And I tried to modify other files, and the same error was reported. This is one of my toy patch:

diff --git a/fs/proc/array.c b/fs/proc/array.c
index 18a4588c35be..170458bbc806 100644
--- a/fs/proc/array.c
+++ b/fs/proc/array.c
@@ -450,6 +450,7 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
        unsigned long rsslim = 0;
        unsigned long flags;

+       pr_info("test klp ...\n");
        state = *get_task_state(task);
        vsize = eip = esp = 0;
        permitted = ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS | PTRACE_MODE_NOAUDIT);

I wouldn't think that a change to fs/proc/array.c would affect the vdso, however 1) kpatch version v0.6.1 is over 3 years old and 2) upstream kpatch doesn't support the arm64 arch (patches welcome!) so I can only speculate on this.

According your answer, when we create a livepatch for vmlinux, it seems that arm64 vdso is not supported livepatchable. Can I submit a patch to the community to ignore arm64 vsdo?

We would need support for arm64 for that patch to be useful. There is some interest (see #1216), so you may want to check in on that issue if you have patches or ideas for supporting that architecture.

[Conleyfree]

Thanks for your answer， @joe-lawrence
I've learned these knowledge.