linux uml segfault

[Anton Ivanov]

On 04/03/2021 07:47, Johannes Berg wrote:
> On Thu, 2021-03-04 at 14:38 +0900, Hajime Tazaki wrote:
> 
>>> Now, I don't know how to fix it (short of changing your nsswitch
>>> configuration) - maybe we could somehow rename sem_init()? Or maybe we
>>> can somehow give the kernel binary a lower symbol resolution than the
>>> libc/libpthread.
>>
>> objcopy (from binutils) can localize symbols (i.e., objcopy -L
>> sem_init $orig_file $new_file).  It also does renaming symbols.  But
>> not sure this is the ideal solution.
> 
> Yes, we started thinking about it but it was too late at night when I
> replied ...
> 
> I think there's basically a way to have an external list of symbols to
> export, for symbol versioning, that we could/should use to basically not
> export any of the kernel symbols out to libs.
> 
>> How does UML handle symbol conflicts between userspace code and Linux
>> kernel (like this case sem_init) ?  AFAIK, libnl has a same symbol as
>> Linux kernel (genlmsg_put) and others can possibly do as well.
> 
> I fear it doesn't?

Let's assume it does not, and try to fix this by de-conflicting the symbol.
For the time being, also, let's aim for a Debian specific patch just to go into their "patches" dir for build so that UML is not dropped out of the release.

This should make all internal uses of sem_init be um_sem_init in the actual object files. I will chase the issue of it picking up glibc memcpy separately.
Upon close inspection it looks like a different issue - it is in the other direction (picking a dynamic symbol instead of the one from the tree). I spent all day chasing it today and I cannot reproduce it. At the same time it was reproducible yesterday without any problems :(

Ritesh, can you give the following a spin - it renames sem_init as um_sem_init for UML only?

diff --git a/ipc/sem.c b/ipc/sem.c
index f6c30a85dadf..5157796daf54 100644
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -263,7 +263,11 @@ void sem_exit_ns(struct ipc_namespace *ns)
  }
  #endif

+#ifdef CONFIG_UML
+void __init um_sem_init(void)
+#else
  void __init sem_init(void)
+#endif
  {
         sem_init_ns(&init_ipc_ns);
         ipc_init_proc_interface("sysvipc/sem",
diff --git a/ipc/util.h b/ipc/util.h
index 5766c61aed0e..b3356efb3c96 100644
--- a/ipc/util.h
+++ b/ipc/util.h
@@ -47,7 +47,12 @@ extern int ipc_min_cycle;
  #define IPCMNI_IDX_MASK                ((1 << IPCMNI_SHIFT) - 1)
  #endif /* CONFIG_SYSVIPC_SYSCTL */

+#ifdef CONFIG_UML
+void um_sem_init(void);
+#define sem_init() um_sem_init()
+#else
  void sem_init(void);
+#endif
  void msg_init(void);
  void shm_init(void);



> 
> johannes
> 
> 

-- 
Anton R. Ivanov
https://www.kot-begemot.co.uk/