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Date: Thu, 04 Mar 2021 14:38:10 +0900
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From: Hajime Tazaki<thehajime@gmail.com>
Subject: Re: linux uml segfault
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On Thu, 04 Mar 2021 07:40:00 +0900,
Johannes Berg wrote:
> 
> I think the problem is here:
> 
> > #24 0x000000006080f234 in ipc_init_ids (ids=0x60c60de8 <init_ipc_ns+8>)
> > at ipc/util.c:119
> > #25 0x0000000060813c6d in sem_init_ns (ns=0x60d895bb <textbuf+91>) at
> > ipc/sem.c:254
> > #26 0x0000000060015b5d in sem_init () at ipc/sem.c:268
> > #27 0x00007f89906d92f7 in ?? () from /lib/x86_64-linux-
> > gnu/libcom_err.so.2
> 
> You're in the init of libcom_err.so.2, which is loaded by
> 
> > "libnss_nis.so.2"
> 
> which is loaded by normal NSS code (getgrnam):
> 
> > #40 0x00007f89909bf3a6 in nss_load_library (ni=ni@entry=0x61497db0) at
> > nsswitch.c:359
> > #41 0x00007f89909bfc39 in __GI___nss_lookup_function (ni=0x61497db0,
> > fct_name=<optimized out>, fct_name@entry=0x7f899089b020 "setgrent") at
> > nsswitch.c:467
> > #42 0x00007f899089554b in init_nss_interface () at nss_compat/compat-
> > grp.c:83
> > #43 init_nss_interface () at nss_compat/compat-grp.c:79
> > #44 0x00007f8990895e35 in _nss_compat_getgrnam_r (name=0x7f8990a2a1e0
> > "tty", grp=0x7ffe3e7a2910, buffer=0x7ffe3e7a24e0 "", buflen=1024,
> > errnop=0x7f899089eb00) at nss_compat/compat-grp.c:486
> > #45 0x00007f8990968b85 in __getgrnam_r (name=name@entry=0x7f8990a2a1e0
> > "tty", resbuf=resbuf@entry=0x7ffe3e7a2910,
> > buffer=buffer@entry=0x7ffe3e7a24e0 "", buflen=1024,
> > result=result@entry=0x7ffe3e7a2908)
> >     at ../nss/getXXbyYY_r.c:315
> 
> 
> You have a strange nsswitch configuration that causes all of this
> (libnss_nis.so.2 -> libcom_err.so.2) to get loaded.
> 
> Now libcom_err.so.2 is trying to call sem_init(), and that gets ... tada
> ... Linux's sem_init() instead of libpthread's.
> 
> And then the crash.
> 
> 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.

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.


-- Hajime

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