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Subject: Re: linux uml segfault
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From: Anton Ivanov <anton.ivanov@kot-begemot.co.uk>
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Date: Thu, 4 Mar 2021 07:28:54 +0000
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To: Johannes Berg <johannes@sipsolutions.net>, rrs@debian.org, Christopher Obbard <chris.obbard@collabora.com>, "linux-um@lists.infradead.org" <linux-um@lists.infradead.org>
Cc: 983379@bugs.debian.org

On 03/03/2021 22:40, 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.

I have not looked in depth in how the linking process works, but it 
should have picked up the sem_init from the kernel library, not libc.

We are already supposed to do that regarding kernel vs libc string.h 
functions - memcpy, etc.

Though for all of them the libc does the same so invoking the wrong one 
does not kill you so this may have been broken for a while and we were 
simply not noticing it.

> 
> 
> johannes
> 
> 


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

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