Bug 225583 - UML on FC6 fails because of utrace patch
UML on FC6 fails because of utrace patch
Product: Fedora
Classification: Fedora
Component: kernel (Show other bugs)
All Linux
medium Severity medium
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Assigned To: Roland McGrath
Brian Brock
Depends On:
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Reported: 2007-01-31 09:51 EST by JuanJo Ciarlante
Modified: 2007-11-30 17:11 EST (History)
3 users (show)

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Fixed In Version:
Doc Type: Bug Fix
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Last Closed: 2007-09-04 16:45:23 EDT
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RHEL 7.3 requirements from Atomic Host:
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Description JuanJo Ciarlante 2007-01-31 09:51:05 EST
Description of problem:
Running any user-mode-linux kernel (did try self compiled 2.6.17 and dloaded
2.6.18, umls) fails: it boots and stops responding after 
"VFS: Mounted root (ext3 filesystem) readonly."

I tracked back and narrowed the kernel versions where the problem
started to show:
   OK:  kernel-smp-2.6.17-1.2187_FC5 (last FC5 upd. kernel I know to run ok)
   BAD: kernel-smp-2.6.18-1.2239.fc5 (IIRC 1st non-working FC5 kernel)
   BAD: kernel-2.6.19-1.2895.fc6     (current FC6 kernel)

Note: this is a FC6 uptodate distro, booted with these 3 diff. kernels.
Because those FC5 kernels have same selinux policyversion , FC6 "environment"
runs ok AFAICT.

To see at least if the command gets "enough" CPU to halt the UML, I tried:     
 ./linux init=/sbin/halt -f                                                    
It didn't, "ps lx" on the host shows:                                          
   $ ps lx                                                                     
   0 240 4284 4152 15 0 33000 11000 - R+ pts/1 3:31 ./linux (Jj9kkn) /sbin/halt]
   1 240 4293 4284 25 0 33000 11000 - S+ pts/1 0:00 ./linux (Jj9kkn) /sbin/halt]
   1 240 4294 4284 24 0 33000 11000 - S+ pts/1 0:00 ./linux (Jj9kkn) /sbin/halt]
   1 240 4295 4284 15 0 33000 11000 - S+ pts/1 0:00 ./linux (Jj9kkn) /sbin/halt]
   1 240 4296 4284 15 0 60 56 -  R+ pts/1 2:42 [linux]                         
, where cpu time for those "R+" procs almost equals wall time;                 
strace'ing repeats eternally these lines:                                      
   $ strace -p 4284                                                            
   -- SIGCHLD (Child exited) @ 0 (0) ---                                       
   waitpid(4296, [{WIFSTOPPED(s) && WSTOPSIG(s) == SIGSEGV}], WSTOPPED) = 4296 
   ptrace(PTRACE_GETREGS, 4296, 0, 0xa06ab82c) = 0                             
   ptrace(PTRACE_GETFPXREGS, 4296, 0, 0xa06ab8dc) = 0                          
   ptrace(PTRACE_CONT, 4296, 0, SIGSEGV) = 0                                   

Version-Release number of selected component (if applicable):

How reproducible:
Check that you are running a stock FC6 kernel
Compile or download an UML guest kernel (eg: http://uml.nagafix.co.uk/)
and any root_fs image, boot UML: it will stop after VFS mount.

Steps to Reproduce:
1. Download/compile UML guest kernel, eg: 
   set -e
   wget http://uml.nagafix.co.uk/kernels/kernel32-
   mv kernel32- linux
   chmod +x linux

2. Download/build UML guest root_fs, eg: 
   wget http://uml.nagafix.co.uk/DSL-2.2/DSL-2.2-root_fs.bz2
   bunzip2 DSL-2.2-root_fs.bz2
   mv DSL-2.2-root_fs root_fs

3. ./linux
Actual results:
UML stops actually running after:
"VFS: Mounted root (ext3 filesystem) readonly."

Expected results:
To continue normal guest boot (run /sbin/init)

Additional info:
I've posted the FULL report uml-users mailing list:
Comment 1 JuanJo Ciarlante 2007-01-31 09:53:05 EST
I forgot metioning I did more testing:

I did the following tests, based on released utrace patch (OK means UML runs
w/no problem, BAD means UML stops after VFS root mounting )
#1 vanilla: OK
#2,CONFIG_{UP}TRACE=y    : BAD (same behaviour as FC6 stock kernel)
#3,CONFIG_{UP}TRACE unset: BAD (idem)

Curiously, #3 did the very same as #2 (even wrt strace process disposition
and strace "behaviour"), I'd expected #3 to simply fail at boot with
EINVAL or alike (given that no ptrace support was compiled in).

Comment 2 Roland McGrath 2007-02-06 00:12:14 EST
Would it be possible for you to test against vanilla 2.6.20?
The utrace patches in http://redhat.com/~roland/utrace/2.6-current/ are now
against 2.6.20, and have some fixes not in the 2.6.18 backport.  If utrace is
breaking UML, I'll need to debug it against the current kernel.
Comment 3 JuanJo Ciarlante 2007-02-06 15:22:05 EST
I've tested against 2.6.20: SAME (bad) behaviour as

FYI ss with previous kernels, I've downloaded  2.6.20 vanilla, copied .config
make && make install && boot ; later applied utrace patch , edited EXTRA_VERSION
&& ... && boot.
Comment 4 Mark Wielaard 2007-03-08 17:50:32 EST
According to Jeff Dike "UML runs on the utrace in -mm (i.e. utrace on the host),
which it didn't with several Fedora kernels."
Comment 5 Roland McGrath 2007-03-08 17:52:56 EST
Indeed, apparently problems with UML are now seen only on a CONFIG_PREEMPT=y
kernel, which Fedora does not ship.  However, it may be only the luck of the
timing, as the bug biting UML was never isolated.
Comment 6 Roland McGrath 2007-09-04 16:45:23 EDT
Several old bugs that might have affected this have been fixed.  Reopen if the
problem reoccurs.

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